Experimental evidence
3 reviewed G2 Science Chemistry objectives with assessment and delayed parallel re-test.
G2 Science Chemistry Revision Guide
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K223 / K225 G2 Science Chemistry revision guide covering definitions, formulae, observations, tests, reaction patterns and exam traps.
Fast revision method
Recall each definition, formula, observation and reaction condition before revealing the table entry. Everything here is limited to K223/K225 G2 Chemistry; G3-only depth is deliberately excluded.
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; they can be separated by physical methods. |
| Isotopes | Atoms of the same element with the same proton number but different numbers of neutrons. |
| Ion | A charged particle formed when an atom or group of atoms gains or loses electrons. |
| 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. |
| Hydrocarbon | A compound containing carbon and hydrogen only. |
2. Experimental Chemistry and Separation
| Need | Best choice | Mark-scoring detail |
|---|---|---|
| Measure time | stopwatch | Record a suitable unit and precision. |
| Measure temperature | thermometer | Read at eye level after the reading settles. |
| Measure mass | electronic balance | Use the appropriate balance precision. |
| Measure liquid volume | measuring cylinder | Read the bottom of the meniscus at eye level for a colourless liquid. |
| Collect a water-insoluble gas | over water | Use only when the gas does not dissolve or react significantly with water. |
| Separate insoluble solid from liquid | filtration | Residue remains; filtrate passes through. |
| Obtain dissolved solid | evaporation | The solvent is not collected. |
| Obtain solvent from solution | simple distillation | Vapour is cooled to form the distillate. |
| Separate miscible liquids | fractional distillation | Different boiling points allow repeated evaporation and condensation. |
| Separate coloured solutes | paper chromatography | One spot suggests a pure substance; several spots show a mixture. |
A pure substance has a sharp melting point or boiling point. Impurities usually lower and broaden a melting range and change a boiling point.
3. Particle Model, Atoms and Bonding
- In a solid, particles are closely packed and vibrate about fixed positions.
- In a liquid, particles remain close but move past one another.
- In a gas, particles are far apart and move rapidly in random directions.
- Melting, boiling and sublimation absorb energy; freezing and condensation release energy.
- Proton number = number of protons. Nucleon number = protons + neutrons.
- A neutral atom has equal numbers of protons and electrons.
- Metals form positive ions by losing electrons; non-metals form negative ions by gaining electrons.
| Structure | Property explanation |
|---|---|
| ionic compound | High melting point because strong attractions between ions require much energy to overcome; conducts only when molten or aqueous because ions can then move. |
| simple covalent substance | Low melting and boiling points because weak attractions between molecules require little energy to overcome. |
| metal | Conducts electricity and heat; layers can slide, so metals are malleable. |
| alloy | Different-sized atoms disrupt regular layers, making sliding more difficult than in a pure metal. |
4. Chemical Calculations
| Quantity | Relationship | Unit check |
|---|---|---|
| relative molecular/formula mass | add all relative atomic masses | no unit |
| amount from mass | n = dfracmM | mass in g; molar mass in g mol⁻¹ |
| mass from amount | m = nM | answer in g |
Always balance an equation by changing coefficients, never the small numbers inside a formula. At G2, convert directly between mass, amount and molar mass; reacting-mass ratios, gas-volume stoichiometry and solution concentration are G3 extensions.
5. Acids, Bases and Reaction Patterns
| Reaction | General equation |
|---|---|
| acid + metal | salt + hydrogen |
| acid + base | salt + water |
| acid + carbonate | salt + water + carbon dioxide |
| acid + reactive metal oxide | salt + water |
| Indicator | Acid | Neutral | Alkali |
|---|---|---|---|
| litmus | red | no change | blue |
| methyl orange | red | orange | yellow |
| universal indicator | red/orange/yellow | green | blue/violet |
Neutralisation is represented by:
H + (aq) + OH-(aq) → H₂O(l)
- Basic oxides react with acids.
- Acidic oxides react with bases.
- Amphoteric oxides react with both acids and bases.
- Farmers may add lime to neutralise acidic soil.
6. Prescribed Gas Tests
| Gas | Test | Positive observation |
|---|---|---|
| hydrogen, H₂ | place a lighted splint at the mouth of the container | a squeaky pop |
| oxygen, O₂ | insert a glowing splint | the splint relights |
| carbon dioxide, CO₂ | bubble the gas through limewater | limewater turns milky; a white precipitate forms |
G2 qualitative-analysis boundary
K223/K225 requires these three gas tests. Cation precipitates and the wider anion-test table belong to G3 Science / Pure Chemistry, not this guide.
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 metals become more reactive down the group.
- Group 17 halogens become less reactive down the group; a more reactive halogen displaces a less reactive halide.
- Group 18 elements are unreactive because they have full outer electron shells.
- Reactivity series: K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag.
- Metals above carbon are extracted by electrolysis; less reactive metals may be extracted by reduction with carbon.
- Rusting requires both oxygen and water. Barrier coating, galvanising and sacrificial protection prevent rust by removing a required condition or protecting the iron.
8. Organic Chemistry
| Series | General formula | Required members |
|---|---|---|
| alkanes | CₙH₂ₙ₊₂ | methane, ethane, propane |
| alkenes | CₙH₂ₙ | ethene, propene |
- Crude oil is a mixture separated into fractions by fractional distillation.
- Complete combustion forms carbon dioxide and water; incomplete combustion can form carbon monoxide and soot.
- Alkanes undergo substitution with halogens in ultraviolet light.
- Alkenes turn bromine water from orange to colourless.
- Hydrogenation adds hydrogen across C = C using a nickel catalyst and can harden vegetable oils.
- Addition polymerisation opens the alkene double bond; no small molecule is lost.
9. Maintaining Air Quality
| Substance | Important source or effect |
|---|---|
| carbon monoxide, CO | incomplete combustion; toxic because it reduces the blood’s oxygen-carrying ability |
| sulfur dioxide, SO₂ | sulfur-containing fuels; contributes to acid rain |
| nitrogen oxides, NOₓ | high-temperature engines; contribute to acid rain and photochemical smog |
| unburnt hydrocarbons and particulates | incomplete combustion; smog and respiratory harm |
| carbon dioxide and methane | greenhouse gases that absorb outgoing infrared radiation |
Clean, dry air is about 78% nitrogen, 21% oxygen, 0.9% noble gases and 0.04% carbon dioxide by volume. In the carbon cycle, photosynthesis removes carbon dioxide while respiration, decomposition and combustion return it.
10. High-Value Exam Traps
- State an observation before an inference: “limewater turns milky”, then “carbon dioxide is present”.
- Explain properties using particles and forces, not labels alone.
- Balance equations with coefficients; do not alter chemical formulae.
- Write the correct gas test and its exact positive result.
- Do not import G3 material such as reacting-mass stoichiometry, ion tests, redox, energetics, rates, alcohols or carboxylic acids into a G2 answer.
Diagnose and Repair Weak Objectives
Choose a course-filtered diagnostic. Your answers stay on this device and can create a 20-, 40- or 60-minute repair plan.
Matter and quantitative chemistry
3 reviewed G2 Science Chemistry objectives with assessment and delayed parallel re-test.
Patterns and applications
4 reviewed G2 Science Chemistry objectives with assessment and delayed parallel re-test.
Topic-by-Topic Revision
Use the summary above for recall, then open the full lesson when a point needs explanation or worked practice. Every topic below comes from this course’s reviewed objective map.
1. Experimental Chemistry
Study the complete G2 composition for measurement, gas collection, separation and purity. 2 reviewed objectives.
2. The Particulate Nature of Matter
States and changes of state, atomic structure, nuclide notation, isotopes and ions. Diffusion is not required. 1 reviewed objective.
3. Chemical Bonding and Structure
Ionic and covalent bonding, structure-property links, metals and alloys; Pure-only metallic and giant-structure depth is excluded. 1 reviewed objective.
4. Chemical Calculations
Formulae, equations, relative masses and direct mass-mole-molar-mass calculations; not G3 stoichiometry. 1 reviewed objective.
5. Acid-Base Chemistry
Ions, pH, reactions, neutralisation, soil treatment, bases and oxide classification. 1 reviewed objective.
6. Qualitative Analysis
The three prescribed gas tests: carbon dioxide, hydrogen and oxygen. 1 reviewed objective.
7. Patterns in the Periodic Table
Periodic trends, Groups 1, 17 and 18, the reactivity series, extraction and rust prevention. 1 reviewed objective.
8. Organic Chemistry
Fuels, unbranched C1–C3 hydrocarbons, core alkane/alkene reactions and addition polymers. 1 reviewed objective.
9. Maintaining Air Quality
Air composition, pollutant sources and effects, the carbon cycle and greenhouse gases. 1 reviewed objective.