Composition of dry air by volume
Approximate percentages by volume of the four most abundant gases in dry air: nitrogen () ~78%, oxygen () ~21%, argon (Ar) ~1%, carbon dioxide () ~0.04% (a trace). Check: , so nitrogen, oxygen and argon make up nearly all of dry air. These are by VOLUME of DRY air — water vapour is excluded because its amount varies.
Burning magnesium, hydrogen and sulfur
Combustion is reaction with oxygen that releases energy (the element burns). Three named elements (spec 2.11): magnesium, (bright white flame); hydrogen, (squeaky pop); sulfur, (blue flame). Magnesium and sulfur gain oxygen, so both are oxidised; sulfur dioxide is acidic (acid rain), magnesium oxide basic.
Carbonates decompose to oxide + CO₂
Thermal decomposition is the breakdown of one compound using heat alone. A metal carbonate gives a metal oxide + carbon dioxide; e.g. green copper(II) carbonate to black copper(II) oxide, (spec 2.12). No is added. The turns limewater milky. It is a greenhouse gas (spec 2.13): it absorbs and re-emits heat radiation, so rising amounts may cause global warming — environmental, not the ozone layer.
Drawn from real examiner reports.
No O₂ in a thermal-decomposition equation
Thermal decomposition uses heat only — nothing reacts WITH the carbonate, so must not appear on either side. The correct equation is (already balanced). Watch the formulae: copper(II) carbonate is (not ) and copper(II) oxide is .
June 2023 Paper 2C Q2(b)(iii): common errors were adding oxygen to the left-hand side of the copper(II) carbonate decomposition equation, or giving the wrong formula for copper(II) carbonate or copper(II) oxide.
-ide has no oxygen; -ate does
A name ending in -ide (oxide, ) contains no oxygen beyond what its formula already shows, whereas a name ending in -ate (carbonate, ) does contain oxygen. So copper(II) oxide is just ; do not write in an extra . Read the -ide / -ate ending carefully before writing a formula.
June 2023 Paper 2C Q1(a)(v): candidates wrongly assumed that a compound whose name ends in -ide contains oxygen.
Greenhouse effect is not the ozone layer
For the greenhouse-gas mark, link carbon dioxide -> greenhouse gas -> global warming / climate change (ENVIRONMENTAL). Two traps: (1) the ozone layer — a different issue (CFCs), no credit; (2) rising as a breathing/health problem — the percentage is still tiny and there is plenty of oxygen. Keep it environmental: it traps heat radiation, warming the Earth.
June 2023 Paper 2C Q2(d): a few candidates lost a mark by referring to the ozone layer; one treated increased carbon dioxide as a human breathing/health problem rather than an environmental one.
Say the element burns, not just reacts
Combustion answers must state the element burns / combusts (reacts with oxygen, releasing energy). Just "reacts with oxygen", or only naming the product, is often too vague. For the sulfur impurity in fuels you must say sulfur burns to form sulfur dioxide, — naming only "acid rain" without the step loses the mark.
June 2024 Paper 1CR Q8(c): candidates had to state that the sulfur impurity BURNS / reacts with oxygen to form sulfur dioxide; the burning wording was required.
MgO is a solid — state symbols
In , magnesium oxide is a solid (s). Writing it as (aq) or (l) is a common, avoidable error — the balancing is usually right, but the state symbol is where the mark is lost. A solid oxide formed by burning a metal in oxygen is always (s).
June 2024 Paper 2CR Q3(b)(i): marks were lost mostly on the state symbol for MgO, often wrongly given as (aq) or (l), not on the balancing.
Oxidation = gains oxygen, not electrons
When magnesium burns it is oxidised because it gains oxygen (or loses electrons). Writing that magnesium gains electrons is wrong — that would be reduction. And just saying it "reacts with oxygen" is not enough for the oxidation mark: state that it gains oxygen (or loses electrons).
June 2024 Paper 2CR Q3(b)(ii): for oxidation, gains electrons was wrong; candidates needed gains oxygen or loses electrons, and reacts with oxygen alone was insufficient.
% oxygen in air: used up ÷ start × 100
Pass a fixed volume of air over a substance that removes oxygen only (e.g. heated copper) in sealed apparatus. Note the starting volume, then the final volume once it stops changing. Oxygen used up = start − final; % oxygen = (used up ÷ start) × 100. Aim for ~21%.
Gas tests: give method and result
A gas test needs both a method and the result. Carbon dioxide: bubble through limewater — it turns milky. Hydrogen: lighted splint at the tube mouth — squeaky pop. Oxygen: glowing splint — it relights. Giving only the result, or only the method, drops a mark.
Explain climate change as a chain
Build it as cause -> mechanism -> effect -> consequence: extra (burning fossil fuels) -> a greenhouse gas absorbs and re-emits heat radiation -> more heat trapped, so global temperature rises -> climate change. Keep it environmental — not ozone, not breathing.
Reacting-mass steps: moles = mass ÷ Mr
For a reacting-mass question on : moles = mass ÷ Mr, apply the 1:1 ratio, then mass = moles × Mr. Use the data-sheet (not 64), so Mr of = 123.5 and Mr of = 44.
Composition of dry air (approximate percentages by volume — learn these):
| Gas | Formula | Approx. % by volume of dry air |
|---|---|---|
| Nitrogen | ~78% | |
| Oxygen | ~21% | |
| Argon (a noble gas) | ~1% | |
| Carbon dioxide | ~0.04% (a trace) |
Check: , so nitrogen, oxygen and argon make up effectively all of dry air. "Dry" air excludes water vapour, whose amount varies.
Two-element mark-scheme definitions:
Combustion of elements in oxygen (spec 2.11):
Carbon dioxide from thermal decomposition of metal carbonates (spec 2.12):
No appears — heat alone splits the carbonate. The released turns limewater milky/cloudy (the test for carbon dioxide).
Carbon dioxide as a greenhouse gas (spec 2.13): absorbs and re-emits heat radiation. Increasing amounts in the atmosphere may contribute to global warming / climate change — an environmental effect (not the ozone layer).
State the four most abundant gases in dry air and their approximate percentages by volume.
Dry air is a mixture of gases.
State the two most abundant gases in dry air and give the approximate percentage by volume of each.
