OCR GCSE Chemistry Revision
Explore OCR GCSE Chemistry revision topics. Check the official specification for complete requirements for your exam year and route.
Qualification code: J248
Course details
OCR · GCSE ChemistryJ248Exam years · 2027
Assessment For assessment details, see the official specification.
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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.
Options and source
Paper tier
- Foundation tier
- Higher tier
§2a, §3a and §3d; J248/01–/02 Foundation and J248/03–/04 Higher
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Topic group
Reviewed requirements
- Higher
Scope: Higher · Paper tier: Higher tier · Bold-only learning-outcome text and the bold extensions in C3.1b, C3.4d, CM3.1iv and CM5.1iii; source tiering key, §2c pp.10–54 and §5f pp.74–81
Open revision guidance
What to know
- This is a Higher-tier supplement, not Foundation content. OCR marks Higher-only material in bold; † means separate-science content, not Higher tier. The whole Higher-only outcomes are C1.1c; C3.1e,g,h,k,l; C3.2d; C3.3b,g,i,j; C5.1a,c,d,e,f,k; C5.3c; C6.1c,d,e,f; C6.2e,f; CM3.1ii and CM5.1i. The Higher-only portions of otherwise shared outcomes are “and half equations” in C3.1b, “half equations” in C3.4d, and “particularly from mass to moles” in CM3.1iv and CM5.1iii.
- C1.1c: explain why inelastic-sphere particle models cannot represent attractions between particles, their size, or the empty space between them. A model may explain an observation yet still omit features needed to explain changes of state.
- C3.1e: balance full ionic equations for atoms and charge, identify spectator ions unchanged on both sides, then cancel those ions to form the net ionic equation. C3.1b extension: construct and balance half equations as well as full symbol equations.
- C3.1g–h, C3.1k–l and CM3.1ii/CM5.1i: one mole contains 6.02 × 10²³ entities. Use n = m/Mr, the balanced equation coefficients and the limiting reactant to calculate reaction amounts, theoretical product masses and the mass of one atom or molecule. Example: one 24.0 g mole of Mg contains 6.02 × 10²³ atoms, so one Mg atom has mass 24.0/6.02 × 10²³ = 3.99 × 10⁻²³ g. Keep units consistent and do not define a mole as a mass.
- C3.2d: calculate reaction enthalpy from bond energies using ΔH = Σ(bonds broken) − Σ(bonds formed). If bond breaking totals 800 kJ mol⁻¹ and bond formation totals 950 kJ mol⁻¹, ΔH = −150 kJ mol⁻¹, so the reaction is exothermic.
- C3.3b: track electron loss and gain to identify oxidation and reduction. For example, Mg → Mg²⁺ + 2e⁻ is oxidation; Cu²⁺ + 2e⁻ → Cu is reduction. Name the electron donor and acceptor from the balanced transfer. C3.3g: calculate concentration from amount and volume, keeping concentration distinct from acid strength. C3.3i,j: interpret titration-curve pH and use the logarithmic scale; a tenfold increase in hydrogen-ion concentration lowers pH by one.
- C3.4d extension: write half equations with electrons on the correct side; at a cathode reduction consumes electrons and at an anode oxidation releases them. Balance atoms and charge before combining electrode products.
- C5.1a,c–f: relate molar concentration to solute mass, Mr and solution volume; use c = n/V and n = cV; convert cm³ to dm³ by dividing by 1000. In titration calculations, use the balanced equation coefficient ratio. At RTP use 24 dm³ mol⁻¹ (24,000 cm³ mol⁻¹) for gas volumes. Example: 25.0 cm³ of 0.100 mol dm⁻³ NaOH contains 0.100 × 0.0250 = 0.00250 mol; for HCl + NaOH → NaCl + H₂O, a 1:1 ratio means this amount of HCl neutralises it. C5.1k: justify pathway choice with the supplied yield, rate, equilibrium, atom-economy (if supplied) and by-product data.
- C5.3c: predict equilibrium shifts after changing concentration, temperature or pressure; for pressure compare gas-molecule counts, and for temperature identify the endothermic direction. Explain a production choice using the required product amount as well as rate.
- C6.1c,d–f: evaluate bioleaching and phytoextraction against conventional extraction; explain Haber and Contact process rate/equilibrium trade-offs; interpret rate-condition graphs; and relate operating conditions to raw-material/energy availability, cost, equilibrium and rate.
- C6.2e–f: condensation polymerisation joins suitable bifunctional monomers and eliminates a small molecule; identify the repeat-unit linkage and required functional groups. For the source-described nylon practical, layer the reactant solutions carefully, draw out the film using forceps, wash and record it, following local risk controls.
- Mixed mathematics extensions CM3.1iv and CM5.1iii: apply unit conversion and multistep arithmetic, especially when converting mass to moles. They extend shared mathematics but do not make the entire shared outcomes Higher-only.
What to do in an assessment
- Check the source tier label before practising a question. For every calculation, write the equation, convert units, substitute with a mole ratio where needed, show working and state a sensible answer with units.
- For electron-transfer questions, show where electrons are lost or gained, balance both atoms and charge, and identify the oxidised/reduced species.
In plain language
These extra calculations and explanations are only required for Higher tier. Use the source's bold text—not the dagger symbol—to decide what belongs here.
Self-check
At Higher tier, 0.100 mol of Mg reacts with 0.0400 mol O₂: 2Mg + O₂ → 2MgO. Which reactant limits the product, and what mass of MgO can form? (Ar: Mg 24, O 16)
Show the answer
The equation needs 2 mol Mg per 1 mol O₂. 0.0400 mol O₂ needs 0.0800 mol Mg, so Mg is in excess and O₂ limits. It forms 0.0800 mol MgO; Mr(MgO)=40, so theoretical mass = 0.0800 × 40 = 3.20 g.
- Core
Open revision guidance
What to know
- J248 is a linear separate Chemistry A (Gateway Science) GCSE. Foundation candidates take J248/01 and J248/02; Higher candidates take J248/03 and J248/04. The papers are not interchangeable between tiers.
- Paper 1 tests C1–C3 plus C7 practical skills; Paper 2 tests C4–C6 while assuming C1–C3 and C7 knowledge. Each paper is 90 marks and 1 hour 45 minutes; both papers in the chosen tier contribute 50% each. Each has 15 multiple-choice marks and 75 marks of structured work, including practical/maths questions and a six-mark level-of-response item; Paper 2 also includes synoptic questions. Paper 2 and Paper 4 connect earlier topics to C4–C6.
- At least 15% of written assessment tests practical skills and at least 20% tests mathematics. AO1 assesses knowledge/understanding (40%), AO2 application (40%) and AO3 analysis (20%) at both tiers; the AO3 sub-elements are AO3.1a interpretation, AO3.1b evaluation, AO3.2a judgements, AO3.2b conclusions, AO3.3a developing experimental procedures and AO3.3b improving them. Each paper contributes 20, 20 and 10 percentage points to the full GCSE. All four papers are external; a scientific or graphical calculator is permitted. Foundation assesses grades 1–5 and Higher grades 4–9; on the Higher Tier option, an allowed grade 3 may be awarded to candidates a small number of marks below the grade 3/4 boundary. The learner enters one tier for both papers.
- Complete source command-word guide: analyse breaks information into elements and can require discussing a topic or argument's pros and cons before making a reasoned comment; calculate obtains a numerical value through calculations; choose selects; classify groups by shared features; compare/contrast states similarities/differences; complete supplies missing parts; conclude gives a supported final answer; construct creates a requested equation/model; convert changes units/forms; deduce infers from knowledge/data; define gives a precise meaning; describe gives facts/sequence; design plans a method; determine/find obtain a solution (find is more common at Foundation); discuss addresses a range of ideas/arguments; draw makes a sufficiently detailed labelled diagram; estimate gives an approximate value; evaluate makes a qualitative judgement using factors/evidence; explain gives reasons/mechanisms; give is a short response; how asks in what way; identify recognises/lists/names; illustrate uses examples/diagrams; justify gives a reasoned case; label adds identifying words/symbols; measure uses a suitable instrument/technique; name supplies a term; outline gives main points; plan communicates what will be done; plot graphs data with labelled axes; predict judges what would happen from knowledge/evidence; recall retrieves a relevant fact; select chooses the most suitable; show supplies steps/details to prove an answer; sketch gives a simple freehand shape without detail; state/define expresses meaning precisely; suggest proposes possibilities; use/using bases the answer on supplied information; what asks for contextualised information; which identifies an object/word/explanation; why asks for a reason; write presents the requested information. Follow the command in its full question context.
- Every externally assessed paper includes an asterisk-marked extended-response question assessed with a level-of-response mark scheme. Build sustained, coherent, relevant, substantiated and logically structured reasoning. Paper 2/4 synoptic assessment explicitly connects areas of Chemistry; it can require applying more than one topic, planning/investigating and analysing/evaluating data, or combining concepts.
- Qualification context, not additional Chemistry lesson content: TQT is 140 hours and guided learning time is 120–140 hours; no previous qualification is required and learners are likely to have followed Key Stage 3. The qualification is available in England, while Wales/Northern Ireland availability must be checked with QIW/NIEFQAN. Qualification, assessment and candidate work are in English. Assessment is available in the May/June series, with components taken in the same series at course end; a retake requires all papers for the chosen tier. The overall grade uses the sum of the two paper marks compared with the entry-option and series grade boundaries. Other source context: §5a provides grade descriptors; §5b identifies overlap with other qualifications; §5c explains accessibility; §5d specifies science units; §5g makes safety paramount and places practical safety responsibility on centres; §5h supplies the Periodic Table. Sections 4a–4f describe pre-assessment, special consideration, external assessment arrangements, results/certificates, post-results services and malpractice procedures; these are administrative context, not Chemistry teaching outcomes.
What to do in an assessment
- Read the paper number and tier code before deciding which content and grade demand apply.
- For a calculation, show the equation or relationship, substitution, units and a sensible significant-figure answer; for evaluation, select evidence, explain its consequence and make a justified conclusion.
In plain language
The four J248 papers make one Chemistry GCSE. You sit two papers at one tier; practical and maths skills appear inside the written papers.
Self-check
Which content and practical topic can appear on J248/04, and what tier is it?
Show the answer
J248/04 is the Higher-tier Paper 2; it tests C4–C6 and C7 practical skills.
- Core
Scope: Core · Topic C1, C1.1a–c and C1.2a–e
Open revision guidance
What to know
- In solids, particles are close together in a regular arrangement and vibrate about fixed positions; liquids have close particles that move past one another; gas particles are far apart and move rapidly and randomly. Heating increases particle kinetic energy and can produce melting, boiling or sublimation; cooling reverses the changes. In a physical state change the particles themselves do not become a different substance.
- The particle model explains flow, shape, compressibility and diffusion, but its circles are representations: they are not to scale and do not show the forces, internal structure or detailed motion of real particles.
- Atomic ideas developed as evidence changed: Dalton's atoms were indivisible; Thomson's cathode-ray evidence led to electrons and a positive sphere model; Geiger and Marsden's alpha-scattering evidence implied a tiny dense positive nucleus; Bohr placed electrons in shells; the modern model describes a nucleus containing protons and neutrons with electrons in shells.
- An atom has a tiny nucleus compared with the whole atom; a typical atomic radius or bond length is of order 10⁻¹⁰ m. Proton charge is +1 and relative mass about 1; neutron charge 0 and relative mass about 1; electron charge −1 and relative mass about 1/1840. Atomic number is proton number; mass number is proton plus neutron number. Isotopes have the same proton number but different neutron numbers. For a neutral atom, electron number equals proton number; for an ion, adjust electrons by the magnitude and sign of the charge. Standard nuclide notation places mass number above atomic number beside the element symbol.
- C1.1a–b: distinguish the fixed, vibrating arrangement in a solid, close particles that move past one another in a liquid, and widely spaced, rapidly random gas particles. During melting, boiling, freezing or condensing, energy transfer changes particle kinetic energy or the arrangement against attractions; a temperature plateau during a pure-substance state change reflects energy changing arrangement, not temperature. A physical change preserves particle identity; a chemical change makes new particle combinations. Dissolving is not melting: solvent particles separate and surround solute particles to make a mixture, without turning the solute into a new substance. A sphere model cannot show particle size, empty gaps or attractive forces, and identical spheres hide differences between substances.
- C1.2a–e: Dalton proposed indivisible atoms; cathode-ray evidence led Thomson to electrons in a positive sphere; alpha scattering by Geiger and Marsden supported a small, dense positive nucleus; Bohr proposed shells; later evidence established nuclear protons and neutrons. The nucleus contains nearly all atomic mass but occupies a tiny fraction of atomic volume. Typical atom/bond scale is 10⁻¹⁰ m. An isotope changes neutron count, not proton count; form a positive or negative ion by removing or adding electrons, never by changing the nucleus.
What to do in an assessment
- Explain an observation using particle spacing, arrangement, motion and energy, then name a limitation if a model is being evaluated.
- For nuclide notation, calculate neutrons = mass number − atomic number; use proton number to identify an element and charge to calculate electrons. Give an atom-model change as evidence → inference → revised model.
In plain language
Heating makes particles move more; it does not normally turn one chemical into another. The atom model changed when experiments revealed smaller particles and a compact nucleus.
Self-check
How many protons, neutrons and electrons are in ³⁵Cl⁻?
Show the answer
17 protons, 18 neutrons and 18 electrons: 35−17 neutrons, and one extra electron for the −1 charge.
- Core
Scope: Core · Topic C2.1, C2.1a–k
Open revision guidance
What to know
- A pure substance has a sharp melting point; impurities usually broaden and lower a solid's melting range. Formula mass Mr is the sum of relative atomic masses in the formula. Empirical formula is the simplest whole-number atom ratio; determine it by converting masses or percentages to moles, dividing by the smallest amount, then scaling to whole numbers.
- Filtration separates an insoluble solid from a liquid; crystallisation recovers a dissolved solid by concentrating a solution and cooling it; simple distillation collects a solvent or liquid with a sufficiently different boiling point; fractional distillation separates miscible liquids with closer boiling points using repeated condensation and vaporisation in a fractionating column.
- In paper/TLC chromatography, a solvent is the mobile phase and the paper or coated plate is stationary. Draw a pencil baseline above the solvent; spot the mixture, develop in a covered vessel and mark the solvent front. Components travel different distances because their relative attraction to the two phases differs. Rf = distance travelled by component ÷ distance travelled by solvent front; compare only under matching conditions. Gas chromatography separates volatile components and produces a chromatogram.
- A separation method is selected by the physical properties that differ: solubility, particle size, boiling point or affinity for chromatographic phases. Alloys are mixtures of elements (at least one metal) and can have useful properties unlike a pure metal.
- C2.1a–e: scientific purity means one element or compound, unlike the everyday sense of clean or natural; a pure solid has a sharp melting point while an impurity generally lowers and broadens its melting range. Relative formula mass is the sum of relative atomic masses in the stated formula, including coefficients when calculating a balanced equation. Empirical formula can be deduced in either direction between a particle/model diagram and atom ratio; e.g. a 1:2:1 ratio is CH₂O. Useful formulations are mixtures designed to have selected properties; brass and steel are alloy examples.
- C2.1f–k: filtration leaves an insoluble residue on filter paper and passes filtrate; crystallisation partially evaporates a solution then cools it, filters and dries crystals; simple distillation heats a mixture, condenses the more volatile vapour and collects the distillate; fractional distillation uses a fractionating column and repeated vaporisation/condensation for closer boiling points. In paper/TLC chromatography the pencil baseline remains above solvent, samples are spotted and dried, the covered chamber is developed, and solvent front is marked immediately. Locate invisible spots with a suitable locating agent; aqueous or non-aqueous solvents separate according to partition between stationary and mobile phases. Choose paper, TLC or gas chromatography to distinguish the particular components; calculate Rf as component distance / solvent-front distance.
What to do in an assessment
- Choose the separation method that matches the mixture, and describe apparatus, order of operations, what is collected and why that fraction is pure.
- For Rf, measure from the baseline in the direction of solvent travel, divide by the solvent-front distance, keep units consistent and report a ratio without units.
In plain language
Separation works because substances differ in physical properties. The right method depends on whether you need to separate a solid, a liquid or components that travel differently with a solvent.
Self-check
A spot travels 3.6 cm and the solvent front 6.0 cm. What is its Rf, and what does that value tell you?
Show the answer
Rf = 3.6/6.0 = 0.60. It is a dimensionless comparison for that chromatographic setup; by itself it does not identify a substance unless compared with a reference under the same conditions.
- Core
Scope: Core · Topic C2.2, C2.2a–i
Open revision guidance
What to know
- The periodic table is ordered by atomic number. A period identifies the number of occupied electron shells for the first 20 elements; group position reflects outer-shell electron structure. Metals and non-metals differ in common physical properties and in the ways they form ions or bonds.
- Ionic bonding follows electron transfer, commonly from a metal to a non-metal: oppositely charged ions attract electrostatically in all directions in a giant lattice. Covalent bonding is sharing pairs of electrons between non-metal atoms. Metallic bonding is attraction between positive metal ions and delocalised electrons. Dot-and-cross diagrams show outer-shell electrons and transfer/sharing, not particle scale or the full three-dimensional lattice.
- For a simple ionic formula, choose ion numbers so total positive and negative charge sum to zero; show brackets and charges for ions in diagrams. For covalent molecules, shared pairs complete outer shells for the atoms shown. A bonding model must be judged by the feature it explains; no 2-D drawing is the real 3-D structure.
- An element's reactions can be predicted from outer-shell structure and periodic position; distinguish periodic trends supported by evidence from explanations at particle level.
- C2.2a–c, h–i: metals are generally lustrous, malleable and good conductors, and tend to lose outer electrons; non-metals have varied physical properties and commonly gain or share electrons. Metal oxides are generally basic and non-metal oxides generally acidic: for example 2Mg + O₂ → 2MgO, while C + O₂ → CO₂. For the first 20 elements, group number corresponds to outer-shell electron count (allowing for noble gases) and period number to occupied shells; proton number fixes the electron count and hence the neutral atom's structure. Mendeleev arranged by properties and left gaps, sometimes not strict mass order; the modern table is ordered by atomic number, resolving anomalous placements.
- C2.2d–g: ionic solids are repeating lattices of oppositely charged ions, simple molecules contain discrete covalent molecules, giant covalent networks extend throughout a structure, polymers are long covalent chains, and metals are positive ions in delocalised electrons. Ionic bond is electrostatic attraction after electron transfer; covalent bond is attraction between nuclei and shared electrons; metallic bonding is attraction between ions and delocalised electrons. Construct outer-electron dot-and-cross diagrams for binary ionic compounds and simple covalent molecules. State diagram limits: dot-and-cross omits scale/3D and may not show attractions; ball-and-stick overstates bonds as rods/spacing; 2D views cannot fully show 3D structure.
What to do in an assessment
- Draw a dot-and-cross model with correct atom labels, transferred or shared outer electrons, ion brackets/charges where relevant, and a neutral overall formula.
- When comparing elements, cite their periodic position, identify the outer-shell change, then connect it to electron gain/loss and the observed reaction.
In plain language
Elements' positions help predict their outer electrons. Those electrons explain why atoms transfer, share or pool electrons when they bond.
Self-check
What formula results when Mg²⁺ combines with Cl⁻, and why?
Show the answer
MgCl₂. Two chloride ions provide total charge −2 to balance one magnesium ion at +2.
- Core
Scope: Core · Topic C2.3, C2.3a–j
Open revision guidance
What to know
- Carbon forms four covalent bonds. Diamond is a giant covalent structure in which each carbon bonds to four others, making it hard with a high melting point; graphite has layers of hexagonally arranged carbons, one delocalised electron per carbon and weak forces between layers, so layers slide and it conducts electricity.
- In simple molecular solids, strong covalent bonds hold each molecule together but intermolecular attractions are weaker; melting or boiling usually separates molecules rather than breaking covalent bonds. In ionic solids, a giant lattice of oppositely charged ions requires substantial energy to disrupt; molten or dissolved ions can move and carry charge.
- Use data such as melting/boiling temperatures and density to compare materials and predict state at a stated temperature. Link the property to particle arrangement and bonding rather than treating a substance name as an explanation.
- Nanoparticles have dimensions about 1–100 nm; a high surface-area-to-volume ratio can make their behaviour differ from bulk material. Compare the dimensions using powers of ten and balance potential usefulness with health/environmental uncertainty when evaluating applications.
- C2.3a–f: carbon makes four covalent bonds, enabling diverse chain/ring organic families. Diamond is a rigid 3D network (each carbon bonded to four), graphite is a layered hexagonal network (three bonds and a mobile electron per carbon), graphene is one graphite layer, and fullerenes are hollow carbon cages/tubes; use their structures to explain hardness, conduction and uses. To change state, energy is required to overcome intermolecular attractions, not usually to break covalent bonds within molecules; a giant lattice instead requires disruption of many strong bonds. Compare melting/boiling data to ambient temperature to infer solid/liquid/gas and explain ionic, molecular, giant-covalent, polymer and metallic material properties by bond strength, arrangement and mobile particles; atoms themselves do not possess bulk conductivity or hardness.
- C2.3g–j (both tiers; dagger marks separate-science, not Higher-only): nano means approximately 1–100 nm, compared with atom/bond scales around 0.1 nm, small molecules around 1 nm and everyday objects many orders larger; convert all to one unit and powers of ten. For a cube of side L, surface area = 6L² and volume = L³, so SA:V = 6/L; reducing size increases exposed area per volume and can change reaction rate or optical/catalytic behaviour. Connect nanoparticle properties to a specified use (for example titanium dioxide/zinc oxide sunscreen, silver antimicrobial coatings or catalysts), then evaluate possible risk from exposure, persistence and uncertain evidence without presenting a hazard as a proven harm.
What to do in an assessment
- Build a property explanation as structure → particle interaction/mobility → measured bulk property → use.
- For state questions, compare the stated temperature with the melting and boiling points, include the correct boundary condition, and read units/scale from the data.
In plain language
A material's particles and bonds determine how much energy is needed to change its shape, melt it or conduct electricity.
Self-check
Why does graphite conduct electricity but diamond does not?
Show the answer
Each graphite carbon contributes a delocalised electron that can move along the layers; in diamond all four outer electrons per carbon are used in covalent bonds.
- Core
Scope: Core · Topic C3.1, C3.1a–l; C3.2, C3.2a–d
Open revision guidance
What to know
- A chemical reaction rearranges atoms: total atoms of each element and total mass are conserved in a closed system. Write formulae first, then balance by changing coefficients only; state symbols distinguish solid, liquid, gas and aqueous substances.
- A reaction can be described by observations and represented by a word, symbol or ionic equation. Use state and charge information to identify spectator ions before cancelling them in a net ionic equation. An unfamiliar reaction can be predicted from the reactants and known patterns, but predictions need evidence.
- Breaking bonds takes in energy; forming bonds releases energy. The overall energy change depends on the difference between energy absorbed and released. Exothermic reactions transfer energy to the surroundings; endothermic reactions take energy in. A temperature change is evidence of energy transfer, not a direct measurement of every bond energy.
- A reaction profile shows reactant/product energy levels and activation energy. A catalyst offers an alternative pathway with lower activation energy; it does not change the energy difference between reactants and products.
- C3.1a–f: use correct symbols/formulae for elements, simple covalent substances and ionic compounds; use the supplied periodic table for the first 20 elements, Groups 1, 7 and 0, and other named specification elements. Use ion charges to construct a neutral compound formula, then balance equations by coefficients while keeping subscripts fixed. Balance full ionic equations for atoms and charge and cancel spectator ions only to form a net ionic equation (C3.1e is Higher only). State symbols are (s), (l), (g) and (aq). In C3.1b, names/symbols, conservation of mass, formulae and balanced chemical equations are both-tier; the specifically bold extension “and half equations” is Higher only.
- C3.1g–l: the mole is the amount containing 6.02 × 10²³ entities (also accept the carbon-12 definition); Mr or Ar grams is the mass of one mole, not the definition of a mole. n = m/Mr and m = nMr; mass of one specified atom/molecule = molar mass in grams ÷ 6.02 × 10²³ (or use kg consistently). These mole/atom-mass outcomes C3.1g–h are Higher only. For mass changes in an open system, account for matter entering/leaving: escaping CO₂ can lower measured mass, while oxygen entering a heated metal can raise it; atoms and total mass remain conserved in the whole system. Convert each known mass to moles, divide by the smallest amount to find formula ratios, use balanced coefficients and limiting-reactant ratios, then convert product moles to mass. Deduce reaction stoichiometry/limiting quantity (C3.1k) and calculate reaction masses (C3.1l) at Higher tier; for example, 2Mg + O₂ → 2MgO has 2:1:2 mole ratio; if O₂ is insufficient it limits product, leaving excess Mg.
- C3.2a–c apply at both tiers; C3.2d bond-energy calculations are Higher only. A rise in surroundings temperature indicates exothermic transfer; a fall indicates endothermic transfer. Draw energy against reaction progress, with reactant/product levels, activation-energy arrow from reactants to peak and ΔH arrow between levels. For bond-energy data, ΔH = Σ(bonds broken) − Σ(bonds formed); breaking bonds absorbs energy, forming bonds releases it. Example: if breaking totals 800 kJ/mol and forming totals 950 kJ/mol, ΔH = −150 kJ/mol (exothermic).
What to do in an assessment
- Balance each element, including oxygen and hydrogen last if useful; check atom count, state symbols and ionic charge.
- On a reaction profile label axes, reactants, products, activation energy and the energy change; infer exo/endo from relative energy levels and explain catalyst effect without changing ΔH.
In plain language
Atoms are rearranged, not created or lost. Energy is transferred when bonds break and form, so the reaction can warm or cool its surroundings.
Self-check
Is bond breaking exothermic or endothermic, and what lowers the activation energy?
Show the answer
Bond breaking is endothermic because energy is required; a catalyst supplies a lower-activation-energy pathway.
- Core
Scope: Core · Topics C3.3–C3.4; redox, acids, neutralisation, pH and electrolysis
Open revision guidance
What to know
- Acids provide H⁺ ions in aqueous solution; alkalis provide OH⁻ ions. Neutralisation is H⁺ + OH⁻ → H₂O. Dilute/concentrated describes amount per volume; weak/strong describes degree of ionisation, so these are different ideas. pH measures acidity: a tenfold increase in H⁺ concentration lowers pH by one whole number. Universal indicator estimates pH; a calibrated pH meter/probe measures it more precisely. A salt's name and formula depend on the acid's anion and the base/cation.
- Electrolysis uses direct current to decompose an ionic substance when its ions are mobile in a molten or aqueous electrolyte. Positive ions move to the negative cathode and gain electrons; negative ions move to the positive anode and lose electrons. Product prediction in aqueous solutions must consider competing ions and the specified electrodes.
- In molten or aqueous ionic compounds, cations move to the cathode and anions to the anode. At an inert cathode the metal (or hydrogen) is formed; at an inert anode a non-metal is formed. In aqueous solutions, predict competing products by listing the ions present; use half-equations to show electron gain or loss. Inert and non-inert electrodes can have different roles.
- Oxidation and reduction can be described as oxygen gain/loss or electron loss/gain. Acids react with reactive metals to form salt and hydrogen, and with carbonates to form salt, water and carbon dioxide; write balanced equations with correct salt names and state symbols.
- C3.3a–k: C3.3b (electron-transfer redox), C3.3g (concentration/strength), C3.3i (pH and titration curves) and C3.3j (tenfold H⁺ change) are Higher only; other content in this outcome range is both-tier. Oxidation is oxygen gain/electron loss and reduction is oxygen loss/electron gain; name the species and oxidising/reducing agent from the transfer. Acids form H⁺(aq), alkali solutions contain OH⁻(aq); acid + base → salt + water and H⁺ + OH⁻ → H₂O. Acid + metal → salt + H₂; acid + carbonate → salt + H₂O + CO₂ (e.g. 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂). Choose the salt anion from the acid (hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate) and balance formulae. Concentrated means much solute per volume; dilute means little; strong/weak describes extent of ionisation, not concentration. pH is a whole-number logarithmic scale: neutral at 7, lower is more acidic and higher more alkaline; a tenfold rise in [H⁺] lowers pH by one. On a titration pH curve read both axes, identify initial/final pH and the steep neutralisation region; neutrality means [H⁺]=[OH⁻] and pH 7 at 25°C, not necessarily that the curve's end point always lies at 7. Use universal indicator for an approximate colour range or rinse/calibrate a pH meter/probe, immerse the probe, wait for a stable reading and report pH without a unit.
- C3.4a–e: in electrolysis of molten lead bromide, Pb²⁺ moves to the negative cathode and is reduced (Pb²⁺ + 2e⁻ → Pb); Br⁻ moves to the positive anode and is oxidised (2Br⁻ → Br₂ + 2e⁻). Molten binary salts produce metal at cathode and non-metal at anode. In aqueous NaCl with inert electrodes, competing H⁺/water and Cl⁻/OH⁻ produce H₂ at the cathode and chlorine at the anode under specified concentrated brine conditions, leaving NaOH; in aqueous CuSO₄ with inert electrodes, Cu²⁺ is reduced to copper and water/OH⁻ is oxidised to oxygen. With copper electrodes in CuSO₄, the copper anode dissolves (Cu → Cu²⁺ + 2e⁻) and copper deposits at the cathode. Set up a low-voltage d.c. supply, inert or copper electrodes in electrolyte, identify gas/metal products safely, and record observations. Ionic solids do not conduct because ions are fixed; molten/aqueous electrolytes conduct by ion movement, not electron flow through the liquid. In C3.4d the general method of predicting ions/products applies to both tiers; the specifically bold extension “half equations” is Higher only.
What to do in an assessment
- Write half-equations with electrons on the correct side: oxidation releases electrons, reduction consumes them; balance atoms and charge.
- For electrolysis, identify the ions, direction of migration and electrode reaction; balance the half-equation for atoms and charge, then combine products into an overall reaction if required.
In plain language
Acids, bases and redox reactions can be represented with ions and equations; electrolysis uses mobile ions to produce substances at electrodes.
Self-check
What happens at the cathode during electrolysis of molten lead bromide?
Show the answer
Pb²⁺ ions gain two electrons and form lead: Pb²⁺ + 2e⁻ → Pb.
- Core
Scope: Core · Topic C4.1, C4.1a–f
Open revision guidance
What to know
- Group 1 elements are soft metals; their atoms have one outer electron and react increasingly vigorously down the group because that electron is farther from the nucleus and more easily lost. With water, an alkali metal produces hydrogen and a metal hydroxide.
- Group 7 elements are diatomic non-metals. Their colour darkens and melting/boiling points rise down the group, while their tendency to gain an electron falls. A more reactive halogen displaces a less reactive halide from solution: chlorine can oxidise bromide to bromine, but bromine cannot displace chloride.
- Group 0 atoms have full outer shells and are very unreactive; boiling points rise down the group as atoms get larger. Transition metals typically have higher densities and melting points than Group 1 metals, form coloured compounds and ions with different charges, and some act as catalysts.
- A metal higher in the reactivity series loses electrons more readily. Evidence from reactions with water, dilute acid or another metal's salt can establish an order; describe the observation, write the reaction where possible and infer the relative reactivity.
- C4.1a–b: Group 1 alkali metals are soft, low-density metals; they react with water to give an alkaline hydroxide and hydrogen, with vigour increasing down the group. Group 7 halogens are coloured diatomic non-metals; colour darkens and melting/boiling points rise down the group, while reactivity falls. Group 0 noble gases are monatomic, colourless gases with very low chemical reactivity and boiling points that rise down the group. Explain these observed trends from outer-shell structure: Group 1 loses its one outer electron more easily down the group; Group 7 gains an electron less readily; Group 0 has a full outer shell. Use provided trend data rather than inventing unmeasured values.
- C4.1c–f (both tiers): transition metals such as iron, copper and nickel are typically dense, have high melting points, form coloured compounds and ions of different charges, and some catalyse reactions (iron in Haber synthesis; nickel in hydrogenation). Predict possible reaction from group/period and known patterns. Metals react by losing electrons to form positive ions: potassium + water → potassium hydroxide + hydrogen; magnesium + dilute acid → magnesium salt + hydrogen. Compare rate/evidence, not just the final appearance. In displacement tests a more reactive metal displaces a less reactive metal ion (Fe + CuSO₄ → FeSO₄ + Cu); repeat across metals and infer an order from actual reaction/no-reaction results.
What to do in an assessment
- Predict a group trend by separating observation from explanation: state the trend, identify the outer-shell/electron change, then connect it to the reaction.
- For a displacement, identify which metal/halogen is oxidised and which ion is reduced, then write a balanced equation and use the result to order reactivity.
In plain language
Periodic position gives clues about electron behaviour. Those clues, checked against reaction evidence, help predict which element reacts or displaces another.
Self-check
Chlorine is added to potassium bromide solution. What evidence and reaction support the prediction?
Show the answer
Orange/brown bromine forms as chlorine displaces bromide: Cl₂ + 2KBr → 2KCl + Br₂. Chlorine is the more reactive halogen.
- Core
Scope: Core · Topic C4.2, C4.2a–g
Open revision guidance
What to know
- Use a lit splint for hydrogen (squeaky pop); a glowing splint for oxygen (relights); bubble a gas through limewater for carbon dioxide (turns milky); damp indicator paper is bleached by chlorine.
- Add aqueous sodium hydroxide to test selected cations: calcium gives a white precipitate, copper(II) a blue precipitate, iron(II) a green precipitate, iron(III) a brown precipitate and zinc a white precipitate that dissolves in excess alkali. Acid plus carbonate releases carbon dioxide; acidified silver nitrate gives white, cream or yellow precipitates with chloride, bromide or iodide respectively; acidified barium chloride/nitrate gives a white sulfate precipitate.
- A clean wire loop dipped in a sample and placed in a blue Bunsen flame gives characteristic colours: lithium crimson, sodium yellow, potassium lilac, calcium orange-red/brick red and copper blue-green. Use a known comparison and avoid cross-contamination when inferring an ion.
- Instrumental methods can offer greater sensitivity, accuracy and speed than simple chemical tests. In a mass spectrum, compare the supplied pattern with a reference chart; interpret only what the axes, peaks and reference data support.
- C4.2a–g (all Foundation and Higher): use a lit splint for hydrogen (squeaky pop), a glowing splint for oxygen (relights), bubble CO₂ through limewater (milky CaCO₃ forms), and hold damp blue litmus/indicator paper in chlorine (bleached). For cation tests, add aqueous NaOH dropwise: Ca²⁺ white precipitate; Cu²⁺ blue; Fe²⁺ green; Fe³⁺ brown; Zn²⁺ white, soluble in excess NaOH. Add dilute acid to carbonate to release effervescence/CO₂. Add dilute nitric acid followed by silver nitrate: chloride white AgCl, bromide cream AgBr, iodide yellow AgI; acidify with dilute hydrochloric or nitric acid as specified, then add barium chloride or barium nitrate for white BaSO₄ from sulfate. Use clean separate samples, droppers and apparatus to avoid false positives. The source-listed C4.2b anion tests do not include nitrate; do not make an additional nitrate test a required J248 outcome.
- C4.2c–e: clean a nichrome/platinum loop by dipping in hydrochloric acid and heating until no flame colour remains; moisten a fresh sample, place in a non-luminous blue Bunsen flame and compare against a known standard—Li⁺ crimson, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red/brick red and Cu²⁺ blue-green. Record the observed colour before identifying the ion; sodium contamination can mask other colours.
- C4.2f–g: instrumental tests can be faster, more sensitive and more accurate/reproducible than visual spot tests, and can detect smaller amounts, but need equipment, calibration and interpretation. Read a supplied mass-spectrum chart's axes and units, compare the unknown peak pattern with a reference collected in the same form, match the stated isotope/ion peaks and justify the identification; do not claim more than the reference supports.
What to do in an assessment
- Report the reagent and procedure, the actual observation, then the species it supports; do not treat a colourless solution or a single ambiguous precipitate as proof. Mass is the amount of matter; density is mass per unit volume. Forming a precipitate does not create extra mass or make the complete products heavier than all the starting materials: in a closed system, the total mass of precipitate plus remaining solution equals the total initial mass of the mixed solutions. A change in density or the appearance of a solid is not an increase in total mass. If a gas escapes, the measured open apparatus may lose mass even though atoms and total mass are conserved across the complete reaction system; dissolution makes a mixture rather than a pure new substance.
- For an instrumental-data question, read axes and units, match the relevant peak/pattern to the supplied standard and state any limitation in the inference.
In plain language
A chemical test is a small experiment: use a selective reagent or instrument, observe carefully, and draw only the conclusion supported by that result.
Self-check
An unknown solution gives a green precipitate with sodium hydroxide. Which ion is indicated by this test?
Show the answer
Iron(II), Fe²⁺, is indicated; identify the ion from the green precipitate observation.
- Core
Scope: Core · Topic C5.1, C5.1a–k
Open revision guidance
What to know
- A mole measures amount of substance, not mass; one mole contains the Avogadro constant (about 6.02 × 10²³) particles. Convert mass to amount with n = m/Mr, using a balanced equation to obtain reactant/product mole ratios.
- Molar concentration c in mol/dm³ is amount n divided by solution volume V in dm³; therefore n = cV. Convert cm³ to dm³ by dividing by 1000. Mass concentration in g/dm³ is solute mass divided by solution volume; convert it to mol/dm³ using Mr.
- In an acid–alkali titration, rinse/fill a burette with one solution, use a pipette for a measured aliquot, add a suitable indicator, approach the end point dropwise while swirling, and record concordant titres. The balanced equation's coefficients connect the known concentration/volume to the amount of the other reactant.
- At room temperature and pressure, use the specification's molar gas volume of 24 dm³ per mole: gas volume = moles × 24 dm³, with consistent volume units. Use the balanced equation and limiting reactant to calculate theoretical product mass. Percentage yield = actual mass ÷ theoretical mass × 100%. Atom economy = Mr of desired product ÷ total Mr of products × 100%; a higher value means less reactant mass becomes unwanted product.
- C5.1a and c–f are Higher-only; C5.1b (titration technique) is both tiers. Make a standard solution by accurately weighing solute, dissolving it in a beaker, quantitatively transferring through a funnel to a volumetric flask, rinsing the beaker/funnel into the flask, making up to the calibration line at eye level, stopper and invert to mix. Concentration c = n/V = m/(MrV), with V in dm³; mass concentration (g/dm³) = solute mass (g)/solution volume (dm³), so m = cV (also the shared C3.1j relation). Convert cm³ to dm³ by ÷1000. In a titration rinse the burette with titrant and pipette with analyte, use a pipette filler to deliver a fixed aliquot to a conical flask, add a few drops of suitable indicator, record initial/final burette readings, swirl and add titrant rapidly then dropwise at the end-point, repeat until concordant titres and average concordant results. For C5.1c Higher-tier calculations, use the balanced equation: for HCl + NaOH → NaCl + H₂O the mole ratio is 1:1; n=cV, then use coefficient ratios to find unknown concentration. C5.1d–e Higher tier: at RTP, 1 mol gas occupies 24 dm³ (24,000 cm³); use V=n×24 dm³ or n=V/24 and scale by equation coefficients. C5.1f Higher tier: use c=m/(MrV) and rearrangements with consistent units.
- C5.1g–j are assessed at both tiers; C5.1k is Higher only. For a theoretical mass, convert known reactant mass to moles, use the balanced equation's ratio to find product moles, then multiply by product Mr. Identify the limiting reactant by comparing available moles divided by coefficients. Percentage yield = actual product mass / theoretical product mass ×100%; explain low yield with reversible reaction, incomplete reaction, side reactions or product loss during transfer/purification. Atom economy = (Mr of desired product / sum Mr of all products)×100%; include coefficients in both totals. To select a pathway, compare provided yield, atom economy, rate, equilibrium, usefulness/value of by-products, cost, energy and waste; calculate when data allow, and justify a choice with evidence rather than one metric.
What to do in an assessment
- Write the balanced equation, convert each datum to moles, use the coefficient ratio, then convert to the requested mass, concentration or gas volume. Show unit conversions and sensible significant figures.
- For yield/atom economy, identify the desired product and the relevant theoretical amount from the balanced equation before substituting; explain a pathway choice using waste, yield and practical constraints.
In plain language
Balanced equations give the mole ratios. Convert the measured mass, concentration or volume into moles, use that ratio, then convert to the quantity the question asks for.
Self-check
What is the concentration in g/dm³ of 5.0 g solute dissolved to make 0.50 dm³ solution?
Show the answer
Mass concentration = mass ÷ solution volume = 5.0 g ÷ 0.50 dm³ = 10 g/dm³. This is a Foundation-shared concentration calculation.
- Core
Scope: Core · Topics C5.2–C5.3, C5.2a–i and C5.3a–c
Open revision guidance
What to know
- Rate is change in amount of reactant used or product made per unit time. Measure a suitable signal—gas volume, mass, colour or concentration—at known time intervals; the initial gradient is the initial rate. A fair comparison controls quantities such as temperature, volume and surface area while changing one variable.
- Collision theory explains rate: particles must collide, and only collisions with sufficient energy and suitable orientation react. Increasing temperature raises particle kinetic energy and the fraction above activation energy; increasing concentration or gas pressure increases collision frequency; increasing solid surface area exposes more particles.
- A catalyst changes pathway and lowers activation energy, increasing rate without being used up overall. Reaction profiles compare activation-energy barriers; enzymes are biological catalysts whose activity depends on conditions and can fall if structure is disrupted.
- A reversible reaction can proceed in both directions. In a closed system at dynamic equilibrium the forward and reverse rates are equal, so macroscopic concentrations remain constant. A changed temperature, pressure (for gaseous equilibria) or concentration shifts the equilibrium position; explain the shift by predicting how it opposes the imposed change.
- C5.2a–e: measure a rate by collecting gas with a gas syringe, measuring mass loss on a balance, recording colour/pH/concentration at set times, or timing a visible endpoint such as a disappearing cross; keep total volumes, starting amounts and temperature controlled while changing one independent variable. For marble + acid measure CO₂ volume over time or time to obscure a cross; for magnesium + acid measure H₂ volume. Rate = change/time, and 1/t is proportional to rate for the same endpoint. Plot labelled axes with units; the gradient is rate (draw a tangent for instantaneous rate), not reaction order. Higher concentration/pressure gives more frequent particle collisions; higher temperature gives more frequent and more energetic collisions, so a greater fraction exceed activation energy; smaller solid pieces increase exposed surface area/volume and collision opportunities. Compare trends with a control and repeat.
- C5.2f–i: a catalyst increases rate by providing an alternative pathway of lower activation energy and is chemically unchanged overall. Identify catalysts by comparing equal reactions with/without the candidate and measuring rate; examples in the specification include MnO₂ catalysing hydrogen peroxide decomposition and copper powder catalysing zinc + sulfuric acid. Enzymes are biological catalysts; temperature or pH outside their working range changes the active-site shape and reduces activity. On a reaction profile show a lower peak for catalysed and uncatalysed routes but unchanged reactant/product energy difference.
- C5.3a–b are assessed at both tiers; C5.3c is Higher-only. Some reactions reverse when conditions change. A dynamic equilibrium exists only in a closed system when forward and reverse rates are equal; concentrations are constant, not necessarily equal, and both reactions continue. Apply Le Chatelier: increased reactant concentration shifts toward products; increased product concentration shifts toward reactants; increasing pressure favours the side with fewer gas molecules; increasing temperature favours the endothermic direction. Decreasing a condition reverses the corresponding shift. For industrial yield, state both equilibrium direction and rate compromise; changing a catalyst changes how quickly equilibrium is reached, not its position.
What to do in an assessment
- Calculate mean rate as amount changed ÷ time; take a tangent for an instantaneous rate if required, state units and compare gradients rather than only final amounts.
- For an equilibrium prediction, name the imposed change, state the direction that reduces its effect and identify which reactant/product amount increases. Separate a rate change from a final equilibrium-yield change.
In plain language
Reactions speed up when successful collisions happen more often. At equilibrium, both directions continue but happen at equal rates.
Self-check
At equilibrium, a catalyst is added. What changes?
Show the answer
The forward and reverse rates both increase, so equilibrium is reached faster; the equilibrium composition does not change.
- Core
Scope: Core · Topic C6.1, C6.1a–s
Open revision guidance
What to know
- Metal extraction depends on reactivity: carbon can reduce oxides of less-reactive metals, while very reactive metals require electrolysis. Biological extraction (bioleaching or phytoextraction) may use less energy or lower-grade ores but can be slower and requires subsequent separation.
- Industrial conditions trade off rate, yield, energy cost, safety and environmental impact. For the Haber process, nitrogen and hydrogen form ammonia reversibly; iron catalyst, elevated temperature and pressure are selected as a practical compromise, and unreacted gases are recycled. Fertiliser manufacture combines ammonia with acids and supplies nitrogen, phosphorus or potassium nutrients.
- Life-cycle assessment compares raw-material extraction, manufacture, use and disposal; include energy, emissions, water, transport, durability and recycling limits. A recycling choice is justified by data and practical constraints, not by assuming it is always impact-free.
- Alloys alter pure-metal properties because different-sized atoms disrupt regular layers. Corrosion is chemical deterioration; coatings or sacrificial protection can restrict contact with water/oxygen or oxidise preferentially. Glass, clay, alloys and composites are selected for properties matched to their application.
- C6.1a–b are both-tier; C6.1c is Higher-only. Carbon is below iron in the reactivity series, so carbon reduces iron oxide in a blast furnace; copper can be extracted by heating copper oxide with carbon (2CuO + C → 2Cu + CO₂). A non-ferrous metal such as copper can also be extracted from a suitable ore using carbon where reactivity permits. More reactive metals (for example aluminium) cannot be reduced by carbon; electrolysis of molten aluminium oxide dissolved in cryolite separates Al at the cathode and oxygen at the anode, consumes carbon anodes, and uses substantial electrical energy. Higher-tier C6.1c biological extraction: bioleaching uses bacteria to produce soluble metal compounds from low-grade ore; phytoextraction grows plants that absorb metal ions, then harvests/incinerates biomass and recovers metal from ash. Compare lower-grade resources/energy and reduced mine waste against slow rates, land/time, dilute product and later purification.
- C6.1d–f are Higher-only; C6.1g–j are both tiers. In the exothermic Haber equilibrium N₂(g)+3H₂(g) ⇌ 2NH₃(g), higher pressure favours the side with fewer gas molecules and lower temperature favours ammonia, but low temperature is slow; industry uses about 450°C, high pressure (~200 atm), iron catalyst and recycles unreacted gases. Contact process: 2SO₂(g)+O₂(g) ⇌ 2SO₃(g), exothermic; about 450°C, low pressure (around 1–2 atm) and vanadium(V) oxide catalyst balance rate, yield, cost and safety. Read supplied rate-versus-condition graphs by identifying axes, units, maxima/slopes and comparing equilibrium yield separately from rate. Explain operating conditions through availability/cost of feedstocks and energy, equilibrium, rate, safety and plant constraints. Haber ammonia enables large-scale fertiliser and food production. N, P and K compounds supply nutrients for growth; compare making ammonium sulfate or potassium sulfate in industry with a small laboratory neutralisation/titration. Industrial fertiliser manufacture integrates ammonia from nitrogen and hydrogen feedstocks with nitric acid (itself made by oxidising ammonia) to produce ammonium nitrate, and ammonia with sulfuric acid to produce ammonium sulfate; e.g. NH₃ + HNO₃ → NH₄NO₃ and 2NH₃ + H₂SO₄ → (NH₄)₂SO₄. For laboratory potassium sulfate, neutralise H₂SO₄ with KOH (2KOH + H₂SO₄ → K₂SO₄ + 2H₂O); titrate to endpoint, repeat using indicator to find the exact neutralisation volume, then make a fresh stoichiometric batch without indicator, evaporate/concentrate, cool, filter, wash and dry pure crystals.
- C6.1k–n: a life-cycle assessment follows raw-material extraction and processing, manufacture, use, transport and end-of-life disposal; compare resource/energy use and environmental impacts at each stage. Interpret supplied tables/graphs quantitatively, use consistent functional units and boundaries, note missing data and avoid selecting a material from one stage only. Recycling can turn recovered glass, metals or polymers into a new product where collection, sorting, purity, energy and market make it viable; evaluate contamination, transport, quality loss, energy and demand rather than assuming recycling always saves impact.
- C6.1o–s are assessed at both tiers: steel is mainly iron with carbon (and sometimes other alloying elements); brass is copper + zinc, bronze copper + tin, solder commonly tin-based alloys, and duralumin is aluminium alloyed mainly with copper plus other elements. Link composition to strength, hardness, corrosion resistance, density and use. Corrosion is oxidation of a metal; iron rusting requires both water and oxygen. Paint/oil/plastic/metal coating creates a physical barrier; galvanising coats iron with zinc; sacrificial protection attaches a more reactive metal such as zinc or magnesium, which oxidises instead. Compare numerical data for glass and clay ceramics (e.g. density, strength, heat resistance) and use actual values/units to justify suitable uses; ceramics are hard/brittle and heat-resistant, polymers are light and mouldable, composites combine phases to improve properties, and metals conduct and can be shaped. Explain each material selection through measured properties and application requirements.
What to do in an assessment
- Read yield/rate graphs by comparing axes, units and values; state the numerical advantage and the cost or environmental trade-off.
- Structure an evaluation as criteria → evidence → consequence → justified choice, including uncertainty or missing life-cycle stages.
In plain language
Making useful chemicals and materials involves trade-offs. A good evaluation uses evidence about output, energy, cost, safety and environmental consequences.
Self-check
Why is the Haber process not run at the lowest possible temperature if that gives the greatest equilibrium yield?
Show the answer
Low temperature slows the rate. Industry chooses a compromise temperature that produces ammonia at a useful rate while maintaining a reasonable yield.
- Core
Scope: Core · Topics C6.2–C6.3, C6.2a–q and C6.3a–g
Open revision guidance
What to know
- A homologous series has a common functional group, general formula and similar chemical reactions, with gradual physical-property trends. Alkanes are saturated hydrocarbons; alkenes contain a C=C bond. Crude oil is a finite mixture separated by fractional distillation because fractions have different boiling ranges; cracking converts longer-chain hydrocarbons into shorter alkanes and useful alkenes.
- Addition polymerisation opens alkene double bonds to form a long saturated chain; the repeat unit must preserve the correct atoms and bonds. Condensation polymers form from monomers with suitable functional groups, with a small molecule eliminated. Evaluate plastics, fuels and alternative feedstocks by useful properties, disposal, emissions and resource use.
- Fuel cells convert chemical energy directly into electrical energy; a hydrogen–oxygen cell produces water overall. Compare advantages with production, storage, infrastructure and life-cycle emissions rather than calling hydrogen automatically carbon-free.
- Atmospheric composition changed through geological processes and photosynthesis. Greenhouse gases absorb outgoing infrared radiation; evidence for climate change includes multiple datasets and possible human causes. Evaluate trends, correlation, timescale, uncertainty and alternative evidence. Atmospheric change can affect climate, oceans and ecosystems.
- C6.2a–d and g–i are assessed at both tiers. C6.2e–f (condensation polymerisation principles and practical method) are Higher only. Recognise alkane (CₙH₂ₙ₊₂), alkene (CₙH₂ₙ with C=C), alcohol (–OH) and carboxylic acid (–COOH) functional groups; members share a general formula and characteristic reactions. Name/draw the first four straight-chain alkanes: methane CH₄, ethane CH₃–CH₃, propane CH₃–CH₂–CH₃, butane CH₃–CH₂–CH₂–CH₃; alkenes: ethene CH₂=CH₂, propene CH₃CH=CH₂, but-1-ene CH₂=CHCH₂CH₃, pent-1-ene CH₂=CHCH₂CH₂CH₃; alcohols: methanol CH₃OH, ethanol CH₃CH₂OH, propan-1-ol CH₃CH₂CH₂OH, butan-1-ol CH₃CH₂CH₂CH₂OH; carboxylic acids: methanoic HCOOH, ethanoic CH₃COOH, propanoic CH₃CH₂COOH, butanoic CH₃CH₂CH₂COOH. These are condensed formulae: draw fully displayed formulae showing every atom and covalent bond; use supplied unfamiliar members to continue each homologous series. Complete combustion forms CO₂ + H₂O; incomplete combustion can form CO/soot. Alkene double bonds add bromine (orange to colourless) or hydrogen (hydrogenation, commonly nickel catalyst) across C=C. Oxidise a primary alcohol to a carboxylic acid with acidified potassium manganate(VII), warming/reflux as appropriate. Addition polymerisation opens C=C without loss of atoms; repeat units join with continuation bonds and brackets/subscript n. To reverse-deduce an alkene monomer, identify the repeat-unit backbone, change one C–C single bond to C=C and restore terminal bonds while preserving each substituent. For the Higher-only C6.2e–f, condensation polymerisation joins bifunctional monomers into polyester/polyamide and eliminates a small molecule (commonly water or HCl); count functional groups and repeat units in block diagrams. Make nylon at a liquid–liquid interface by carefully layering a diamine solution and diacid chloride solution, lift and draw out the forming polymer film with forceps, wash and record; use eye protection and local risk controls. DNA is a polymer of four nucleotide monomer types named for their bases—adenine, cytosine, guanine and thymine; each nucleotide also has a sugar and phosphate group. Starch/cellulose are sugar-based polymers and proteins are amino-acid polymers. General reactions follow functional groups rather than carbon-chain length.
- C6.2j–o: crude oil is a finite mixture and petrochemical feedstock. Fractional distillation heats it to vapour and sends it into a column with a temperature gradient; vapours condense at different heights/boiling ranges. Know refinery gases, petrol/gasoline, kerosene, diesel, fuel oil and bitumen as fractions; smaller molecules have lower boiling points, lower viscosity and are more volatile/flammable because intermolecular forces are weaker. Fractions are largely alkane mixtures with CₙH₂ₙ₊₂ formula. Explain dependence of modern transport, heating, electricity and manufacture on hydrocarbons, alongside resource limits. Crack surplus long-chain hydrocarbons by vaporising and passing vapour over a hot catalyst or through high temperature; thermal cracking uses high temperature/pressure, catalytic cracking uses a zeolite catalyst at lower temperature. This makes shorter alkanes and valuable alkenes (e.g. C₁₀H₂₂ → C₈H₁₈ + C₂H₄); choose conditions/products for demand and explain feedstock use.
- C6.2p–q are assessed at both tiers: a chemical cell separates two electrode reactions and produces a potential difference while reactants remain; it falls when reactants are used up. In an acidic hydrogen/oxygen fuel cell, H₂ → 2H⁺ + 2e⁻ at the anode and ½O₂ + 2H⁺ + 2e⁻ → H₂O at the cathode, giving overall 2H₂ + O₂ → 2H₂O. It can operate continuously while fuel/oxygen are supplied and has water at point of use, but hydrogen production, compression/storage, transport, catalyst/materials, cost, safety and electricity source determine whole-life impact. Compare with other fuel cells and combustion for the specified use, using efficiency, emissions, refuelling, portability, range and infrastructure evidence rather than a blanket claim.
- C6.3a–g: evidence from volcanic gases and early rock records supports an initially oxygen-poor atmosphere formed by volcanic outgassing, mainly water vapour, carbon dioxide and nitrogen; water vapour condensed as Earth cooled, oceans formed, and carbon dioxide dissolved/reacted or became locked in carbonate/sedimentary rocks. Photosynthetic algae/plants released oxygen over geological time, producing an oxygen-rich atmosphere; modern air is about 78% nitrogen, 21% oxygen and small amounts of argon, CO₂ and other gases. Interpret timeline/composition charts with units and scale. Incoming short-wave solar radiation passes through the atmosphere and warms Earth; the surface emits infrared radiation, greenhouse gases absorb and re-emit some IR, warming the lower atmosphere. Human fossil-fuel combustion correlates with increased atmospheric CO₂; evaluate multiple time series, independent measurements, timescales, natural variation and uncertainty, and do not confuse greenhouse warming with ozone depletion or assert correlation alone proves causation. More CO₂/CH₄ can raise mean temperature and alter rainfall, sea level, ice, oceans and ecosystems; mitigation includes reducing fossil use, energy efficiency, renewables, reforestation and carbon capture, weighing scale, risk and environmental consequences. CO from incomplete combustion is poisonous; SO₂ from sulfur impurities causes respiratory harm/acid deposition; NOₓ from high-temperature engines contributes to smog/acid deposition; particulates from combustion harm lungs and reduce visibility. Make potable water from freshwater by sedimentation/filtration and sterilisation (for example chlorine/UV); ground water may require filtration/treatment; salt water requires distillation or reverse osmosis, with energy, cost and brine disposal considered.
What to do in an assessment
- Identify a functional group from a displayed or structural formula; write a balanced reaction and show the repeating unit with continuation bonds for polymer questions.
- For climate or fuel claims, quote data, distinguish observation from causal explanation, assess timescale/uncertainty and state a proportionate conclusion.
In plain language
Organic molecules can be rearranged into fuels and materials. Their uses bring environmental consequences that should be judged with evidence.
Self-check
What structural feature makes an alkene unsaturated, and what happens to it during addition polymerisation?
Show the answer
An alkene has a carbon–carbon double bond. The double bond opens and links monomers into a carbon-chain polymer.
- Core
Scope: Core · §5e–5f; Topic C7 PAG1–PAG8; M1–M5 and WS1/WS2 outcomes
Open revision guidance
What to know
- C7 requires at least eight practical activities and opportunities to use every listed apparatus/technique; the eight PAGs group practical work as reactivity trends, electrolysis, separation, distillation, identifying species, titration, salt production and rate measurement. Learners keep a contemporaneous record of activities, and the centre submits the practical-science statement/declaration; written papers also assess understanding of apparatus, method, safety and data.
- The chemistry mathematical scope includes decimal and standard form, ratios/fractions/percentages and estimates; significant figures, means, tables/charts and order of magnitude; symbols, rearranging/substitution; interpreting and plotting graphs, gradients and tangents; and 2D/3D representations. Apply each skill in chemical contexts, with units and justified precision.
- WS1 asks learners to use models, hypotheses, evidence, data and scientific language; plan methods; select apparatus; interpret results; assess accuracy, precision, repeatability, reproducibility, error and risk; and make evidence-based conclusions. WS2 covers practical execution, accurate records, data presentation and communicating rationale.
- Practical work is not a separate learner-marked practical endorsement or NEA component of the written grade: practical knowledge is assessed in the written papers. Separately, the centre must provide the required practical opportunities and submit the Practical Science Statement within the NEA Centre Declaration Form; this is a centre declaration, not a learner NEA assessment.
- PAG1 Reactivity trends: compare controlled displacement reactions between halogens/halides (for example chlorine water added separately to KBr and KI); handle gases, liquids and solids safely, use clean labelled wells/test tubes, record colour change and infer order from evidence. PAG2 Electrolysis: set up an electrolyte, d.c. supply and labelled inert electrodes; compare products for aqueous NaCl/CuSO₄, collect/test gases and note electrode mass/deposition, with electrical and chemical safety. PAG3 Separation: separate unknown ink dyes using pencil baseline, spotted paper/TLC, solvent below baseline, covered chamber, solvent-front mark and locating agent; identify components from chromatogram/Rf. PAG4 Distillation: use a Bunsen burner and water bath or electric heater safely, condenser and receiver to distil a mixture; record temperatures, time, mass/volume and fractions. PAG5 Identification: use clean samples/reagents for gas, flame, cation precipitate and anion tests, record positive/negative observations, and heat safely. PAG6 Titration: accurately measure strong acid/alkali using pipette, burette, indicator or pH probe; record initial/final readings, obtain concordant titres and calculate concentration. PAG7 Salt production: prepare a pure dry soluble salt by titrating acid and alkali to find exact volume, repeat without indicator, evaporate/concentrate, cool, filter, wash and dry crystals; for an insoluble salt use precipitation then filtration/washing. PAG8 Rate measurement: change one factor (surface area or concentration) for acid/carbonate reaction, measure gas with a gas syringe or colour change/time, repeat, calculate rate and compare graph gradients. Use suitable PPE, small scale, risk assessment, safe heating and gas handling; record diagrams, quantities, units, hazards and disposal.
- Apparatus/technique coverage: measure mass on a balance, time with a stopwatch, temperature with a thermometer/probe, liquid volume with a measuring cylinder/pipette/burette and gas volume with a gas syringe; choose resolution suited to the question. Safely use Bunsen/electric heating and water baths; conduct and monitor reactions and pH; purify/separate by evaporation, filtration, crystallisation, chromatography and distillation; collect observations and measure reaction rate by gas or colour change. Electrochemical cells and qualitative analysis include gas tests, flame tests, precipitation and measurement of strong acid/alkali concentration (DfE technique 8 in the specification table; distinct from PAG8). Each numbered technique 1, 2, 3, 4 and 8 is deliberately used across the PAGs.
- Chemistry mathematical learning outcomes CM1.1i, CM1.2ii, CM2.2ii, CM2.3i and CM6.2i require translating 3D chemical structures (diamond, graphite, molecules) into 2D drawings and reconstructing 3D from conventions; CM1.2i and CM2.3ii compare atomic/nanoparticle scale with familiar physical lengths; CM1.2ii, CM2.2i and CM2.3iii estimate atom/nanoparticle size; CM2.3iv orders/calculates standard-form nanoparticle sizes; CM2.3v compares ratios and surface-area:volume; CM2.3vi calculates cube areas/volumes. Show unit conversion and powers-of-ten explicitly.
- CM2.1i, CM3.3i, CM5.1iv, CM5.2i, CM5.3i and CM6.1i apply arithmetic, ratios, percentages and multi-step computation in chemical quantities/rates; CM2.1ii, CM3.1iii and CM5.1ii select significant figures; CM2.1iii and CM5.1vi rearrange an equation; CM2.1iv and CM3.1i/CM4.1i/CM3.4i use ratios to obtain empirical formulae and balance equations; CM3.1ii and CM5.1i are Higher-only source outcomes and use Avogadro's constant in standard form; CM3.1iv and CM5.1iii are shared outcomes with a bold Higher-only extension particularly from mass to moles; CM3.2ii calculates energy changes; CM3.2i and CM5.2ii/CM5.3ii/CM6.1ii read or construct graphs; CM5.2iii/CM5.3iii/CM6.1iii determine gradients; CM5.2iv/CM5.3iv/CM6.1iv compare proportional effects; CM4.2i interprets spectroscopy charts; CM2.2iii translates diagrammatic and numerical representations; CM6.3i extracts/interprets chart, graph and table data; CM6.3ii uses order of magnitude to judge significance.
- CM5.1v calculates reaction yield and atom economy; CM5.1vi rearranges equations in calculation contexts; CM2.1iv/CM3.1i/CM4.1i/CM3.4i link atom ratio to formula/equation rather than treating a coefficient as a subscript. CM3.2i reads reaction-profile energy levels and CM3.2ii calculates the difference. For every calculation show the governing equation, unit conversion, substitution, answer unit and appropriate significant figures; estimate first and check the result is chemically plausible.
- M1a–d: use decimal notation in concentrations/measurements, standard form for Avogadro/nanoscale, fractions/ratios/percentages for empirical formula/yield, and estimate a result before multistep calculation. M2a–c,h: choose justified significant figures, calculate an arithmetic mean from repeats, construct/read frequency tables, diagrams, bar charts and histograms, and compare orders of magnitude. M3a–c: interpret =, <, ≪, ≫, >, ∝ and ≈; rearrange equations (e.g. n=m/Mr, c=n/V); substitute values with compatible units. M4a–e: translate graph and numeric data, recognise y=mx+c, plot independent/dependent experimental data with labels/units and suitable scales, calculate line slope/intercept, and draw a tangent to a curve to estimate instantaneous rate. M5b–c: represent 3D lattices/molecules in 2D and infer 3D structure; calculate cube surface area and volume for nano comparisons.
- WS1.1a–i: explain that new technology/new evidence can revise scientific models; use representational, spatial, descriptive, computational and mathematical models to predict/explain; state science's benefits and limits/unanswered questions; discuss ethical issues; explain everyday/technological applications; evaluate personal, social, economic and environmental implications; choose from evidence/arguments; assess practical and societal risk using likelihood, consequence and perception; and explain why peer review and audience-appropriate communication matter.
- WS1.2a–e: turn a collision/particle explanation into a testable hypothesis; plan controlled experiments/procedures to observe, produce/characterise substances or test hypotheses; select suitable apparatus/reagents and measurement resolution; sample representative material when a sample could vary; evaluate a method and give a feasible improvement or follow-up with the reason it improves validity, precision or range.
- WS1.3a–i: present observations in prose, tables, diagrams and graphs; translate between words, symbols, numbers and diagrams; calculate mean, mode and median where appropriate; display result spread and estimate uncertainty; interpret patterns, anomalies and trends, infer and conclude; relate data to hypothesis; distinguish accuracy from precision and repeatability from reproducibility; identify random scatter versus systematic bias; and communicate rationale, method, results and evidence-linked conclusion in diagrammatic, graphical, numerical or symbolic form.
- WS1.4a–f and WS2a–d: use correct chemical vocabulary/definitions and IUPAC names; identify how quantities such as mass, pH, volume and rate are measured; use SI/base/derived units and chemical nomenclature; apply tera, giga, mega, kilo, deci, centi, milli, micro and nano as powers of ten; interconvert units; round calculations appropriately. In practical work manipulate apparatus accurately and safely following written instructions, make and contemporaneously record observations/readings, present them descriptively/tabularly/diagrammatically/graphically, then communicate the scientific rationale, method, results and reasoned conclusion in a written or electronic report/presentation.
- C7 minimum: centres provide reasonable opportunities for learners to complete at least eight practical activities covering all five chemistry-applicable apparatus/technique categories; each learner's contemporaneous record documents the practical work undertaken and the knowledge, skills and understanding derived from it. The centre submits the Practical Science Statement within the NEA Centre Declaration Form. This is a centre compliance declaration, not a learner NEA assessment or a separate practical endorsement. The written papers assess the 15% practical weighting, so revise equipment choice, method order, variables, risks, expected observations, processing and evaluation—not only the activity name.
What to do in an assessment
- For an investigation, identify independent/dependent/control variables, justify the apparatus and range, repeat readings, record units, plot a labelled graph and evaluate outliers, uncertainty and improvements.
- Choose a safe, feasible method; distinguish random scatter from a systematic shift, and make conclusions no stronger than the data support.
In plain language
You must understand how an experiment works as well as the chemistry result: plan it, measure carefully, process the data and explain what the evidence can show.
Self-check
A rate graph becomes less steep with time. What does the gradient mean, and why might it fall?
Show the answer
The gradient is the rate at that time. It may fall because reactant concentration decreases, so successful collision frequency decreases.
Earlier topic checklist
These earlier topic ticks are separate from the reviewed points above and do not count towards that progress.
Chemistry topic list
This is a topic overview, not a complete specification checklist. Check the official course for all assessed content, options and your exam year.
The initial headings match Chemistry A (Gateway Science, J248), not Chemistry B. Other headings are broader than the official specification; confirm your course before using this overview.
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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