Chemistry Revision: WJEC GCSE Science (Double Award) · teaching from 2016
This guide is for the legacy qualification taught from 2016. The page shows Chemistry requirements plus shared skills and Unit 7; choose another science above to switch scope.
Qualification code: 60182362
Course details
Assessment Six 75-minute, 60-mark written exams, each worth 15%, plus Unit 7 Practical Assessment worth 10%; summer 2027 is the final full assessment, with a January 2028 resit subject to demand.
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Reviewed course guidance
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Open a point to read what to know, how to use it in an assessment, a plain-language explanation and a self-check. Answers stay hidden until you choose to reveal them.
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Topic group
Unit 2 · 2.1 · Elements, Compounds, and Chemical Change
- Core
Scope: Core · Topic 2.1 (core)
Open revision guidance
What to know
- Elements are substances consisting of only one type of atom and cannot be broken down by chemical means [2.1(a), 2.1(b)].
- Compounds are substances made of two or more different types of atoms chemically joined; they exhibit properties distinct from their constituent elements [2.1(c)].
- Chemical symbols represent elements (e.g., O for oxygen) and chemical formulae represent molecules (e.g., H2O) [2.1(d)].
- Relative atomic mass (Ar) is the weighted average mass of an atom compared to 1/12th of a carbon-12 atom; relative formula mass (Mr) is the sum of Ar values in a formula [2.1(g)].
- Percentage composition = (total mass of element in compound / Mr of compound) × 100 [2.1(h)].
- Mixtures contain substances not chemically joined, separable by filtration, evaporation, chromatography, or distillation [2.1(i)].
- Rf value = distance moved by substance / distance moved by solvent [2.1(j)].
- Chemical reactions involve rearranging atoms; the total number and type of atoms remain constant [2.1(k)].
- Signs of reaction include colour changes, temperature shifts (exothermic release or endothermic absorption), and effervescence [2.1(l)].
- Balanced equations ensure total relative mass is conserved [2.1(n)].
- Percentage yield = (actual yield / theoretical yield) × 100 [2.1(o)].
What to do in an assessment
- Draw diagrams for simple molecules using keys for different atoms [2.1(e)].
- Write ionic formulae by balancing charges (e.g., Na+ and Cl- become NaCl) [2.1(f)].
- Calculate Mr (e.g., H2SO4 = (2*1) + 32 + (4*16) = 98) [2.1(g)].
- Perform separations based on solubility, boiling point, or polarity [2.1(i)].
- Construct word and balanced symbol equations [2.1(m), 2.1(n)].
In plain language
Elements are the simplest building blocks, while compounds are chemically bonded combinations. Chemical reactions rearrange atoms without creating or destroying them, which we track using balanced equations. Mixtures are physical combinations that can be physically separated, unlike compounds which require chemical energy to split.
Self-check
Calculate the percentage composition of oxygen in calcium carbonate (CaCO3). Ar: Ca=40, C=12, O=16.
Show the answer
Mr = 40 + 12 + (3*16) = 100. Mass of O = 3 * 16 = 48. Percentage = (48/100) * 100 = 48%.
Topic group
Unit 2 · 2.1 · Advanced Quantitative Chemistry
- Higher
Scope: Higher · Unit 2 tier: Higher · Topic 2.1 (higher)
Open revision guidance
What to know
- The mole is defined by the Avogadro constant (6.02 x 10^23 particles per mole) [2.1(r)].
- Mole calculation: Moles = Mass (g) / Molar Mass (g/mol) [2.1(r)].
- Calculating reacting masses: Use balanced equations to find the molar ratio, convert given mass to moles, use the ratio to find target moles, and convert back to mass [2.1(q)].
- Empirical formula calculation: Divide mass of each element by its Ar, find the simplest whole number ratio [2.1(p)].
What to do in an assessment
- Convert between grams and moles using molar mass [2.1(r)].
- Calculate the empirical formula from experimental mass data [2.1(p)].
- Use stoichiometry to find the mass of a product produced from a known mass of reactant [2.1(q)].
In plain language
The mole acts as a bridge between the microscopic world of atoms and the macroscopic world of grams. By using molar ratios from balanced equations, we can predict exactly how much material is required for a reaction.
Self-check
How many moles are in 22 grams of CO2 (Mr=44)?
Show the answer
Moles = 22g / 44g/mol = 0.5 moles.
Topic group
Unit 2 · 2.2 · Atomic Structure and The Periodic Table
- Core
Scope: Core · Topic 2.2 (core)
Open revision guidance
What to know
- Atoms consist of a positively charged nucleus containing protons and neutrons, orbited by negatively charged electrons [L9, L19].
- Relative mass: Proton (1), Neutron (1), Electron (1/1840). Relative charge: Proton (+1), Neutron (0), Electron (-1).
- Atoms are electrically neutral as proton count equals electron count.
- Atomic Number: Number of protons. Mass Number: Total protons and neutrons. Isotopes: Atoms of the same element with different neutron numbers [L49, L59].
- Periodic Table: Arranged by increasing atomic number. Metals (left/centre), Non-metals (right), intermediate properties between [L69, L79].
- Electronic structure rules: 1st shell max 2, 2nd shell max 8, 3rd shell max 8. Position: Period = shells, Group = outer electrons [L89, L99].
- Group 1 (Alkali Metals): Very reactive, form +1 ions. Group 7 (Halogens): Reactive non-metals, form -1 ions. Group 0 (Noble Gases): Unreactive, full outer shells [L109, L119, L205].
- Reactions: Alkali metals react with air, water, and halogens. Halogens react with alkali metals and iron [L142, L162].
- Identification: Hydrogen gas gives a squeaky pop with a lit splint. Flame tests: Li+(red), Na+(yellow), K+(lilac), Ca2+(brick red), Ba2+(apple green). Silver nitrate test for halide ions (Cl-, Br-, I-) forms white, cream, and yellow precipitates respectively [L152, L195].
What to do in an assessment
- Calculate neutrons: Mass Number - Atomic Number.
- Draw electronic shells: For elements 1-20.
- Write ionic equations for halide precipitation: Ag+(aq) + X-(aq) -> AgX(s).
In plain language
Everything is made of atoms, which have a central core of protons and neutrons surrounded by electrons. Elements are organised in the Periodic Table based on their proton count and how their electrons are arranged, which determines their chemical behaviour.
Self-check
How many protons, neutrons, and electrons are in an atom of Carbon-14 (Atomic number 6)?
Show the answer
Protons: 6, Electrons: 6, Neutrons: 14 - 6 = 8.
Topic group
Unit 2 · 2.2 · Advanced Trends in Reactivity
- Higher
Scope: Higher · Unit 2 tier: Higher · Topic 2.2 (higher)
Open revision guidance
What to know
- Group 1 reactivity increases down the group because the outer electron is further from the nucleus, shielded by inner shells, making it easier to lose [L125, L132].
- Group 7 reactivity decreases down the group because the outer shell is further from the nucleus, making it harder to attract and gain an electron [L126, L132].
- Displacement reactions: A more reactive halogen will displace a less reactive halogen from a solution of its salt (e.g., Cl2 + 2KBr -> 2KCl + Br2) [L168, L175].
What to do in an assessment
- Compare relative reactivity of Cl2, Br2, and I2 using displacement observations.
In plain language
In groups 1 and 7, reactivity changes as you move down the group. Metals find it easier to lose electrons the larger they get, while non-metals find it harder to capture new electrons as they get larger.
Self-check
Why is Potassium more reactive than Lithium?
Show the answer
Potassium has more electron shells, so the outer electron is further from the nucleus and experiences less electrostatic attraction, making it easier to lose.
Topic group
Unit 2 · 2.3 · Water Supplies, Treatment, and Hardness
- Core
Scope: Core · Topic 2.3 (core)
Open revision guidance
What to know
- Natural water contains dissolved gases (e.g., CO2, O2), mineral ions (e.g., Ca2+, Mg2+), microorganisms, and pollutants.
- Sustainable water management involves reducing consumption and minimising environmental damage during extraction and treatment.
- Public water treatment: Sedimentation (particles settle), Filtration (passing through sand/gravel to remove solids), and Chlorination (kills pathogens).
- Fluoridation of water is used to reduce tooth decay; arguments involve dental benefits vs. medical ethics/potential toxicity.
- Desalination uses distillation or reverse osmosis to turn seawater into drinking water; it is energy-intensive and expensive.
- Miscible liquids are separated by distillation based on boiling points.
- Solubility is the maximum mass of solute that dissolves in 100g of solvent at a specific temperature. Solubility curves show how solubility changes with temperature.
- Hard water contains high concentrations of dissolved Ca2+ or Mg2+ ions. It forms a scum with soap and requires more soap to create a lather than soft water.
- Temporary hardness is caused by dissolved calcium hydrogencarbonate, removed by boiling. Permanent hardness is caused by other dissolved calcium/magnesium salts and is not removed by boiling.
- Softening methods include: Boiling (temporary only), adding sodium carbonate (precipitates ions as carbonates), and ion exchange (replaces Ca2+/Mg2+ with Na+).
- Hard water helps bone/teeth health but causes 'fur' or limescale in boilers and kettles.
What to do in an assessment
- Determine solubility: Mix solute with solvent until saturation, weigh solid and liquid, calculate solubility in g/100g water.
- Interpret solubility curves: Identify the solubility of a substance at temperature X by reading the y-axis (g/100g) for a given x-axis (temperature).
- Test for hardness: Add soap solution to a water sample and shake. If a stable, thick foam forms immediately, the water is soft. If it produces cloudy 'scum' and requires significant additional soap to foam, it is hard.
In plain language
Water from natural sources requires treatment to be safe. We must use it wisely to protect our planet. Treatment removes dirt and germs, while chemistry helps us manage water 'hardness'—the mineral content that leaves limescale in pipes and affects how soap works. Chemists use solubility data and distillation techniques to purify and analyse water supplies.
Self-check
How does the action of soap distinguish between hard and soft water, and why?
Show the answer
Hard water contains calcium or magnesium ions that react with soap to form an insoluble precipitate called 'scum'. Soft water does not contain these ions, allowing the soap to form a stable lather immediately without forming a precipitate.
Topic group
Unit 2 · 2.3 · Mechanisms of Water Softening
- Higher
Scope: Higher · Unit 2 tier: Higher · Topic 2.3 (higher)
Open revision guidance
What to know
- Boiling: Calcium hydrogencarbonate decomposes on heating to form calcium carbonate (insoluble precipitate), removing temporary hardness.
- Adding Sodium Carbonate (Washing Soda): Na2CO3 reacts with Ca2+ or Mg2+ ions in the water to form insoluble calcium/magnesium carbonate, removing the ions from the solution.
- Ion Exchange: Water passes through a column containing ion-exchange resin beads. The resin holds sodium ions (Na+). As hard water flows over, the resin captures Ca2+ or Mg2+ ions and releases Na+ ions into the water, resulting in soft water.
What to do in an assessment
- Compare costs and convenience of softening methods.
- Evaluate the advantages of ion exchange (continuous process, efficient) versus the disadvantages of adding chemicals (residue, ongoing cost).
In plain language
At the higher level, we look at exactly how we remove hardness. We focus on the chemical reaction of boiling, the precipitation chemistry when adding washing soda, and the ion-swapping mechanism of synthetic resin columns used in domestic water softeners.
Self-check
Describe the ion exchange process in terms of ion displacement.
Show the answer
An ion-exchange resin is loaded with sodium ions. When hard water passes through, the calcium (or magnesium) ions in the water are more strongly attracted to the resin than the sodium ions. The resin captures the calcium ions and releases an equivalent number of sodium ions into the water, effectively removing the hardness.
Topic group
Unit 2 · 2.4 · Earth's Structure and Atmospheric Chemistry
- Core
Scope: Core · Topic 2.4 (core)
Open revision guidance
What to know
- The Earth is composed of a solid iron inner core, a molten iron outer core, a rocky mantle, and a thin outer crust.
- Plate tectonics evolved from Alfred Wegener's 'continental drift' theory, which proposed continents move slowly over time.
- Plate boundaries include conservative (sliding past), destructive (moving towards each other), and constructive (moving apart).
- The early atmosphere was formed by gases like CO2 and water vapor released from volcanic activity.
- Atmospheric composition changes over time; present air contains nitrogen, oxygen, argon, and neon [L49, L89].
- Respiration and combustion consume O2 and produce CO2; photosynthesis consumes CO2 and produces O2.
- Fossil fuel combustion releases CO2 (causing global warming) and SO2 (causing acid rain).
- Global warming is addressed by reducing greenhouse gas emissions; acid rain is addressed by desulfurizing fuels and emissions.
- Oxygen test: relights a glowing splint; CO2 test: turns limewater cloudy.
What to do in an assessment
- Draw and label the layers of the Earth starting from the centre outward.
- Compare tectonic boundary types in a table showing direction of movement.
- Write word equations for photosynthesis (6CO2 + 6H2O -> C6H12O6 + 6O2) and respiration.
- Describe how acid rain forms when SO2 dissolves in atmospheric water to form sulfuric acid.
- Perform the gas tests: bubble gas through limewater to identify CO2; insert a glowing splint to identify O2.
In plain language
This lesson covers how Earth is structured like a layered ball, how continents move via plate tectonics, how volcanic gases made our early air, and how humans impact air quality through burning fuels.
Self-check
What is the result of the limewater test for carbon dioxide, and what process releases oxygen into the atmosphere?
Show the answer
Limewater turns cloudy; photosynthesis releases oxygen.
Topic group
Unit 2 · 2.5 · Rates of Reaction: Measurement, Theory, and Catalysis
- Core
Scope: Core · Topic 2.5 (core)
Open revision guidance
What to know
- Rate of reaction is the change in concentration of a reactant or product per unit time.
- Practical measurement methods include gas collection, mass loss, and precipitation.
- Particle theory states that collisions must occur with sufficient energy for a reaction to happen.
- Increasing temperature, concentration/pressure, or surface area increases the frequency of collisions.
- Catalysts increase reaction rates by providing an alternative pathway with lower activation energy.
What to do in an assessment
- For gas collection: Connect a flask to a gas syringe and record volume over time.
- For mass loss: Place a reaction vessel on a balance and record the decrease in mass as gas escapes.
- For precipitation: Observe the disappearance of a cross through a beaker containing a mixture of reagents.
- Use data-logging sensors to record pH, pressure, or temperature changes in real-time.
In plain language
Chemical reactions happen at different speeds. We measure these speeds by tracking how fast reactants disappear or products form. If particles hit each other more often or with more force, the reaction happens faster. Catalysts are helpful chemicals that act like a shortcut, letting reactions happen easier without being used up themselves.
Self-check
How does increasing the surface area of a solid reactant increase the rate of reaction according to particle theory?
Show the answer
Increasing the surface area exposes more particles of the solid reactant to the liquid or gas particles, resulting in a higher frequency of successful collisions.
Topic group
Unit 2 · 2.5 · Principles of Catalysis
- Higher
Scope: Higher · Unit 2 tier: Higher · Topic 2.5 (higher)
Open revision guidance
What to know
- Catalysts are substances that increase the rate of reaction while remaining chemically unchanged.
- Catalysts function by lowering the energy threshold required for a collision to be successful.
What to do in an assessment
- Explain the function of a catalyst as providing a low-energy pathway for effective collisions.
In plain language
Catalysts make chemical reactions work faster because they reduce the minimum energy needed for atoms to bond together when they collide, ensuring more collisions have enough power to result in a reaction.
Self-check
What happens to the chemical composition of a catalyst at the end of a reaction?
Show the answer
The catalyst remains chemically unchanged.
Topic group
Unit 5 · 5.1 · Chemical Bonding, Structures, and Nanomaterials
- Core
Scope: Core · Topic 5.1 (core)
Open revision guidance
What to know
- Metals consist of a giant lattice of positive ions in a 'sea' of delocalized electrons, explaining conductivity and malleability.
- Ionic bonds form via electron transfer between metals and non-metals, creating giant ionic lattices with high melting points.
- Covalent bonds involve electron sharing between non-metals to form molecules or giant covalent structures.
- Simple molecular substances have weak intermolecular forces, leading to low melting points, while giant covalent structures (diamond/graphite) have strong covalent bonds throughout.
- Carbon allotropes like graphene, fullerenes, and nanotubes exhibit unique properties distinct from bulk carbon.
- Nanoscale silver and titanium dioxide have unique properties compared to bulk forms.
- Smart materials change properties based on environment: thermochromic (heat), photochromic (light), polymer gels (swelling), and shape memory alloys/polymers (reverting to set shapes).
What to do in an assessment
- Draw dot and cross diagrams for ionic compounds like NaCl and covalent molecules like H2O.
- Explain high melting points in giant structures by referencing the strength of ionic/covalent bonds.
- Contrast the electrical conductivity of diamond (insulator) with graphite (conductor due to delocalized electrons).
- Describe uses for nanoparticles (e.g., silver for antibacterial, TiO2 for sunscreens).
In plain language
Chemical bonding determines how atoms arrange themselves, which in turn defines the properties of a material. Metals flow because of free electrons, while ionic compounds stay solid and rigid in lattices. Nanomaterials are tiny particles that behave differently than the bulk material they came from, and smart materials are engineered to react to external conditions like light or temperature.
Self-check
Why does graphite conduct electricity while diamond does not, despite both being made only of carbon atoms?
Show the answer
In graphite, each carbon atom is bonded to three others, leaving one delocalized electron per atom that can move freely and carry current. In diamond, each carbon atom is bonded to four others, with no delocalized electrons.
View diagrams
Sodium chloride: ionic transferBefore transfer, sodium is 2,8,1 and chlorine is 2,8,7. The sodium outer electron transfers to chlorine. Separate bracketed ions result: Na⁺ has configuration 2,8 and Cl⁻ has 2,8,8. Dots identify sodium-origin electrons; crosses identify chlorine-origin electrons. This is electron transfer, not a shared covalent pair.
Hydrogen · H₂Two hydrogen atoms contribute one electron each to one shared pair. The dot comes from the left H atom and the cross from the right H atom; each hydrogen now has a full first shell.
In dot-and-cross diagrams, a dot is an electron from the atom named in the explanation and a cross is from the other atom. The symbols show electron origin, not a charge. Shared pairs belong between bonded atoms; lone pairs stay on one atom.
The dot-and-cross and ionic shell sketches show outer-shell electrons only; inner-shell electrons are omitted from those sketches. Full sodium and chlorine configurations are printed beside the ions. Displayed structural formulae instead draw every atom, including each hydrogen, and every covalent bond.
Compare the diagram with the explanation, then use practice mode before opening the key.
Topic group
Unit 5 · 5.1 · Risks of Nanoscale Particles
- Higher
Scope: Higher · Unit 5 tier: Higher · Topic 5.1 (higher)
Open revision guidance
What to know
- Nanoscale particles (1-100 nm) have a massive surface area-to-volume ratio compared to bulk materials.
- This high reactivity poses potential health risks, as nanoparticles can penetrate biological membranes (like the skin or blood-brain barrier) more easily than bulk particles.
- Long-term toxicity of silver and titanium dioxide nanoparticles in the environment remains a concern due to their persistence and potential to disrupt biological systems.
- Future nanoscience developments require rigorous testing to ensure unintended biological interactions are identified before commercial release.
What to do in an assessment
- Evaluate the trade-offs between the beneficial uses of nanoparticles and the potential safety hazards.
- Discuss why a substance known to be inert in bulk form might become reactive at the nanoscale.
In plain language
Because nanoparticles are so small and have such a large surface area for their size, they can interact with the human body and the environment in ways that bulk materials cannot. We must carefully weigh their innovative benefits against the risks of them entering cells or accumulating in biological systems.
Self-check
Why is the high surface area-to-volume ratio of nanoparticles a significant factor in evaluating their potential risk?
Show the answer
It increases the reactivity of the substance and allows more opportunities for the particles to interact with and potentially enter biological cells or catalyze chemical reactions that are not observed in bulk materials.
Topic group
Unit 5 · 5.2 · Acids, Alkalis, and Salt Formation
- Core
Scope: Core · Topic 5.2 (core)
Open revision guidance
What to know
- The pH scale (0-14) measures acidity/alkalinity: pH < 7 is acidic, pH 7 is neutral, pH > 7 is alkaline.
- Acids release H+(aq) ions in solution; alkalis release OH-(aq) ions.
- Strong acids/alkalis fully dissociate into ions; weak acids/alkalis partially dissociate.
- Acid + Metal -> Salt + Hydrogen gas.
- Acid + Base (or Alkali) -> Salt + Water (Neutralisation).
- Acid + Carbonate -> Salt + Water + Carbon Dioxide.
- Hydrochloric acid forms chlorides, sulfuric acid forms sulfates, and nitric acid forms nitrates.
- The test for CO3 2- ions involves adding acid and checking for gas that turns limewater cloudy (CO2).
- The test for SO4 2- ions is adding barium chloride solution, which forms a white precipitate of barium sulfate.
- Preparation of soluble salts involves reacting an insoluble base or carbonate with an acid, filtering excess solid, and evaporating the filtrate.
- Titration is used to determine unknown concentrations by reacting precise volumes of acid and alkali using an indicator.
What to do in an assessment
- Use the general equation: Metal + Acid -> Salt + Hydrogen.
- Calculate the salt name based on the acid: e.g., Magnesium + Sulfuric Acid -> Magnesium Sulfate + Hydrogen.
- Perform a titration by adding titrant from a burette until the indicator changes colour at the endpoint.
- Filter excess metal oxide/carbonate from a salt solution to ensure only soluble salt remains.
In plain language
Acids provide hydrogen ions, while alkalis provide hydroxide ions. When they react, they cancel each other out in a process called neutralisation. We can create useful salts by reacting acids with specific bases, metals, or carbonates, and identify unknown solutions using indicators or chemical tests.
Self-check
What product is formed when hydrochloric acid reacts with copper(II) oxide, and what is the test for the sulfate ion?
Show the answer
Copper(II) chloride and water are formed. The test for the sulfate ion is adding barium chloride solution; a white precipitate indicates its presence.
Topic group
Unit 5 · 5.2 · Ionic Mechanisms of Neutralisation
- Higher
Scope: Higher · Unit 5 tier: Higher · Topic 5.2 (higher)
Open revision guidance
What to know
- Neutralisation is specifically the reaction where hydrogen ions (H+) from an acid combine with hydroxide ions (OH-) from an alkali to form water molecules.
- The ionic equation for neutralisation is: H+(aq) + OH-(aq) -> H2O(l).
What to do in an assessment
- Write full balanced ionic equations representing the net movement of ions in neutralisation reactions.
- Identify that spectator ions (ions not changing state or identity) are omitted from the net ionic equation.
In plain language
At a molecular level, the actual neutralisation reaction between any strong acid and strong alkali is just the hydrogen ion and hydroxide ion merging to become a stable water molecule.
Self-check
Write the net ionic equation for the reaction of nitric acid with potassium hydroxide.
Show the answer
H+(aq) + OH-(aq) -> H2O(l).
Topic group
Unit 5 · 5.3 · Extraction, Properties, and Reactivity of Metals
- Core
Scope: Core · Topic 5.3 (core)
Open revision guidance
What to know
- Metals exist as ores in the Earth's crust; extraction requires chemical reactions.
- Unreactive metals (e.g., gold) are found native; higher reactivity increases extraction difficulty.
- Displacement occurs when a more reactive metal replaces a less reactive one in solution (e.g., Fe + CuCl2 -> FeCl2 + Cu).
- The thermit reaction (Al + Fe2O3 -> Al2O3 + Fe) demonstrates competition for oxygen.
- Oxidation is gain of oxygen/loss of electrons; reduction is loss of oxygen/gain of electrons [L38-39, L68-69].
- Blast furnace: Combustion (C + O2 -> CO2), Reduction (Fe2O3 + 3CO -> 2Fe + 3CO2), Decomposition (CaCO3 -> CaO + CO2), Neutralisation (CaO + SiO2 -> CaSiO3).
- Electrolysis of molten lead(II) bromide: Pb2+ + 2e- -> Pb (cathode); 2Br- -> Br2 + 2e- (anode).
- Aluminium extraction uses electrolysis; alumina is dissolved in molten cryolite to lower its melting point.
- Transition metals form ions with different charges; they are hard, dense, and catalysts.
- Alloys are mixtures of molten metals; properties are modified by changing the mixture composition.
- Sustainability factors: Energy costs, greenhouse gases, plant location, and recycling.
What to do in an assessment
- Predict displacement reactions using the reactivity series.
- Write word and symbol equations for furnace reactions.
- Calculate the mass of metals extracted from ores.
- Compare properties of steel, Al, Cu, and Ti.
In plain language
Metals are extracted from ores using chemistry. Reactive metals are harder to extract and often require electrolysis, while less reactive metals can be separated using carbon. Transition metals have unique properties like variable ion charges. Alloys allow us to customise metal strength for specific uses while considering environmental impacts.
Self-check
Why is cryolite used in the industrial extraction of aluminium?
Show the answer
Cryolite is used to dissolve alumina (aluminium oxide) and significantly lower its melting point, reducing the high energy costs required for the electrolysis process.
Topic group
Unit 5 · 5.4 · Energy Changes in Chemical Reactions
- Core
Scope: Core · Topic 5.4 (core)
Open revision guidance
What to know
- Exothermic reactions release thermal energy to the surroundings, causing a temperature increase.
- Endothermic reactions absorb thermal energy from the surroundings, causing a temperature decrease.
- Activation energy (Ea) is the minimum energy required for colliding particles to react.
- An energy profile diagram maps the relative energy levels of reactants and products over the course of a reaction.
- Bond energy represents the energy required to break a specific chemical bond (measured in kJ/mol).
- Overall energy change = (Energy to break bonds) - (Energy released to form bonds).
What to do in an assessment
- In an exothermic profile, the reactant energy level is higher than the product energy level.
- In an endothermic profile, the reactant energy level is lower than the product energy level.
- Calculate total bond energy of reactants (bond breaking) and total bond energy of products (bond forming).
- If (Energy in - Energy out) is negative, the reaction is exothermic; if positive, it is endothermic.
- Example: For H2 + Cl2 -> 2HCl; Bond energies (kJ/mol): H-H 436, Cl-Cl 242, H-Cl 431. Breaking: (436 + 242) = 678 kJ/mol. Forming: 2 * 431 = 862 kJ/mol. Overall change: 678 - 862 = -184 kJ/mol. Since it is negative, it is exothermic.
In plain language
Chemical reactions involve energy changes because breaking existing chemical bonds takes energy, and forming new ones releases it. If more energy is released than absorbed, the reaction gets hot (exothermic). If more energy is absorbed than released, the reaction gets cold (endothermic). Activation energy is the 'hump' of energy needed to start the process.
Self-check
If a reaction requires 500 kJ to break bonds and releases 400 kJ when forming new bonds, is it exothermic or endothermic and what is the energy change?
Show the answer
It is endothermic because 500 - 400 = +100 kJ/mol. The positive value indicates energy absorption.
Topic group
Unit 5 · 5.5 · Crude Oil, Hydrocarbons, and Polymers
- Core
Scope: Core · Topic 5.5 (core)
Open revision guidance
What to know
- Crude oil is a complex mixture of hydrocarbons formed from marine organism remains.
- Fractional distillation separates crude oil into fractions of hydrocarbons with similar boiling points [L18-L19, L28-L29].
- Longer chains have higher boiling points and are more viscous.
- Alkanes follow CnH2n+2 and alkenes follow CnH2n.
- Combustion of hydrocarbons releases energy; hydrogen combustion produces water [L58-L59, L78-L79].
- The fire triangle consists of fuel, oxygen, and heat.
- Cracking breaks large hydrocarbons into smaller alkanes and useful alkenes.
- Alkenes undergo addition reactions: they turn orange bromine water colourless.
- Addition polymerization creates plastics like poly(ethene).
- Plastics are non-biodegradable, creating landfill issues addressed by recycling.
What to do in an assessment
- Calculate energy per gram: Energy (J) = mass of water (g) × 4.2 × temperature rise (°C). Then divide by mass of fuel burned (g).
- Write molecular/structural formulas for methane (CH4), ethene (C2H4), and propene (C3H6).
- Draw addition polymerization equations: n(C2H4) -> -(CH2-CH2)n-.
In plain language
Crude oil is a finite resource processed to make fuels and plastics. We separate it by boiling point, crack it to get smaller molecules, and turn those into useful materials like plastics. Burning fuels releases energy, but we must manage environmental and waste impacts.
Self-check
What is the test for an alkene?
Show the answer
Add bromine water; it will change from orange to colourless if an alkene is present.
Topic group
Unit 5 · 5.5 · Isomerism in Hydrocarbons
- Higher
Scope: Higher · Unit 5 tier: Higher · Topic 5.5 (higher)
Open revision guidance
What to know
- Isomers are compounds with the same molecular formula but different structural arrangements.
- Structural isomers occur in alkanes and alkenes when carbon atoms are arranged differently (e.g., branched vs straight chain).
What to do in an assessment
- Draw structures for butane (C4H10) and its isomer, 2-methylpropane (C4H10).
In plain language
Isomerism happens when molecules have the same chemical 'ingredients' (atoms) but are put together in a different shape, changing their structure.
Self-check
Do isomers have the same molecular formula?
Show the answer
Yes, they have the same molecular formula but different structural arrangements.
Topic group
Skills across the sciences
- Core
Scope: Core · shared-enquiry; core
Open revision guidance
What to know
- Develop hypotheses and devise procedures [L125]
- Select appropriate apparatus and techniques [L125]
- Use SI units, prefixes, and powers of ten [L136]
- Understand random and systematic errors [L136]
- Data quality: accuracy, precision, repeatability, and reproducibility [L136]
- Risk evaluation and ethical considerations [L125]
What to do in an assessment
- Plan investigations to ensure samples are representative [L125]
- Collect/record data accurately including uncertainty estimation [L125, L136]
- Present data via tables, graphs, and symbolic forms [L136]
- Translate data between forms and identify patterns/trends [L136]
- Evaluate methods to suggest improvements [L125]
In plain language
Scientific enquiry involves planning experiments, selecting equipment, and collecting data with awareness of errors. You must ensure results are repeatable and reproducible, use proper SI units, and evaluate your findings to draw conclusions based on evidence.
Self-check
What is the difference between repeatability and reproducibility?
Show the answer
Repeatability is achieving the same results using the same methods/equipment by the same investigator, while reproducibility involves different conditions or investigators reaching similar findings.
- Core
Scope: Core · shared-maths; core
Open revision guidance
What to know
- Arithmetic: decimal form, standard form, ratios, fractions, percentages [L158]
- Handling data: significant figures, arithmetic means, tables/diagrams [L158]
- Probability: understanding simple probability [L158]
- Algebra: substitute numerical values and solve equations, change the subject [L158]
- Graphs: y=mx+c linear relationship, plotting variables, interpret slope and intercept [L158]
- Geometry: angular measures, area of triangles/rectangles, surface area/volume [L158]
What to do in an assessment
- Convert between units (e.g., km to m, g to kg) [L136]
- Perform order of magnitude calculations [L158]
- Calculate area of a rectangle (base x height) or volume of a cube (side³) [L158]
- Determine the gradient (m) and y-intercept (c) from a linear graph [L158]
- Solve for variables in physical formulae [L158]
In plain language
Mathematical skills are essential for science. You must be able to handle arithmetic, work with significant figures and units, use algebra to manipulate formulae, and construct/interpret linear graphs and geometric shapes.
Self-check
If y = 3x + 2, what is the gradient and the y-intercept?
Show the answer
The gradient (m) is 3 and the y-intercept (c) is 2.
- Higher
Scope: Higher · Higher in either Unit 1 tier or Unit 2 tier or Unit 3 tier or Unit 4 tier or Unit 5 tier or Unit 6 tier · shared-maths; higher
Open revision guidance
What to know
- Tangents: Using the gradient of a tangent to determine instantaneous rate of change [L158]
- Area under curves: Physical significance of area between a curve and the x-axis [L158]
What to do in an assessment
- Draw a tangent to a curve at a point and calculate gradient (dy/dx ≈ rise/run) [L158]
- Estimate area under a curve by counting squares [L158]
In plain language
Higher tier maths requires interpreting curved graphs. You must find the rate of change by drawing a tangent and calculating its gradient, and estimate physical quantities (like displacement) by counting squares under a curve.
Self-check
How do you find the instantaneous rate of change from a curved graph?
Show the answer
Draw a tangent to the curve at the specific point and calculate the gradient (slope) of that straight line.
Topic group
Unit 7 Practical Assessment Guidance
- Core
Scope: Core · unit-7; core
Reading this guidance does not complete practical assessment. It is not counted as written-exam progress.
Open revision guidance
What to know
- Assessment structure: Two tasks chosen by the centre (Biology/Chemistry/Physics) [L114]
- Section A: Obtaining results, 60 minutes, group work allowed (max 3), 6 marks [L114]
- Section B: Analysis and evaluation, 60 minutes, individual work, 24 marks [L114]
What to do in an assessment
- Perform experiments to obtain results in Section A [L114]
- Analyze/evaluate data and answer questions individually in Section B [L114]
- Ensure all data presented follows scientific notation and significant figure rules [L136]
In plain language
Unit 7 is an externally marked practical assessment worth 10% of your grade. You collect data in groups for Section A, then analyse that data individually under formal exam conditions in Section B.
Self-check
Are you allowed to receive teacher help during Section B?
Show the answer
No, Section B is conducted under high control with no teacher feedback or assistance allowed.
Topic group
Unit 2 · practical:2.3 · Determining Water Hardness Using Soap Solution
- Core
Scope: Core · Topic practical:2.3 (core)
Open revision guidance
What to know
- Hard water contains dissolved calcium and magnesium ions.
- Soap reacts with these ions to form a scum, preventing lathering until the ions are precipitated.
- The volume of soap solution required to produce a stable, lasting lather is proportional to the concentration of hardness-causing ions.
- Control variables include the initial volume of water sample and the concentration of the soap solution.
What to do in an assessment
- Measure 50 cm³ of the water sample using a measuring cylinder and pour it into a conical flask.
- Add soap solution from a burette in 0.5 cm³ increments, stoppering and shaking the flask vigorously between additions.
- Stop adding soap when a stable lather is formed that persists for at least 30 seconds.
- Record the final burette reading. Repeat three times for each sample to calculate a mean volume.
- Wear safety goggles to protect against splashes and ensure laboratory supervision during the use of glassware.
In plain language
We measure how much soap it takes to make a permanent lather in different water samples. More soap needed means the water is harder.
Self-check
If 10 cm³ of soap is needed for Sample A and 5 cm³ for Sample B, which is harder?
Show the answer
Sample A is harder because it required more soap to overcome the calcium and magnesium ions.
Topic group
Unit 2 · practical:2.5 · Investigating Factors Affecting Reaction Rates
- Core
Scope: Core · Topic practical:2.5 (core)
Open revision guidance
What to know
- Gas collection measures the volume of gas produced over time.
- Disappearance of a cross measures the time taken for a cloudy precipitate to obscure a mark.
- Factors affecting rate include concentration, temperature, surface area, and catalysts.
What to do in an assessment
- Gas collection: Attach a gas syringe to a conical flask containing reactants. Record volume produced at regular time intervals.
- Precipitation: Place a conical flask over a black cross on paper. Add reactants and time how long it takes for the cross to disappear from view.
- Controls: Keep all variables constant except the factor being investigated.
- Safety: Wear safety goggles; handle acids with care and dispose of sodium thiosulfate waste correctly.
In plain language
We measure how fast a reaction happens either by catching gas in a syringe or timing how long it takes for a liquid to turn opaque. Always change only one factor at a time to keep results fair.
Self-check
If you increase the concentration of hydrochloric acid, what happens to the rate of reaction with sodium thiosulfate?
Show the answer
The rate increases, so the cross disappears in a shorter amount of time.
Topic group
Unit 5 · practical:5.2 · Preparation of Soluble Salts and Acid-Base Titration
- Core
Scope: Core · Topic practical:5.2 (core)
Open revision guidance
What to know
- A soluble salt is prepared by reacting an acid with an insoluble base or carbonate until no more reacts, then filtering off the excess.
- Titration determines the concentration of an unknown solution by finding the exact volume of acid needed to neutralize a specific volume of base using an indicator.
- Acid-base indicators like phenolphthalein or methyl orange change colour at the neutralization point (endpoint).
What to do in an assessment
- For salt preparation: Heat acid, add base/carbonate until excess remains, filter, and crystallize filtrate by evaporating solvent.
- For titration: Use a pipette to place base in a conical flask, add indicator, fill a burette with acid, add acid dropwise until colour change, and record volume.
- Control variables: Keep concentrations and volumes consistent for repeated trials; ensure precise meniscus reading at eye level.
- Safety: Wear safety goggles, use heat-resistant mats, and handle acids/bases with caution under teacher supervision.
In plain language
This practical teaches how to make a salt by mixing acid with a base, and how to measure the exact amount of acid needed to cancel out a base using a color-changing indicator.
Self-check
If 20 cm3 of acid is needed to neutralize 25 cm3 of base, and the concentration of the base is 0.1 mol/dm3, what is the volume of acid used?
Show the answer
The volume of acid used is 20 cm3.
Topic group
Unit 5 · practical:5.3 · Determining Metal Reactivity via Displacement
- Core
Scope: Core · Topic practical:5.3 (core)
Open revision guidance
What to know
- A more reactive metal displaces a less reactive metal from its aqueous salt solution.
- The reaction is visible through colour changes or solid deposits.
- Reactivity is determined by the number of successful displacement reactions observed for each metal.
What to do in an assessment
- Gather test tubes, rack, small pieces of metals (e.g., Mg, Zn, Fe, Cu), and 1.0 mol/dm³ solutions of their respective salts.
- Add 5cm³ of salt solution to each of four test tubes.
- Place one metal piece into each tube and observe for 5 minutes.
- Record if a displacement reaction (colour change or solid formation) occurs.
- Clean equipment thoroughly and repeat for other metals.
- Wear safety goggles at all times; school staff must supervise acid handling and metal disposal.
- Calculate reactivity order by counting how many different salts each metal displaced.
In plain language
This experiment finds out which metals are most reactive by testing if a metal can push another metal out of a solution. If a metal creates a solid deposit in a salt solution, it is more reactive than the metal currently in that solution.
Self-check
If Magnesium displaces Copper from Copper Sulfate, but Iron does not displace Magnesium from Magnesium Sulfate, which metal is the most reactive?
Show the answer
Magnesium is the most reactive because it displaces Copper, but cannot be displaced by Iron.
Topic group
Unit 5 · practical:5.5 · Determination of the amount of energy released by a fuel
- Core
Scope: Core · Topic practical:5.5 (core)
Open revision guidance
What to know
- Energy transferred (Q) is calculated as mass (m) × specific heat capacity (c) × temperature change (delta T).
- The energy content of a fuel is determined by heating a known mass of water using the burning fuel.
- Energy loss to surroundings is the primary source of inaccuracy.
- Variables: Independent is the type of fuel; Dependent is temperature rise; Control are mass of water and distance between flame and container.
What to do in an assessment
- Measure 50ml of water into a copper calorimeter or small metal can.
- Measure the starting temperature of the water with a thermometer.
- Weigh the burner containing the fuel before heating.
- Light the burner, place it beneath the can, and heat the water for a fixed time.
- Extinguish the flame and record the final temperature of the water.
- Weigh the burner again to determine the mass of fuel burned.
- Safety: Wear safety goggles, use a heat-proof mat, and ensure adequate ventilation.
In plain language
We find how much energy is in a fuel by using the heat from the burning fuel to warm up a fixed amount of water and measuring how much the temperature increases.
Self-check
If 100g of water increases by 10 degrees Celsius, how much energy is transferred (using specific heat capacity 4.2 J/g°C)?
Show the answer
Energy = 100g × 4.2 J/g°C × 10°C = 4200 Joules.
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