AQA GCSE Combined Science: Trilogy Revision
Explore AQA GCSE Combined Science: Trilogy revision topics. Check the official specification for complete requirements for your exam year and route.
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Reviewed course guidance
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Topic group
Biology · 4.1 Cell biology
- Core
Scope: Core · 4.1.1.1
Open revision guidance
What to know
- Animal and plant cells are eukaryotic: genetic material is enclosed in a nucleus. Bacterial cells are prokaryotic: their DNA is not in a nucleus and includes a single loop; plasmids may also be present.
- Plant and animal cells have a cell membrane, cytoplasm and ribosomes. Plant cells also have a cellulose wall, chloroplasts and a permanent vacuole; mitochondria are the site of aerobic respiration.
- Bacterial cell walls and cell membranes surround the cytoplasm, but bacterial cells are much smaller and do not have the membrane-bound organelles found in eukaryotes.
What to do in an assessment
- In an unseen micrograph, identify the cell type from its structures before explaining a difference. State what the structure does when the question asks for function; do not confuse bacterial cell wall with a plant cellulose wall.
In plain language
The useful test is where the DNA is and whether the cell contains membrane-bound organelles. A bacterium has DNA, even though it has no nucleus: its main DNA loop lies in the cytoplasm, and small plasmids may carry additional genes. A plant cell has a nucleus plus chloroplasts and a large vacuole; the wall outside its membrane gives support.
Self-check
A diagram shows a small cell with a wall, membrane, cytoplasm, a loop of DNA and a small DNA ring but no nucleus. Identify it and name the small ring.
Show the answer
It is a bacterial (prokaryotic) cell. The small extra ring of DNA is a plasmid; absence of a nucleus does not mean absence of genetic material.
- Core
Scope: Core · 4.1.3.1
Open revision guidance
What to know
- Diffusion is the net movement of particles from a region of higher concentration to lower concentration, down a concentration gradient, because particles move randomly.
- Dissolved substances and gases can diffuse through a cell membrane. A larger concentration difference produces a faster net movement, all else equal.
- Diffusion does not require the cell to supply energy; this distinguishes it from active transport.
What to do in an assessment
- For a transport question, name the substance, the direction relative to its concentration gradient, and the process. Use net movement rather than claiming every particle moves one way.
In plain language
Particles keep moving randomly in both directions. When there are more on one side, more cross toward the side with fewer, so the overall result is movement down the gradient. That net change is diffusion.
Self-check
Oxygen concentration is higher outside a cell than inside. Which way is the net diffusion of oxygen?
Show the answer
Into the cell, from higher oxygen concentration outside to lower concentration inside; individual oxygen molecules still move randomly in both directions.
Topic group
Biology · 4.3 Infection and response
- Core
Scope: Core · 4.3.1.1
Open revision guidance
What to know
- Communicable diseases are caused by pathogens and can spread between organisms. AQA names viruses, bacteria, protists and fungi as pathogen groups affecting animals and plants.
- Transmission can occur by direct contact, water, air droplets or vectors; the route depends on the pathogen and host. Plant diseases can spread between plants as well as through contaminated material or vectors.
- A virus reproduces inside cells; bacteria are living cells that can reproduce independently in suitable conditions. Both can cause disease, but the treatment implications are not interchangeable.
What to do in an assessment
- When asked how a disease spreads, link a specific route to how the pathogen reaches a susceptible host. In a comparison, distinguish pathogen type and replication rather than calling every pathogen a bacterium.
In plain language
A pathogen must reach a host it can infect. An airborne pathogen may travel in droplets, while a vector carries a pathogen between hosts. The route matters because reducing contact with that route can interrupt transmission.
Self-check
A crop disease is carried between plants by an insect. Name the transmission route and give one way to interrupt it.
Show the answer
It is vector transmission. Controlling the insect vector can reduce transmission, although the specific control method should match the crop and disease context.
- Core
Scope: Core · 4.3.1.6
Open revision guidance
What to know
- The body has non-specific barriers such as skin, mucus/cilia in the airways and acid in the stomach. These reduce the chance that pathogens enter or survive.
- If pathogens enter, the immune system recognises foreign antigens. White blood cells can engulf pathogens and produce antibodies; memory cells can support a faster later response.
What to do in an assessment
- Source scope: 4.3.1.6.
- Distinguish a physical/chemical barrier from a specific immune response. Explain the sequence from pathogen entry to antibody action rather than saying antibodies kill every pathogen directly.
In plain language
Defence works in layers: barriers reduce entry, while immune cells respond to pathogens that get inside. Antibodies bind to particular antigens, making the pathogen easier to target.
Self-check
Why is skin described as a non-specific defence?
Show the answer
It acts as a general physical barrier against many kinds of pathogen rather than targeting one specific antigen.
- Core
Scope: Core · 4.3.1.7
Open revision guidance
What to know
- Vaccines introduce a small quantity of dead or inactive pathogen, or its antigens, to stimulate an immune response without causing the full disease.
- Lymphocytes produce specific antibodies and memory cells. A later encounter can trigger a faster antibody response; high vaccination coverage can reduce transmission through a population.
What to do in an assessment
- Source scope: 4.3.1.7.
- Explain both individual protection and population spread. Do not state that vaccination guarantees an individual can never become infected.
In plain language
The first exposure trains the immune system. If the pathogen returns, memory cells help the body respond sooner, reducing the chance of illness and spread.
Self-check
How can vaccination reduce pathogen transmission in a population?
Show the answer
When many people are immune, a pathogen has fewer susceptible hosts to infect, reducing onward transmission.
Topic group
Biology · 4.4 Bioenergetics
- Core
Scope: Core · 4.4.1.1
Open revision guidance
What to know
- Photosynthesis transfers energy from light into chemical energy stored in glucose. In plants it occurs in chloroplasts.
- Carbon dioxide and water are reactants; glucose and oxygen are products. The balanced symbol equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- It is an endothermic reaction: energy is transferred from the environment by light.
What to do in an assessment
- Balance the equation before using it quantitatively. In a practical or data question, identify the measured outcome (for example, oxygen produced) and state which factor is changed and which are controlled.
In plain language
A leaf does not turn light directly into plant material. Chloroplasts use light energy to rearrange carbon dioxide and water into glucose and oxygen; the energy is now stored chemically in glucose.
Self-check
Why is photosynthesis described as endothermic, and what two reactants supply the atoms in glucose?
Show the answer
It takes in energy transferred by light, so it is endothermic. Carbon dioxide and water provide the atoms used to make glucose.
- Core
Scope: Core · 4.4.2.1
Open revision guidance
What to know
- Respiration is an exothermic process in cells that transfers energy for cellular activities. Aerobic respiration uses oxygen to release more energy from glucose.
- When oxygen supply is insufficient, anaerobic respiration in muscles releases less energy and produces lactic acid; in yeast, anaerobic respiration produces ethanol and carbon dioxide.
What to do in an assessment
- Source scope: 4.4.2.1.
- Distinguish cellular respiration from breathing and use the organisms/conditions in the question to select the correct anaerobic products.
In plain language
Breathing moves air in and out of lungs; respiration is the set of reactions in cells that transfers energy from glucose.
Self-check
What product accumulates in muscles during anaerobic respiration?
Show the answer
Lactic acid.
Topic group
Chemistry · 5.3 Quantitative chemistry
- Core
Scope: Core · 5.3.1.1
Open revision guidance
What to know
- In a chemical reaction atoms are rearranged; they are not created or destroyed. A closed system therefore has equal total mass before and after reaction.
- A balanced symbol equation has the same number of each type of atom on both sides. Change coefficients in front of formulae, not the formula subscripts.
- An apparent mass change can occur if a gas enters or escapes an open apparatus; this is a change in what is weighed, not failure of conservation.
What to do in an assessment
- Count each element on both sides, adjust whole-number coefficients, then recount. For a practical mass comparison, state whether the apparatus is open or sealed and whether gas can escape.
In plain language
A reaction changes which atoms are joined together, not the atoms themselves. If a gas escapes an open flask, the flask and contents can weigh less even though the total mass of flask, gas and everything else is conserved.
Self-check
Balance H₂ + O₂ → H₂O. Why could the mass measured for an open flask appear to fall?
Show the answer
2H₂ + O₂ → 2H₂O. If a gaseous product leaves the flask, the balance no longer weighs that escaped gas.
- Higher
Scope: Higher · Examination tier: Higher tier · 5.3.2.1
Open revision guidance
What to know
- One mole contains the same number of specified particles as any other mole; the Avogadro constant is 6.02 × 10²³ mol⁻¹.
- The molar mass in grams per mole is numerically equal to relative formula mass. Amount in moles = mass ÷ relative formula mass, and mass = moles × relative formula mass.
What to do in an assessment
- Source scope: 5.3.2.1.
- Identify whether the question asks for moles or mass, calculate Mr from the formula, substitute with compatible units and give a suitable significant-figure answer.
In plain language
A mole is a counting unit for particles. Because different substances have particles with different masses, one mole of each has the same particle count but a different mass.
Self-check
How many moles are in 12 g of carbon with Mr = 12?
Show the answer
n = mass ÷ Mr = 12 ÷ 12 = 1 mol.
Topic group
Chemistry · 5.4 Chemical changes
- Core
Scope: Core · 5.4.1.2
Open revision guidance
What to know
- Metal atoms form positive ions by losing electrons. A more reactive metal has a greater tendency to form positive ions.
- A more reactive metal can displace a less reactive metal from its compound. The observed change supports the relative positions in the reactivity series.
- Use evidence from reactions with water, dilute acids and salt solutions; the outcome depends on the relative reactivity of the two metals.
What to do in an assessment
- For a displacement prediction, compare the two metals first, then identify which becomes ions and which is deposited. Write a word or symbol equation and check atoms and charge if ionic equations are requested.
In plain language
A more reactive metal takes the place of a less reactive one in a compound. The more reactive metal atoms lose electrons and enter solution as positive ions; the displaced metal ions gain electrons and appear as metal.
Self-check
A strip of zinc is placed in copper sulfate solution. Which metal is deposited, and why?
Show the answer
Copper is deposited. Zinc is more reactive than copper, so zinc forms Zn²⁺ ions and displaces copper from the solution.
- Core
Scope: Core · §5.4 Chemical changes (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- Acids in water produce hydrogen ions. Acid reactions with metals can produce a salt and hydrogen; reactions with bases or carbonates produce a salt and water, with carbon dioxide also formed from a carbonate.
- Salt names depend on the acid: hydrochloric acid forms chlorides, sulfuric acid sulfates and nitric acid nitrates. Soluble salts can be prepared by neutralisation followed by crystallisation.
- During electrolysis, mobile ions move to oppositely charged electrodes. Positive ions gain electrons at the cathode; negative ions lose electrons at the anode.
What to do in an assessment
- Predict products from reactants and balance the symbol equation. For electrolysis, identify ions present and state which electrode they move toward before predicting products.
In plain language
An acid supplies hydrogen ions in solution. A base removes those ions, leaving a salt. In electrolysis the electric current drives separated ions to electrodes, where electrons are gained or lost.
Self-check
What salt forms when hydrochloric acid is neutralised by sodium hydroxide?
Show the answer
Sodium chloride and water are formed: hydrochloric acid + sodium hydroxide → sodium chloride + water.
Topic group
Chemistry · 5.6 Rate and extent of chemical change
- Core
Scope: Core · 5.6.1.2
Open revision guidance
What to know
- Rate depends on how often reacting particles collide successfully. Higher concentration or gas pressure, higher temperature, greater solid surface area, or a catalyst can increase rate.
- Increasing temperature makes particles move faster and increases the fraction with energy at least equal to the activation energy. A catalyst provides an alternative pathway with lower activation energy.
- A concentration or surface-area change changes collision frequency; a catalyst is not used up overall.
What to do in an assessment
- Use a rate measure such as gas volume per time or the time for a visible endpoint. Keep other variables controlled and use the particle collision model to explain the observed direction of change.
In plain language
A collision only makes products if the particles meet with enough energy. More particles in the same volume, a hotter mixture, or smaller pieces of solid can raise the number of successful collisions each second; a catalyst lowers the energy barrier.
Self-check
Why does increasing the temperature usually make a reaction faster?
Show the answer
Particles move faster and a larger fraction collide with energy equal to or above the activation energy, so there are more successful collisions per second.
Topic group
Physics · 6.1 Energy
- Core
Scope: Core · 6.1.1.1
Open revision guidance
What to know
- A system is an object or group of objects. When it changes, energy may be transferred between stores, for example by heating, radiation, electrical work or mechanical work.
- Energy is conserved: in a closed system the total energy remains constant, although some energy may become less useful as it spreads to the surroundings.
- For a raised object, energy is transferred to a gravitational potential store; for a moving object, to a kinetic store. Identify the system and the transfer pathway.
What to do in an assessment
- Draw an energy-transfer diagram with initial and final stores and the transfer pathway. Distinguish energy transferred from energy stored, and explain why dissipated energy is less useful rather than destroyed.
In plain language
Energy is not a substance that disappears. It moves between stores. A motor can transfer energy electrically into movement, while some spreads into the surroundings as heating; the total still balances.
Self-check
A falling object speeds up while air resistance warms the air. Which stores gain energy?
Show the answer
The object’s kinetic store gains energy and the thermal stores of the object and surroundings gain dissipated energy; the total energy is conserved.
- Core
Scope: Core · 6.1.1.4
Open revision guidance
What to know
- Power is the rate of energy transfer or work done. P = E/t and P = W/t; power is measured in watts, energy/work in joules and time in seconds.
- A higher power means more energy is transferred each second. Convert time to seconds before using the equations if required.
What to do in an assessment
- Source scope: 6.1.1.4.
- Select the equation matching the known quantities, substitute values with SI units and check whether the answer is a power or an energy.
In plain language
Power tells how quickly energy moves, not how much energy is stored in total.
Self-check
A device transfers 600 J in 20 s. What is its power?
Show the answer
P = E/t = 600/20 = 30 W.
Topic group
Physics · 6.2 Electricity
- Core
Scope: Core · 6.2.1.4
Open revision guidance
What to know
- For an ohmic conductor at constant temperature, current is proportional to potential difference and resistance is constant. The relationship is V = I R.
- For a filament lamp, increasing current heats the filament; its resistance rises, so the current is no longer directly proportional to potential difference.
- A thermistor’s resistance changes with temperature and an LDR’s resistance changes with light intensity; use the specification’s stated component behaviour when interpreting a circuit.
What to do in an assessment
- Read current and voltage axes with units, compare ratios V/I rather than only the graph slope if the axes are reversed, and explain a curved I–V trace by the component’s changing resistance.
In plain language
Resistance tells how much a component opposes current. A metal wire at steady temperature behaves simply: doubling voltage doubles current. A hot lamp filament opposes current more strongly than a cool one, so its graph bends.
Self-check
A resistor has 2.0 V across it and a current of 0.40 A. Calculate its resistance.
Show the answer
R = V/I = 2.0/0.40 = 5.0 Ω.
- Core
Scope: Core · §6.2 Electricity (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- Current is rate of charge flow; potential difference is energy transferred per unit charge. A resistor circuit can be analysed with V = IR and E = QV.
- Series components share one current; potential differences across them add. Parallel branches have the same potential difference and split current.
- Mains electricity is alternating; electrical safety depends on insulation, earthing and circuit protection. Power is the rate of energy transfer, P = IV.
What to do in an assessment
- Mark meter placement, trace conventional current, and show units in each equation. In a domestic-safety explanation identify the fault path and how a fuse or breaker interrupts current.
In plain language
Voltage tells how much energy each coulomb transfers; current tells how many coulombs pass each second. Their product is power, the energy transfer each second.
Self-check
A device operates at 230 V and draws 2.0 A. What is its power?
Show the answer
P = IV = 230 × 2.0 = 460 W.
Topic group
Physics · 6.5 Forces
- Core
Scope: Core · 6.5.4.1
Open revision guidance
What to know
- Distance is the total path travelled; displacement is the change in position in a stated direction. Speed is scalar, whereas velocity includes direction.
- Average speed = distance ÷ time. Acceleration = change in velocity ÷ time; give units and use a signed velocity if direction changes.
- A straight segment on a distance–time graph has constant speed; on a velocity–time graph, gradient gives acceleration and area under the graph gives displacement.
What to do in an assessment
- Convert units before calculating, retain direction where velocity is used, and show the equation, substitution and unit. On a graph identify whether gradient or area has the required meaning.
In plain language
Speed describes how fast something moves; velocity also says which way. If a runner returns to the start, the total distance is not zero even though the final displacement is.
Self-check
A cyclist travels 150 m in 30 s. What is the average speed?
Show the answer
Average speed = 150 m ÷ 30 s = 5.0 m s⁻¹.
- Core
Scope: Core · §6.5 Forces (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- A resultant force changes velocity according to F = ma. A force can change speed, direction or shape; balanced forces mean zero resultant, not necessarily rest.
- Momentum is mass multiplied by velocity. In a closed system total momentum is conserved; increasing collision time reduces the average force for the same momentum change.
- Stopping distance combines thinking and braking distances; speed, fatigue, alcohol, road conditions and vehicle condition affect safety. Pressure is force per unit area.
What to do in an assessment
- Draw force arrows with direction, calculate resultant before acceleration and use units. In collision explanations connect impulse/change in momentum to contact time.
In plain language
If a moving car has balanced forward and backward forces, it can keep moving steadily. A non-zero resultant changes its velocity, and the size of that change depends on the object’s mass.
Self-check
A 2 kg trolley experiences a resultant force of 6 N. What is its acceleration?
Show the answer
a = F/m = 6/2 = 3 m s⁻².
Topic group
Physics · 6.6 Waves
- Core
Scope: Core · 6.6.1.2
Open revision guidance
What to know
- Amplitude is the maximum displacement from the undisturbed position; wavelength is the distance between equivalent points on adjacent waves.
- Frequency is the number of complete waves passing a point per second, measured in hertz. Period is the time for one wave and is measured in seconds.
- Transverse waves oscillate perpendicular to their direction of travel; longitudinal waves oscillate parallel to it.
What to do in an assessment
- Label amplitude and wavelength from a diagram using equivalent points. Read frequency or period with units and explain whether a wave transfers energy without transferring matter overall.
In plain language
A wave shape moves forward while a point on the medium oscillates around its resting position. Wavelength is a distance along the wave; amplitude is the size of the oscillation at one point.
Self-check
A wave has a frequency of 4 Hz. How long does one complete oscillation take?
Show the answer
The period is 1/f = 1/4 s = 0.25 s.
Topic group
Biology · 4.2 Organisation
- Core
Scope: Core · §4.2 Organisation (topic 2; full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- Organ systems are built from cells, tissues and organs. In humans, the digestive system breaks large insoluble food molecules into soluble molecules that can be absorbed through the small intestine.
- The circulatory system moves blood between heart, lungs and tissues. Arteries carry blood away from the heart, veins toward it and capillaries provide thin exchange surfaces; the heart’s chambers and valves support one-way flow.
- Xylem carries water and mineral ions from roots to leaves; phloem transports dissolved sugars around a plant. Stomata permit gas exchange, while guard cells change pore opening.
What to do in an assessment
- For an adaptation, name the structure and show how its feature helps the transport or exchange function. On a diagram, follow the direction of blood or sap rather than memorising isolated parts.
In plain language
Digestion makes molecules small enough to cross the gut wall; blood carries absorbed substances to body cells. Plants use different transport tissues: xylem mainly moves water and minerals, while phloem distributes products such as sugars.
Self-check
Which vessel carries blood away from the heart, and where does exchange with tissues occur?
Show the answer
Arteries carry blood away from the heart; exchange with tissues occurs across capillary walls.
Topic group
Biology · 4.5 Homeostasis and response
- Core
Scope: Core · §4.5 Homeostasis and response (topic 5; full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- Homeostasis keeps internal conditions within limits. Receptors detect a change; coordination centres process information; effectors produce a response that opposes the change.
- The nervous system uses electrical impulses for rapid, targeted responses. Hormones travel in blood and usually act more slowly on cells with suitable receptors.
- The body regulates temperature, blood glucose and water balance; hormone control also coordinates reproductive cycles. Negative feedback reduces a deviation from a set range.
What to do in an assessment
- Trace a control loop as change → receptor → coordinator → effector → response. State whether the response counteracts the change; do not confuse a nerve impulse with a hormone in blood.
In plain language
A thermostat analogy helps: if body temperature rises, sensors and effectors trigger cooling responses that bring it back toward the normal range. A response that reduces the original change is negative feedback.
Self-check
If blood glucose rises after a meal, what should a negative-feedback response do?
Show the answer
It should reduce the blood glucose concentration toward its normal range; insulin release promotes uptake/storage of glucose.
Topic group
Biology · 4.6 Inheritance, variation and evolution
- Core
Scope: Core · §4.6 Inheritance, variation and evolution (topic 6; full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- Meiosis produces haploid gametes with half the usual chromosome number; fertilisation combines gametes and restores the diploid number. DNA, genes and alleles provide inherited information.
- Mutation and sexual reproduction contribute to genetic variation. Natural selection occurs when inherited variants affect survival or reproduction, changing the frequency of alleles over generations.
- Selective breeding chooses parents with desired characteristics; cloning copies an organism; genetic engineering transfers selected genes. The methods differ in how variation is selected or introduced.
What to do in an assessment
- Explain evolution across generations, not as an individual changing because it needs to. A genetic cross should show parental alleles, gametes and possible offspring before calculating probability.
In plain language
An environmental challenge does not direct a useful mutation. Some individuals already carry variants; if a variant helps its carriers reproduce, it may become more common in later generations.
Self-check
What happens to an advantageous inherited allele when carriers leave more offspring over many generations?
Show the answer
The allele can increase in frequency in the population through natural selection.
- Core
Scope: Core · 4.6.2.2
Open revision guidance
What to know
- Populations contain inherited variation. If a characteristic gives some individuals a reproductive advantage in a particular environment, those individuals are more likely to pass it on.
- Over generations, the frequency of the advantageous inherited characteristic can increase; evolution is change in inherited characteristics of a population over time.
What to do in an assessment
- Source scope: 4.6.2.2.
- Describe variation, selection pressure, differential survival/reproduction and inheritance in sequence. Do not claim an individual evolves because it tries to adapt.
In plain language
Natural selection acts on differences already present. Successful individuals leave more offspring, so the population changes over many generations.
Self-check
What changes during evolution by natural selection?
Show the answer
The inherited characteristics of a population, over generations.
Topic group
Biology · 4.7 Ecology
- Core
Scope: Core · §4.7 Ecology (topic 7; full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- An ecosystem contains communities of organisms and their physical environment. Abiotic conditions and biotic interactions affect distribution and abundance.
- Energy enters many ecosystems through sunlight, passes through food chains and is dissipated; materials such as carbon and water are recycled by organisms and physical processes.
- Biodiversity is variation among living organisms. Human activities can reduce it; sustainable resource use, conservation and habitat management can reduce harm.
What to do in an assessment
- Distinguish matter cycling from energy flow. In a food chain use arrows from food to consumer, and use data to support claims about population change or conservation.
In plain language
Carbon atoms can be reused in new molecules: photosynthesis takes in carbon dioxide and respiration, decomposition and combustion return it. Energy is not recycled in the same way; it spreads to the surroundings at each transfer.
Self-check
Is energy in an ecosystem recycled in the same way as carbon atoms?
Show the answer
No. Carbon cycles through organisms and the environment, while energy flows through and is eventually dissipated.
- Core
Scope: Core · 4.7.2.2
Open revision guidance
What to know
- Materials cycle between living organisms and the non-living environment. Photosynthesis removes carbon dioxide from air; respiration, decomposition and combustion return it.
- Water moves through evaporation, condensation and precipitation and between soil, organisms and water bodies. Both cycles connect ecosystems to physical processes.
What to do in an assessment
- Source scope: 4.7.2.2.
- Annotate cycle arrows with the process and direction of transfer. When discussing carbon, distinguish carbon atoms moving between stores from energy flowing through food chains.
In plain language
Cycles mean matter is reused rather than used up. The same carbon atoms may move from air into a plant, through animals, and back to the atmosphere.
Self-check
Which biological process removes carbon dioxide from the atmosphere?
Show the answer
Photosynthesis.
Topic group
Chemistry · 5.1 Atomic structure and periodicity
- Core
Scope: Core · §5.1 Atomic structure and the periodic table (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
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What to know
- An atom has a tiny nucleus containing protons and neutrons with electrons in shells. Proton number identifies the element; atoms are neutral when proton and electron counts match.
- Isotopes have the same number of protons but different neutron numbers. Relative atomic mass is an abundance-weighted mean and may not be a whole number.
- The modern periodic table is ordered by proton number. Elements in a group have related chemical properties; their outer-electron arrangements help explain those patterns.
What to do in an assessment
- Use proton number to identify an element and mass number to find neutrons. For a relative atomic-mass calculation, multiply each isotope mass by its fractional abundance and add.
In plain language
Atoms of one element always have the same proton count. Isotopes are versions of that element with different neutron counts, so a sample’s average atomic mass can fall between whole numbers.
Self-check
A sodium atom has 11 protons and mass number 23. How many neutrons does its nucleus contain?
Show the answer
23 − 11 = 12 neutrons.
Topic group
Chemistry · 5.2 Bonding and structure
- Core
Scope: Core · §5.2 Bonding, structure, and properties of matter (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- Ionic bonding involves electron transfer and electrostatic attraction between oppositely charged ions. Simple molecular substances have strong covalent bonds within molecules but weaker forces between molecules.
- Giant covalent structures contain many atoms joined in a network; metals contain positive ions and delocalised electrons. These different structures explain differences in melting point, conductivity and hardness.
- For a property explanation, connect the particle arrangement and bonding to the energy or charge movement involved.
What to do in an assessment
- State whether a structure is simple molecular, giant covalent, ionic or metallic. Explain conductivity by whether charged particles are free to move, and melting point by the attractions that must be overcome.
In plain language
Solid sodium chloride contains ions but they cannot move freely, so it does not conduct. When molten, ions can move and carry charge. A metal conducts because delocalised electrons move through its structure.
Self-check
Why does solid sodium chloride not conduct electricity but molten sodium chloride does?
Show the answer
The ions are fixed in the solid but mobile in the molten liquid; mobile charged ions carry current.
Topic group
Chemistry · 5.5 Energy changes
- Core
Scope: Core · §5.5 Energy changes (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- Exothermic reactions transfer energy to the surroundings; endothermic reactions take in energy from the surroundings. Temperature change can provide evidence of the direction.
- Breaking bonds requires energy and forming bonds releases energy. Overall reaction energy depends on the balance between energy absorbed and released.
- A reaction profile represents relative energy of reactants and products and the activation energy needed to start a reaction.
What to do in an assessment
- Use surroundings’ temperature change to identify exothermic/endothermic, then compare reactant and product energy levels. For a bond-energy calculation, total bonds broken minus total bonds formed gives the reaction energy change.
In plain language
In an exothermic reaction, bond formation releases more energy than bond breaking takes in, so the surroundings warm. Endothermic reactions absorb more than they release, so surroundings cool.
Self-check
The reaction mixture cools. Is the reaction exothermic or endothermic?
Show the answer
Endothermic: it takes in thermal energy from its surroundings.
Topic group
Chemistry · 5.7 Organic chemistry
- Core
Scope: Core · §5.7 Organic chemistry (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- Crude oil is a finite mixture of hydrocarbons formed from ancient biomass. Fractional distillation separates fractions by their different boiling ranges.
- Hydrocarbons contain hydrogen and carbon only. Shorter-chain alkanes tend to be more volatile and flammable and have lower viscosity than longer-chain alkanes.
- Cracking breaks long-chain hydrocarbons into shorter molecules, including alkenes used to make polymers and other products.
What to do in an assessment
- Use molecular formulae or displayed structures to count atoms and identify homologous-series features. Link a fraction’s properties to a named use rather than simply listing uses.
In plain language
A crude-oil column is hotter at the bottom than at the top. Vapours condense at different heights according to their boiling ranges, separating mixtures without turning them into pure single compounds.
Self-check
Why is crude oil fractionated rather than treated as one pure compound?
Show the answer
It is a mixture of hydrocarbons with different boiling ranges, so they condense at different temperatures/heights.
Topic group
Chemistry · 5.8 Chemical analysis
- Core
Scope: Core · §5.8 Chemical analysis (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- Chemical tests identify ions and gases through characteristic colour changes, precipitates or gas behaviour. A positive result is evidence for an ion, not necessarily proof that no other species are present.
- Chromatography separates mixture components according to their movement in a mobile solvent and attraction to the stationary phase.
- Instrumental methods can analyse small quantities rapidly and sensitively; results should be interpreted against known standards or reference values.
What to do in an assessment
- Describe the reagent and observation separately, then state the supported inference. In chromatography, mark the solvent front and compare spots only when the solvent and conditions match.
In plain language
A test’s colour or precipitate is an observation; naming the ion is an inference based on the test. Keeping the two statements separate makes it clear what the evidence actually showed.
Self-check
In chromatography, why should a pencil line be used for the starting spot?
Show the answer
Graphite is insoluble in the solvent, so it will not travel and contaminate the separated spots.
- Core
Scope: Core · 5.8.1.3
Open revision guidance
What to know
- A solvent carries dissolved substances through a stationary phase; different attractions cause mixture components to move different distances.
- Rf = distance moved by substance ÷ distance moved by solvent front. Use distances from the baseline and the same units; a pure substance gives one spot under the method’s conditions.
What to do in an assessment
- Source scope: 5.8.1.3.
- Keep the baseline above the solvent, mark the solvent front promptly and compare Rf values only when solvent and conditions match.
In plain language
A mixture can separate because its substances travel at different speeds with the solvent. The Rf ratio describes how far a component travelled compared with the solvent.
Self-check
A dye moves 2 cm while the solvent moves 5 cm. Calculate Rf.
Show the answer
Rf = 2 ÷ 5 = 0.4.
- Core
Scope: Core · 5.8.2.2
Open revision guidance
What to know
- Oxygen supports combustion. A glowing splint inserted into a test tube containing the gas relights if oxygen is present.
- Use a small gas sample and distinguish a glowing splint test from the lit splint used to test for hydrogen.
What to do in an assessment
- Source scope: 5.8.2.2.
- State the apparatus and observation precisely: introduce a glowing splint, then record whether it relights.
In plain language
Oxygen helps a glowing ember burn more strongly. The relighting observation is the positive test.
Self-check
What is the positive result for oxygen?
Show the answer
A glowing splint relights.
Topic group
Chemistry · 5.9 Atmosphere
- Core
Scope: Core · §5.9 Chemistry of the atmosphere (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- Earth’s atmosphere changed over geological time through volcanic activity, photosynthesis, respiration and other processes; the present atmosphere is mainly nitrogen and oxygen.
- Combustion of fuels can release carbon dioxide and pollutants such as sulfur dioxide and particulates. Greenhouse gases absorb outgoing infrared radiation and contribute to warming.
- Climate explanations use evidence and account for natural and human influences; forecasts involve uncertainty because many variables interact.
What to do in an assessment
- Distinguish the greenhouse effect from ozone depletion and from local air pollution. Use measured trends and identify a causal mechanism rather than claiming one weather event proves a long-term climate trend.
In plain language
Greenhouse gases let much incoming visible light through but absorb some infrared radiation emitted by Earth, then re-emit it. Increasing greenhouse-gas concentration strengthens this warming effect.
Self-check
Why can a rise in carbon dioxide affect Earth’s average temperature?
Show the answer
Carbon dioxide absorbs and re-emits outgoing infrared radiation, increasing heat retained in the atmosphere.
Topic group
Chemistry · 5.10 Using resources
- Core
Scope: Core · §5.10 Using resources (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- Finite raw materials and energy are used to make products; extraction, manufacture and disposal can cause pollution and habitat damage.
- Life-cycle assessment compares environmental impacts across raw-material extraction, processing, use and disposal. Results depend on assumptions and are not a single unquestionable score.
- Recycling can conserve materials and energy but requires collection, sorting and processing; sustainability balances benefits against impacts.
What to do in an assessment
- Compare the same stages and functional use when assessing options. Support a conclusion with evidence and acknowledge a trade-off such as energy used in recycling versus avoided mining.
In plain language
A reusable or recyclable product can reduce the need for new raw materials, but collection and processing also consume energy. A fair comparison considers its whole life rather than one attractive feature.
Self-check
Why is it misleading to compare two products using only the energy needed to manufacture them?
Show the answer
It ignores raw-material extraction, use, transport and disposal; a whole-life comparison considers all those stages.
Topic group
Physics · 6.3 Particle model
- Core
Scope: Core · §6.3 Particle model of matter (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- In a solid, particles vibrate around fixed positions; in a liquid they move past each other; in a gas they move freely and are far apart. Heating changes particles’ energy and can cause a change of state.
- Density is mass per unit volume. For a fixed mass, expansion lowers density; for an irregular solid, volume can be found from liquid displacement.
- Gas pressure results from particle collisions with container walls. At fixed volume, increasing temperature increases particle kinetic energy and typically pressure.
What to do in an assessment
- Use ρ = m/V with matching units. On a heating curve distinguish temperature increase from a flat change-of-state interval where energy is used to alter particle arrangement.
In plain language
A solid is not made of motionless particles: they vibrate in place. When heated, their energy increases; at melting, added energy changes the arrangement instead of immediately raising temperature.
Self-check
Why does a gas exert pressure on the walls of its container?
Show the answer
Its moving particles collide with the walls and exert forces; many collisions create pressure.
Topic group
Physics · 6.4 Atomic structure
- Core
Scope: Core · §6.4 Atomic structure (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- The nucleus contains protons and neutrons; electrons occupy surrounding regions. Isotopes share proton number but differ in neutron number.
- Unstable nuclei decay randomly, emitting ionising radiation. Alpha, beta and gamma differ in ionising power, range and penetration.
- Activity is measured in becquerels; half-life is the time for activity or number of undecayed nuclei to halve. Ionising radiation can damage cells, so exposure and contamination are controlled.
What to do in an assessment
- Use the stated half-life repeatedly to calculate remaining activity. Distinguish irradiation from contamination and compare shielding with the radiation type.
In plain language
A single nucleus cannot be predicted to decay at a chosen moment; the behaviour of a large sample is described statistically. Half-life gives the repeatable halving interval for that sample.
Self-check
A sample has activity 800 Bq and half-life 5 days. What is its activity after 10 days?
Show the answer
Two half-lives pass: 800 → 400 → 200 Bq.
- Core
Scope: Core · 6.4.2.3
Open revision guidance
What to know
- Radioactive decay is random for an individual nucleus, but large samples show a predictable average pattern. Half-life is the time for the number of undecayed nuclei or activity to halve.
- After each half-life, half of the remaining nuclei/activity remains; activity and count rate decrease over time but do not mean every nucleus decays at a scheduled instant.
What to do in an assessment
- Source scope: 6.4.2.3.
- Count the number of elapsed half-lives, halve the quantity repeatedly or use the decay curve, and keep units consistent.
In plain language
Half-life describes a statistical pattern in a large sample. It does not mean every atom lasts exactly one half-life.
Self-check
A sample has activity 160 counts per minute and a half-life of 3 hours. What activity remains after 6 hours?
Show the answer
Two half-lives pass: 160 → 80 → 40 counts per minute.
Topic group
Physics · 6.7 Magnetism and electromagnetism
- Core
Scope: Core · §6.7 Magnetism and electromagnetism (full section)
This explanation covers part of the source requirement only. Its tick is separate and does not count as a fully revised requirement.
Open revision guidance
What to know
- A current-carrying wire produces a magnetic field; a coil or solenoid concentrates the field and behaves like a bar magnet. An iron core strengthens an electromagnet.
- A current-carrying conductor in a magnetic field experiences a force; this is the motor effect. Reversing current or field reverses force direction.
- A changing magnetic field can induce a potential difference. A generator uses relative motion between a coil and magnet; a transformer changes alternating potential difference using changing magnetic flux.
What to do in an assessment
- Use field direction and conventional current when finding force direction; state which quantity is changed to increase induced voltage. Explain transformer changes through turns ratio and alternating current.
In plain language
Electricity can make magnetism, and changing magnetism can generate electricity. In a motor the field pushes a current-carrying wire; in a generator movement changes magnetic flux through a coil.
Self-check
How can the direction of force on a motor coil be reversed?
Show the answer
Reverse either the current direction or the magnetic-field direction (but not both).
- Core
Scope: Core · 6.7.2.1
Open revision guidance
What to know
- A current in a conductor produces a magnetic field. The field around a straight wire is circular; coiling the wire into a solenoid concentrates the field.
- Increasing current or adding a soft iron core can make an electromagnet stronger. The field can be switched off by stopping current, unlike a permanent magnet.
What to do in an assessment
- Source scope: 6.7.2.1.
- Use field-line direction and circuit context to explain changes in strength. Distinguish an induced magnet from a permanent magnet.
In plain language
An electromagnet is a magnetic field created by electric current; changing the current or coil/core changes the field.
Self-check
How can a solenoid’s magnetic field be made stronger?
Show the answer
Increase the current or add a soft iron core.
Topic group
Required practicals · Biology · 4.1 Cell biology
- Core
Scope: Core · §10.2.1 Required practical activity 1
Open revision guidance
What to know
- Prepare a thin specimen on a slide, focus first at low power, then use higher magnification to resolve details. Record an accurately proportioned scientific drawing with labels and a scale.
- Magnification = image size ÷ actual size; convert both lengths to the same unit. A larger image is not necessarily more detailed if the microscope cannot resolve the structures.
What to do in an assessment
- Use a light microscope to observe, draw and label a selection of plant and animal cells; include a magnification scale. Keep the slide secure, focus carefully and record the observed structures rather than drawing features not visible. Include the stated scale and units; calculate magnification only from matched units.
In plain language
Low power makes the specimen easier to locate; higher power helps inspect detail. The scale links the drawing to the specimen rather than relying on artistic size.
Self-check
A cell is 40 µm long and its image is 8 mm long. What is the magnification?
Show the answer
8 mm = 8,000 µm; magnification = 8,000 ÷ 40 = ×200.
- Core
Scope: Core · §10.2.2 Required practical activity 2
Open revision guidance
What to know
- Cut equivalent pieces of plant tissue, record initial mass, immerse in measured concentrations for the same time and temperature, then blot consistently and record final mass.
- Calculate percentage change = (final mass − initial mass) ÷ initial mass × 100. The concentration with no net mass change estimates the isotonic point.
What to do in an assessment
- Investigate the effect of a range of salt or sugar solution concentrations on plant tissue mass. Use repeats at each concentration, control tissue dimensions and immersion time, and plot concentration against percentage mass change. Handle balance readings consistently.
In plain language
Water crosses cell membranes by osmosis. Tissue gains or loses mass depending on the water-potential difference; blotting removes surface liquid that would distort the measurement.
Self-check
A potato sample changes from 5.0 g to 5.5 g. What is its percentage mass change?
Show the answer
(5.5−5.0) ÷ 5.0 ×100 = 10% gain.
Topic group
Required practicals · Biology · 4.2 Organisation
- Core
Scope: Core · §10.2.3 Required practical activity 3
Open revision guidance
What to know
- Benedict’s reagent is heated with a sample: blue may change through green/yellow/orange to brick-red as reducing sugar evidence increases. Iodine changes from orange-brown to blue-black if starch is present.
- Biuret reagent changes from blue to lilac/purple when protein is detected. The ethanol-emulsion test gives a cloudy white emulsion when lipid is present; use the specified safe procedure.
What to do in an assessment
- Test for sugars with Benedict’s, starch with iodine, protein with Biuret and lipids with an appropriate qualitative test. Test a known positive and negative control when appropriate, use clean apparatus and equal sample amounts, and report the observation before concluding which nutrient is supported.
In plain language
Each test depends on a characteristic chemical reaction. A positive colour is evidence for the named nutrient in the tested sample, not a measurement of every nutrient present.
Self-check
What colour change indicates starch in the iodine test?
Show the answer
Iodine changes from orange-brown to blue-black.
- Core
Scope: Core · §10.2.4 Required practical activity 4
Open revision guidance
What to know
- Mix amylase, starch and buffer at the selected pH; maintain the same temperature with a water bath. Test a small sample with iodine at regular 30-second intervals until iodine no longer indicates starch.
- A shorter digestion time means a greater rate; estimate rate consistently as 1/time. Compare repeated trials and control volumes, concentrations and temperature.
What to do in an assessment
- Measure the time taken for amylase to digest starch across a range of pH values, using iodine sampling every 30 seconds and controlling temperature. Use the same sampling interval, temperature, enzyme/substrate quantities and endpoint rule at each pH. Record all times and plot rate against pH.
In plain language
Amylase has an active site whose function depends on conditions. A pH that changes its shape reduces successful substrate binding; varying temperature at the same time would make the cause unclear.
Self-check
Starch disappears in 60 s at pH 7 and 120 s at pH 5. Using 1/time, which rate is greater?
Show the answer
The pH 7 rate proxy is 1/60, twice 1/120, so the reaction is faster at pH 7.
Topic group
Required practicals · Biology · 4.4 Bioenergetics
- Core
Scope: Core · §10.2.5 Required practical activity 5
Open revision guidance
What to know
- Change light intensity, for example by changing lamp distance, and measure oxygen production per unit time. Keep pondweed length, water conditions and observation interval consistent.
- A lamp can warm the water, changing photosynthesis independently of light; use a water heat shield and monitor temperature. Repeat measurements and compare rate with light intensity.
What to do in an assessment
- Investigate the effect of light intensity on oxygen production by aquatic pondweed; control water temperature with a heat shield. Record lamp distance and light intensity method, keep a fixed counting/collection interval, and plot rate against intensity. For distance from a point source, account for the inverse-square relationship when applicable.
In plain language
Light supplies energy for photosynthesis, but temperature and carbon dioxide can also limit rate. Separating the light change from heating makes the result interpretable.
Self-check
Why should water temperature be monitored when the lamp distance changes?
Show the answer
Moving the lamp changes heating as well as light intensity; temperature itself affects photosynthesis rate.
Topic group
Required practicals · Biology · 4.5 Homeostasis and response
- Core
Scope: Core · §10.2.6 Required practical activity 6
Open revision guidance
What to know
- Use a measurable response, such as ruler-drop distance, to estimate reaction time. Choose one factor to vary and use the same participant/recording method where appropriate.
- Repeat each condition, use a mean and control other influences such as starting position and instructions. Ruler distance can be converted to time using free-fall relationships if required.
What to do in an assessment
- Plan and carry out an investigation into the effect of a factor on human reaction time. Write a testable hypothesis, define the factor and response, plan repeats, address consent/safety and avoid presenting one participant’s result as universal.
In plain language
Reaction time varies between trials. Repeats reduce the effect of random variation, while a controlled comparison helps attribute a difference to the selected factor.
Self-check
Why repeat a ruler-drop test under the same condition?
Show the answer
To check consistency and calculate a more reliable mean reaction time.
Topic group
Required practicals · Biology · 4.7 Ecology
- Core
Scope: Core · §10.2.7 Required practical activity 7
Open revision guidance
What to know
- Use quadrats to sample abundance or percentage cover; use random positions for an unbiased estimate. A transect can investigate change along a gradient.
- Record the environmental factor with the species data, repeat samples and estimate the whole area only when the sampled area and method support it.
What to do in an assessment
- Estimate a common species population and investigate how a factor affects its distribution using sampling techniques. Choose a suitable quadrat size and number, sample systematically or randomly as the question requires, keep area consistent, and graph species measure against the factor.
In plain language
A habitat is too large to count every organism. Replicated sampling estimates a pattern, but uneven distribution and small sample sizes limit precision.
Self-check
Why is random quadrat placement useful when estimating population abundance?
Show the answer
It reduces selection bias by giving locations a more even chance of being sampled.
Topic group
Required practicals · Chemistry · 5.4 Chemical changes
- Core
Scope: Core · §10.2.8 Required practical activity 8
Open revision guidance
What to know
- Warm dilute acid safely, add the insoluble solid in portions until some remains unreacted, and filter away excess solid. The soluble salt stays in the filtrate.
- Gently evaporate some water, cool to grow crystals, then collect and dry the crystals. Avoid evaporating all the solvent or overheating a hydrated salt.
What to do in an assessment
- Prepare a pure, dry soluble salt from an insoluble oxide or carbonate using warm dilute acid, filtration and crystallisation. Use a water bath or electric heater where the method calls for it, wear eye protection, and explain why excess insoluble reactant is removed by filtration.
In plain language
Excess oxide/carbonate ensures the acid is used up. Filtration removes unreacted solid; concentration and cooling allow the dissolved salt to crystallise.
Self-check
Why is the reaction mixture filtered after adding excess insoluble oxide?
Show the answer
To remove unreacted solid while the dissolved salt remains in the filtrate.
- Core
Scope: Core · §10.2.9 Required practical activity 9
Open revision guidance
What to know
- An electrolyte contains mobile ions. Positive ions move to the negative cathode and gain electrons; negative ions move to the positive anode and lose electrons.
- In aqueous solutions, products can depend on the ions present and competition with water. Observe gases or deposits safely and compare predicted products with evidence.
What to do in an assessment
- Investigate electrolysis of aqueous solutions using inert electrodes and develop a testable hypothesis. Use the same voltage/time/electrode arrangement when comparing solutions, record each electrode separately and state how a gas or deposit was identified. Follow electrical and chemical safety instructions.
In plain language
The power supply drives oppositely charged ions to electrodes. At the surface, electron transfer produces neutral atoms or molecules; product prediction depends on the solution composition.
Self-check
Which electrode attracts positive ions during electrolysis?
Show the answer
The negative cathode.
Topic group
Required practicals · Chemistry · 5.5 Energy changes
- Core
Scope: Core · §10.2.10 Required practical activity 10
Open revision guidance
What to know
- Measure starting solution temperature, mix measured quantities in an insulated container and record the maximum or minimum temperature. Calculate temperature change from the initial value.
- Vary one factor and keep volumes, concentrations or solid mass controlled as appropriate. Repeat trials; temperature rise indicates energy released to surroundings, while a fall indicates energy absorbed.
What to do in an assessment
- Investigate how variables affect temperature changes in reacting solutions, such as acid–metal, acid–carbonate, neutralisation or displacement reactions. Use a lid/insulation to reduce heat transfer, stir consistently, record a time series and compare the largest change. Describe measurement precision and likely heat loss.
In plain language
The thermometer records the solution’s response, not all reaction energy: heat exchanged with the cup and air reduces the observed change. Consistent insulation improves comparison.
Self-check
A reacting mixture falls from 21°C to 16°C. Is the reaction exothermic or endothermic?
Show the answer
Endothermic: the solution cooled as energy was taken in from the surroundings.
Topic group
Required practicals · Chemistry · 5.6 Rate and extent of chemical change
- Core
Scope: Core · §10.2.11 Required practical activity 11
Open revision guidance
What to know
- Measure gas volume at intervals or time until a repeatable visual endpoint. Keep total volume, temperature and other reactant conditions constant while changing concentration.
- Compare initial graph gradients or use 1/time for the same endpoint. Repeat conditions and ensure gas collection is sealed and measured consistently.
What to do in an assessment
- Investigate concentration effects using gas-volume measurement or a color/turbidity endpoint and develop a hypothesis. Plot the dependent measure against time, label units and compare rates. State how you controlled temperature and why the endpoint was judged consistently.
In plain language
A more concentrated solution has more reacting particles in each unit volume, increasing collision frequency; only successful collisions form products.
Self-check
If the same cloudiness endpoint takes half as long after concentration increases, what happens to the 1/time rate estimate?
Show the answer
It doubles.
Topic group
Required practicals · Chemistry · 5.8 Chemical analysis
- Core
Scope: Core · §10.2.12 Required practical activity 12
Open revision guidance
What to know
- Place a small spot on a pencil baseline above the solvent level, stand the paper in a covered container and let solvent rise. Mark the solvent front before it evaporates.
- Rf = distance travelled by substance ÷ distance travelled by solvent front. Measure both from the baseline using the same units; comparable Rf values require the same solvent and conditions.
What to do in an assessment
- Separate coloured substances by paper chromatography and calculate Rf values. Use small concentrated spots, keep the baseline above solvent, and record distances accurately. Explain that a pure sample typically gives one spot under the test conditions.
In plain language
Different substances have different attractions to solvent and paper, so move at different rates. The ratio compares travel distances without depending only on the total solvent rise.
Self-check
A spot travels 3.0 cm and the solvent front travels 6.0 cm. Find Rf.
Show the answer
Rf = 3.0/6.0 = 0.50.
Topic group
Required practicals · Chemistry · 5.10 Using resources
- Core
Scope: Core · §10.2.13 Required practical activity 13
Open revision guidance
What to know
- Measure pH with suitable indicator or probe; use a stated method to test for dissolved solids and record sample volumes. Distillation evaporates water and condenses vapor separately from many dissolved solids.
- Use clean apparatus, compare like volumes and distinguish purified distillate from the residue. Distillation alone does not guarantee removal of all volatile contaminants.
What to do in an assessment
- Compare water from different sources using pH, dissolved solids and distillation. Record source, measurements, test method and units. Explain how the distillation apparatus separates substances by boiling/condensation and compare evidence before claiming a sample is purer.
In plain language
Water can look clear while containing dissolved substances. Tests and separation provide evidence, but each method detects only particular properties or contaminants.
Self-check
Why can distillation separate many dissolved salts from water?
Show the answer
Water vaporises and condenses while most non-volatile dissolved salts remain in the original vessel.
Topic group
Required practicals · Physics · 6.1 Energy
- Core
Scope: Core · §10.2.14 Required practical activity 14
Open revision guidance
What to know
- Measure sample mass and temperature change while supplying known energy; electrical energy may be calculated from power × time. Use E = mcΔT and rearrange for c.
- Insulate the sample, place the heater and thermometer correctly and record time and temperature consistently. Energy lost to surroundings makes calculated c less accurate.
What to do in an assessment
- Determine specific heat capacity by linking energy supplied/work done to mass and temperature rise. Show units, keep power input and sample conditions documented, repeat or use a temperature-time graph, and state how insulation and stirring affect reliability.
In plain language
Specific heat capacity is energy needed per kilogram for each degree of temperature rise. Some supplied energy warms the surroundings, so insulation makes more of the input heat the material.
Self-check
A 0.50 kg sample gains 2,000 J and rises 10°C. Find its specific heat capacity.
Show the answer
c = E/(mΔT) = 2000/(0.50×10) = 400 J kg⁻¹ °C⁻¹.
- Core
Scope: Core · §10.2.21 Required practical activity 21
Open revision guidance
What to know
- Compare surfaces that differ in finish or colour while holding area, distance, source, time and temperature conditions constant. Measure temperature change or detector response.
- Dull, dark surfaces are generally better infrared absorbers and emitters than shiny, light surfaces. Repeat readings and prevent convection or conduction differences from dominating.
What to do in an assessment
- Investigate how infrared radiation absorbed or emitted depends on the nature of a surface. Use identical containers or plates and equal water volumes where applicable; arrange them symmetrically, record starting temperature and compare changes over the same interval.
In plain language
The surface changes how effectively infrared is absorbed or emitted. Keeping geometry and material otherwise alike isolates the effect of surface finish.
Self-check
Which is generally the better infrared emitter: a dull black surface or a shiny light surface?
Show the answer
A dull black surface.
Topic group
Required practicals · Physics · 6.2 Electricity
- Core
Scope: Core · §10.2.15 Required practical activity 15
Open revision guidance
What to know
- Measure current and potential difference for a wire of different lengths while keeping its material, thickness and temperature constant; calculate R = V/I.
- In a series circuit the same current flows through each resistor and total resistance increases. In parallel, potential difference is shared equally across branches and total resistance is lower than either branch resistance.
What to do in an assessment
- Use circuit diagrams to investigate wire length at constant temperature and resistor combinations in series/parallel. Use an ammeter in series and voltmeter in parallel, switch off between readings to reduce heating, and plot resistance against wire length or compare arrangements.
In plain language
A longer wire gives electrons more distance over which to interact with the lattice, increasing resistance when other properties stay fixed. Circuit arrangement changes available paths for charge.
Self-check
Where should the ammeter go when measuring current through the test wire?
Show the answer
In series with the wire.
- Core
Scope: Core · §10.2.16 Required practical activity 16
Open revision guidance
What to know
- Change the potential difference and record paired current and voltage readings, including reversed polarity where appropriate. Plot current against potential difference.
- An ohmic resistor at constant temperature gives a straight proportional relation. A filament lamp heats and its resistance changes; a diode conducts mainly in one direction after its forward threshold.
What to do in an assessment
- Measure I–V characteristics of a filament lamp, diode and resistor at constant temperature where required. Use the correct meter placement, vary voltage in small steps and avoid overheating the resistor. Annotate polarity and identify how each graph departs from a straight line.
In plain language
The I–V graph reveals whether resistance remains constant. Heating changes a lamp’s filament resistance, while a diode’s structure restricts current direction.
Self-check
Which component conducts mainly in one direction?
Show the answer
A diode.
Topic group
Required practicals · Physics · 6.3 Particle model of matter
- Core
Scope: Core · §10.2.17 Required practical activity 17
Open revision guidance
What to know
- Measure mass with a balance. Find regular-solid volume from dimensions, irregular-solid volume by displacement, and liquid volume with a graduated instrument; calculate ρ = m/V.
- Use consistent units, read the meniscus at eye level and repeat measurements. Volume uncertainty particularly affects small irregular objects.
What to do in an assessment
- Determine density of regular and irregular solids and liquids using mass and volume. Subtract container mass when necessary, record units and significant figures, and explain why water displacement gives the volume of an irregular object.
In plain language
Density is how much mass occupies a given volume. Measuring the wrong volume or including container mass changes the calculated value.
Self-check
A solid has mass 30 g and displaces 10 cm³. What is its density?
Show the answer
ρ = 30/10 = 3.0 g cm⁻³.
Topic group
Required practicals · Physics · 6.5 Forces
- Core
Scope: Core · §10.2.18 Required practical activity 18
Open revision guidance
What to know
- Measure the spring’s original length, add known forces in steps and record total length; extension is stretched length minus original length.
- Plot force against extension. A straight proportional section supports Hooke’s law; beyond the limit of proportionality, extension is no longer proportional to force.
What to do in an assessment
- Investigate the relationship between force and extension for a spring. Use a clamp stand, pointer and ruler without overloading the spring. Take readings at eye level and wait for oscillations to settle.
In plain language
The extension measures the change in length, not the spring’s full length. The graph shows the range over which force and extension are proportional.
Self-check
A spring length changes from 8 cm to 13 cm. What is its extension?
Show the answer
13−8 = 5 cm.
- Core
Scope: Core · §10.2.19 Required practical activity 19
Open revision guidance
What to know
- Measure the motion of a trolley or other object while changing pulling force with mass fixed, then change mass while keeping resultant force fixed. Use a suitable timing method to calculate acceleration.
- Keep track friction and timing method consistent; repeat trials and graph acceleration against force or reciprocal mass as appropriate.
What to do in an assessment
- Investigate acceleration by varying force at constant mass and varying mass at constant force. State which quantity is independent in each comparison, account for the total moving mass, and relate the data to F = ma.
In plain language
For a fixed mass, increasing resultant force increases acceleration. For a fixed resultant force, increasing mass reduces acceleration.
Self-check
What happens to acceleration if the same resultant force acts on twice the mass?
Show the answer
It halves, since a = F/m.
Topic group
Required practicals · Physics · 6.6 Waves
- Core
Scope: Core · §10.2.20 Required practical activity 20
Open revision guidance
What to know
- For a wave, measure spacing between matching points for wavelength and count cycles in a measured time for frequency. Use v = fλ and appropriate equipment for ripple or solid waves.
- Select apparatus whose scale and response suit the wave speed/frequency. Repeat timing or count multiple cycles to reduce reaction-time error.
What to do in an assessment
- Select apparatus to measure wave frequency, wavelength and speed in a ripple tank and waves in a solid. Label frequency in hertz, wavelength in metres and speed in metres per second. Describe how wave fronts or repeated pulses are observed and identify limitations of the apparatus.
In plain language
Measuring one wave cycle is vulnerable to timing error. Timing several cycles and dividing by the number gives a more reliable frequency estimate.
Self-check
A wave travels at 3.0 m s⁻¹ and has frequency 5 Hz. What is its wavelength?
Show the answer
λ = v/f = 3.0/5 = 0.60 m.
Topic group
Science skills
- Core
Scope: Core · 3 Working scientifically (all assessed strands)
Open revision guidance
What to know
- Scientific thinking includes explaining how methods and theories change when evidence changes; using representational, spatial, descriptive, computational and mathematical models while recognising their limits; considering uncertainty, ethics, applications and social/environmental implications; evaluating measured and perceived risks; and understanding peer review and audience-appropriate communication.
- Develop a testable hypothesis from a scientific explanation. Plan a procedure that identifies independent, dependent and control variables, selects suitable apparatus, includes repeat measurements where useful, considers safety and ethics, and is detailed enough for another person to follow.
- Choose techniques and materials to suit the question. Make and record observations with appropriate resolution and units; use representative sampling when investigating populations; distinguish a measurement from an interpretation.
- Present and translate data using suitable tables, diagrams and graphs. Calculate appropriate statistics and gradients, recognise patterns and anomalies, infer conclusions from evidence, and use evidence to assess whether a hypothesis is supported.
- Every measurement has uncertainty. Use the range around a mean to describe spread; evaluate accuracy, precision, repeatability and reproducibility separately; identify possible random or systematic error and suggest a targeted improvement or follow-up investigation.
- Communicate a logically structured explanation that links scientific vocabulary, the method, the evidence and the conclusion. Distinguish a correlation or trend from a demonstrated cause and explain the limits of the evidence.
What to do in an assessment
- For a method, say what changes, what is measured, which conditions are held constant, what apparatus and scale are appropriate, how repeats/sampling improve the evidence, and how risk is controlled.
- For data analysis, select a presentation and statistic that fit the data, show calculations and units, describe the pattern before explaining it, identify uncertainty/limitations and make a conclusion proportional to the evidence.
- For evaluation, name the limitation, state its likely effect on the result and propose a practical improvement that addresses it. Explain the limitations of a model or the social/ethical trade-offs when interpreting an application.
In plain language
A good practical answer explains what is changed, what is measured and how other influences are controlled. A result becomes an explanation only when linked to evidence and the science model.
Self-check
Repeated readings are close together but all use an uncalibrated instrument that reads high. Which quality is supported by the close readings, and what remains doubtful?
Show the answer
The close readings support repeatability/precision, but accuracy is doubtful because a systematic calibration error may bias every result.
- Core
Scope: Core · 9 Mathematical requirements (all listed skills)
Open revision guidance
What to know
- Arithmetic: recognise decimals and standard form, use ratios/fractions/percentages and estimate simple calculations. Data handling: use suitable significant figures, means, frequency tables/diagrams, bar charts and histograms; know mean, mode and median; make order-of-magnitude calculations.
- Sampling and simple probability are specified for Biology questions. Scatter diagrams and correlation are specified for Biology and Physics questions. Correlation describes association, not proof that one variable caused the other.
- Algebra: interpret =, <, <<, >>, >, proportional-to and approximately-equal symbols; rearrange equations; substitute physical quantities with units in Chemistry/Physics and solve simple equations in Biology/Physics.
- Graphs: translate between numerical and graphical forms; interpret y = mx + c; plot two variables; determine the gradient and intercept of a straight line; use a tangent gradient for rates in Chemistry/Physics; interpret and estimate area under a curve in specified Physics questions.
- Geometry: use degrees and two-dimensional representations of three-dimensional objects in Physics; calculate triangle/rectangle areas and the surface area or volume of a cube where relevant.
- Foundation questions do not fall below the stated Key Stage 3 demand. Higher questions do not fall below the Foundation-tier GCSE Mathematics demand. The science specification's subject-only limits above still apply at the relevant tier.
What to do in an assessment
- Identify the quantity and subject context first, choose the stated relationship, convert units, show substitutions and rearrangement, and give a sensible final value with units/significant figures.
- On a graph use labelled axes and a suitable scale; distinguish the gradient, intercept and area meanings. Only use sampling, probability, correlation, tangents, area or geometric methods in the subject contexts identified by the specification.
In plain language
A science calculation is more than arithmetic: the course says which tools apply in which subject. Convert units, show the model and working, then check the result against the data and physical context.
Self-check
A Chemistry question asks for the gradient of a curve that gets steeper over time. What graph quantity estimates an instantaneous rate, and what must you include in the unit?
Show the answer
Draw a tangent at the specified point and calculate its gradient; divide the vertical-axis unit by the horizontal-axis unit to give the rate unit.
Topic group
Physics · 6.3 Particle model of matter
- Core
Scope: Core · 6.3.2.2
Open revision guidance
What to know
- The energy needed for a temperature change depends on the mass, material and size of the temperature change. ΔE = m c Δθ, where c is specific heat capacity.
- Specific heat capacity is energy transferred per kilogram per degree Celsius (J kg⁻¹ °C⁻¹). It can be found from a heating experiment using mass, energy input and temperature rise.
What to do in an assessment
- Source scope: 6.3.2.2.
- Use a temperature difference, not final temperature, and convert mass to kilograms. Explain heat loss to surroundings when interpreting an experimental result.
In plain language
A material with a high specific heat capacity needs more energy to warm each kilogram by one degree.
Self-check
If 2 kg of a material with c = 500 J kg⁻¹ °C⁻¹ warms by 3°C, how much energy is transferred?
Show the answer
ΔE = m c Δθ = 2×500×3 = 3000 J.
Topic group
Practical and scientific skills · Written papers
- Core
Scope: Core · 10.1 Use of apparatus and techniques
Open revision guidance
What to know
- Biology AT1–AT7: measure length, area, mass, time, temperature, liquid/gas volume and pH accurately; heat safely with a Bunsen, water bath or electric heater; observe and measure biological change; work safely and ethically with living organisms; measure reaction rates from gas, water uptake or indicator colour; sample organism distribution/abundance in the field; and use microscopes/magnification to observe specimens and draw labelled diagrams.
- Chemistry AT1–AT7: accurately measure mass, time, temperature and liquid/gas volume; heat safely; conduct and monitor reactions and measure pH; separate/purify mixtures using evaporation, filtration, crystallisation, chromatography and distillation; record temperature changes and rates from gas production or colour change; handle substances carefully and mix reagents under controlled conditions; and set up/use electrochemical cells to separate or produce substances.
- Physics AT1–AT7: measure length, area, mass, time, volume and temperature and use measurements to determine density; measure force effects including spring extension; measure motion, speed and acceleration; observe waves in solids/fluids and select measurements of speed, frequency and wavelength; observe electromagnetic-wave interactions; measure energy transfer/work; measure current, potential difference and resistance; and construct/check series and parallel circuits from circuit diagrams.
- Safety overrides convenience: identify hazards, use risk controls and suitable apparatus before beginning. Clear scientific diagrams should show the apparatus arrangement and how readings are taken.
What to do in an assessment
- In an unseen practical question, choose an instrument suited to the quantity and required resolution, describe how to read it consistently, name units and controls, and include safe handling.
- Explain why the apparatus answers the question: for example, a voltmeter is connected in parallel to measure potential difference across a component, while an ammeter is in series to measure circuit current.
In plain language
Practical apparatus is chosen to measure the right thing safely and precisely. The specification expects students to understand the technique, not just recognise equipment names.
Self-check
Which circuit connections measure current through a lamp and potential difference across it?
Show the answer
Connect the ammeter in series with the lamp and the voltmeter in parallel across it.
- Core
Scope: Core · 10.2 Required practical activities 1–21 (assessed through written papers)
Open revision guidance
What to know
- All students are expected to carry out the 21 required practical activities. Their written-exam methods, observations, results and scientific ideas can be examined; practical-related questions count for at least 15% of the qualification marks.
- The written exam may use a new practical context. Apply the same skills: select apparatus, identify independent/dependent/control variables, state a safe method, record data with units, process results and draw a conclusion from evidence.
- Each activity has an individual card in this guide covering its source-specific method and relevant data analysis. Practical completion and the centre's practical-science statement are separate from the written-paper progress denominator.
What to do in an assessment
- For any required practical question, identify which measurement or comparison tests the scientific idea, show how variables are controlled and explain what result pattern would support the conclusion.
- Use the named activity cards to revise its precise apparatus, sequence, measurements, calculations, safety and evaluation; do not treat an activity title as a complete method.
In plain language
Practical knowledge is part of the written qualification. You need to understand why a method works and how its results support an idea, not merely remember that an experiment was done.
Self-check
Does the centre's practical-science statement count as one of the six written papers?
Show the answer
No. The statement records required practical completion; practical knowledge can be assessed within written questions on the six papers.
Earlier topic checklist
These earlier topic ticks are separate from the reviewed points above and do not count towards that progress.
Combined Science: Trilogy topic list
Tick each item off as you revise. Ticks are saved on this device only.
These ticks keep your earlier topic checklist. They do not count towards the new reviewed requirements above. Its numbering may be internal; use the linked references above for the exact reviewed source requirements.
Foundation tier topics
Higher tier topics
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