G3 Science Chemistry Revision Guide
K326 / K328 G3 Science Chemistry revision guide covering definitions, formulae, observations, tests, conditions and exam traps without Pure-only depth.
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Test yourself on every definition, formula, observation and condition before reading the answer. This guide follows K326/K328 G3 Science Chemistry and stops before Pure-only depth.
1. Important Definitions
| Term | Exam-safe meaning |
|---|---|
| Element | A substance made of only one type of atom. |
| Compound | A substance containing two or more different elements chemically combined. |
| Mixture | Two or more substances not chemically combined and separable by physical methods. |
| Isotopes | Atoms of the same element with the same proton number but different numbers of neutrons. |
| Ionic bond | The electrostatic attraction between oppositely charged ions. |
| Covalent bond | A shared pair of electrons between atoms. |
| Acid | A substance that produces hydrogen ions, H⁺, in aqueous solution. |
| Alkali | A soluble base that produces hydroxide ions, OH⁻, in aqueous solution. |
| Oxidation | Gain of oxygen, loss of hydrogen, loss of electrons or increase in oxidation state. |
| Reduction | Loss of oxygen, gain of hydrogen, gain of electrons or decrease in oxidation state. |
| Oxidising agent | A substance that oxidises another substance and is itself reduced. |
| Reducing agent | A substance that reduces another substance and is itself oxidised. |
| Rate of reaction | The change in amount of reactant used or product formed per unit time. |
| Hydrocarbon | A compound containing carbon and hydrogen only. |
2. Key Chemistry Maths
| Quantity | Relationship | Unit check |
|---|---|---|
| amount from mass | n = m/M | g and g mol⁻¹ |
| mass from amount | m = nM | answer in g |
| amount in solution | n = cV | convert V to dm³ |
| concentration | c = n/V | mol dm⁻³ |
| gas amount | n = V/Vₘ | use the stated molar gas volume and matching units |
For every stoichiometry question:
- balance the equation;
- convert the known quantity to moles;
- apply the coefficient ratio;
- convert the required moles to the requested quantity;
- include units and sensible precision.
Calculation boundary
Percentage composition, empirical and molecular formulae, limiting reagent, percentage yield and percentage purity are G3 Pure / O-Level extensions.
3. Experimental Chemistry, Particles and Bonding
| Need | Key method or explanation |
|---|---|
| Read liquid volume | Read the bottom of the meniscus at eye level for a colourless liquid. |
| Separate an insoluble solid | Filter; residue remains and filtrate passes through. |
| Obtain a dissolved solid | Evaporate the solvent. |
| Obtain the solvent | Use simple distillation and collect the condensate. |
| Separate miscible liquids | Use fractional distillation because their boiling points differ. |
| Analyse coloured solutes | Use paper chromatography; one spot suggests purity, several spots a mixture. |
| Judge purity | A pure substance has a sharp melting or boiling point. |
- Solid particles vibrate about fixed positions; liquid particles move past one another; gas particles move rapidly and randomly far apart.
- Melting, boiling and sublimation absorb energy. Freezing and condensation release energy.
- Ionic compounds have high melting points and conduct only when molten or aqueous, because their ions are then mobile.
- Simple covalent substances have low melting and boiling points because the attractions between molecules are weak.
- Metals conduct; alloys are harder because different-sized atoms make layer sliding more difficult.
4. Acids, Bases and Neutralisation
| Reaction | General products |
|---|---|
| acid + metal | salt + hydrogen |
| acid + base | salt + water |
| acid + carbonate | salt + water + carbon dioxide |
The net ionic equation for neutralisation is:
H + (aq) + OH-(aq) → H₂O(l)
| Indicator | Acid | Neutral | Alkali |
|---|---|---|---|
| litmus | red | no change | blue |
| methyl orange | red | orange | yellow |
| universal indicator | red/orange/yellow | green | blue/violet |
- Basic oxides react with acids; acidic oxides react with bases.
- Amphoteric oxides react with both acids and bases.
- Bases neutralise acidic soil; alkalis also react with ammonium salts to release ammonia.
- Strong/weak acid theory, salt-preparation methods, solubility rules and the Haber process are Pure Chemistry extensions.
5. Qualitative Analysis
Gas tests
| Gas | Test | Positive observation |
|---|---|---|
| hydrogen, H₂ | lighted splint | a squeaky pop |
| oxygen, O₂ | glowing splint | the splint relights |
| carbon dioxide, CO₂ | bubble through limewater | limewater turns milky; white precipitate forms |
| chlorine, Cl₂ | damp blue litmus paper | turns red, then bleaches white |
| ammonia, NH₃ | damp red litmus paper | turns blue |
| sulfur dioxide, SO₂ | aqueous acidified potassium manganate(VII) | purple solution turns colourless |
Anion tests
| Ion | Test | Positive observation |
|---|---|---|
| carbonate, CO₃²⁻ | add dilute acid; test the gas with limewater | effervescence; limewater turns milky |
| chloride, Cl⁻ | acidify with dilute nitric acid; add aqueous silver nitrate | white precipitate |
| iodide, I⁻ | acidify with dilute nitric acid; add aqueous silver nitrate | yellow precipitate |
| nitrate, NO₃- | add aqueous sodium hydroxide and aluminium foil; warm | ammonia is produced |
| sulfate, SO₄²⁻ | acidify with dilute nitric acid; add aqueous barium nitrate | white precipitate |
Cation precipitates
| Ion | With aqueous sodium hydroxide | With aqueous ammonia |
|---|---|---|
| Cu²⁺ | light blue, insoluble in excess | light blue, soluble in excess to form dark blue solution |
| Fe²⁺ | green, insoluble in excess | green, insoluble in excess |
| Fe³⁺ | red-brown, insoluble in excess | red-brown, insoluble in excess |
| Zn²⁺ | white, soluble in excess | white, soluble in excess |
| Al³⁺ | white, soluble in excess | white, insoluble in excess |
| Pb²⁺ | white, soluble in excess | white, insoluble in excess |
| Ca²⁺ | white, insoluble in excess | no precipitate |
| NH₄ + | warm with aqueous sodium hydroxide: ammonia forms | — |
6. Redox Chemistry
- Oxidation and reduction always occur together.
- Electron loss is oxidation; electron gain is reduction.
- An oxidising agent accepts electrons and is reduced.
- A reducing agent donates electrons and is oxidised.
- Oxidation state increases during oxidation and decreases during reduction.
- Balance atoms and total charge in ionic half-equations.
Electrolysis, electroplating, simple cells and hydrogen fuel cells belong to G3 Pure / O-Level, not K326/K328 G3 Science Chemistry.
7. Periodic Table and Reactivity
- Elements are arranged by increasing proton number.
- Elements in the same group have the same number of outer-shell electrons and similar chemical properties.
- Group 1 reactivity increases down the group.
- Group 17 reactivity decreases down the group; a more reactive halogen displaces a less reactive halide.
- Group 18 elements are unreactive because their atoms have full outer shells.
- Reactivity series: K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag.
- Metals above carbon are extracted by electrolysis; metals below carbon may be extracted by reduction with carbon.
- Rusting needs oxygen and water. Barrier protection, galvanising and sacrificial protection prevent rust in different ways.
8. Energy Changes and Rates
Energy changes
- Exothermic: energy is transferred to the surroundings, so the surroundings warm.
- Endothermic: energy is taken in from the surroundings, so the surroundings cool.
- State the observed temperature change and the direction of energy transfer.
- Enthalpy signs, energy-profile diagrams, bond-breaking/bond-making energy and activation energy are Pure Chemistry extensions.
Rate of reaction
- Increasing concentration or gas pressure gives more frequent collisions.
- Smaller particles have greater total surface area, so collision frequency increases.
- Higher temperature makes particles move faster, causing more frequent collisions and a greater fraction of sufficiently energetic collisions.
- On a product–time graph, a steeper gradient means a faster rate; a plateau means the measured product is no longer increasing.
- Catalyst definitions and alternative activation-energy pathways are Pure Chemistry extensions.
9. Organic Chemistry
| Series | General formula | Required carbon range |
|---|---|---|
| alkanes | CₙH₂ₙ₊₂ | C1–C3 |
| alkenes | CₙH₂ₙ | C2–C3 |
| alcohols | CₙH₂ₙ₊₁OH | C1–C3 |
- Crude oil is separated by fractional distillation.
- Complete combustion forms carbon dioxide and water; incomplete combustion may form carbon monoxide and soot.
- Alkanes undergo substitution with halogens in ultraviolet light.
- Alkenes decolourise bromine water and undergo addition.
- Hydrogenation uses hydrogen and a nickel catalyst.
- Ethanol can be oxidised to ethanoic acid.
- Addition polymerisation opens C = C; no small molecule is lost.
- Esters, C4 compounds, structural isomers and condensation polymers are Pure Chemistry extensions.
10. Maintaining Air Quality
Clean, dry air is approximately 78% nitrogen, 21% oxygen, 0.9% noble gases and 0.04% carbon dioxide by volume.
| Pollutant | Source and effect |
|---|---|
| CO | incomplete combustion; toxic because it reduces oxygen transport in blood |
| SO₂ | sulfur-containing fuels; contributes to acid rain |
| NOₓ | high-temperature engines; acid rain and photochemical smog |
| unburnt hydrocarbons | incomplete combustion; photochemical smog |
| particulates | incomplete combustion; respiratory harm and global dimming |
Carbon dioxide and methane absorb outgoing infrared radiation and contribute to global warming. Photosynthesis removes carbon dioxide; respiration, decomposition and combustion return it. Catalytic converters, flue-gas desulfurisation and ozone depletion are Pure Chemistry extensions.
11. High-Value Exam Traps
- Give the exact observation, then the inference.
- Use coefficients to balance equations; never alter a chemical formula.
- Convert cm³ to dm³ before using n = cV.
- Explain rate changes with collision frequency and, for temperature, the fraction of sufficiently energetic collisions.
- Do not import Pure-only salt preparation, electrolysis, activation-energy, ester or pollution-control content into a G3 Science answer.