Dry air: 78% nitrogen, 21% oxygen
Clean, dry air by volume is about 78% nitrogen (), 21% oxygen (), 0.9% argon (a noble gas) and 0.04% carbon dioxide () — roughly four-fifths nitrogen and one-fifth oxygen. Water vapour is present in ordinary air but is variable, which is why the standard figures are quoted for dry air. Learn 21% and 78% as the key numbers; argon and carbon dioxide are the small print.
Combustion: basic and acidic oxides
Combustion = reacting with oxygen and releasing energy. Elements burn to oxides: magnesium burns with a bright white light, (a basic oxide); sulfur burns with a blue flame, (acidic); carbon gives (acidic). General rule: metals give basic oxides, non-metals give acidic oxides — test with damp indicator (blue/purple basic, red/orange acidic).
Thermal decomposition: carbonate → oxide + CO2
Thermal decomposition = breaking one compound into simpler substances using heat. Metal carbonates give a metal oxide + carbon dioxide: (green → black) and . The turns limewater milky. Carbon dioxide is a greenhouse gas: it absorbs outgoing thermal (infrared) radiation, and rising from burning fossil fuels drives climate change.
Drawn from real examiner reports.
Why carbon monoxide is toxic
Carbon monoxide (from incomplete combustion) is toxic for a specific reason: it reduces the blood''s capacity to carry oxygen. Vague answers such as "it is poisonous" miss the mark — learn the specification wording. State that lowers the amount of oxygen the blood can transport around the body.
Jun24 1CR Q2cii: CO reduces the blood''s oxygen-carrying capacity
Oxygen supports combustion, it does not burn
Oxygen does not burn — it supports combustion, letting other things burn. Calling oxygen "flammable" is a property error and scores nothing; oxygen is also not toxic. Keep it sharp: a fuel is flammable (it burns); oxygen is the oxidiser that makes fuels burn hotter. An oxygen cylinder's hazards are vigorous combustion near a fuel and high pressure.
Percentage of oxygen USED, not gas left
When air is passed over heated copper the oxygen is removed and the gas volume falls. The percentage of oxygen = (volume decrease ÷ starting volume) × 100. The frequent error is finding the percentage of gas remaining, or finding the volume of oxygen used but forgetting to convert it to a percentage. E.g. 100 cm³ falling to 79 cm³ means 21 cm³ oxygen used = 21%.
"Not enough oxygen", never "no oxygen"
Incomplete combustion (producing carbon monoxide and/or soot) happens when there is a limited supply of oxygen — "not enough" oxygen. Writing "no oxygen" is not creditworthy, because with no oxygen there would be no combustion at all. The precise phrase examiners want is "insufficient / limited / not enough oxygen (or air)".
Jun24 1CR Q2ci: "not enough oxygen/air" credited, "no oxygen" not
Decomposition ≠ combustion
Do not confuse the two heat processes. Thermal decomposition takes in heat to split one compound (e.g. a carbonate) and does not need oxygen. Combustion reacts with oxygen and releases heat. A carbonate breaking into a metal oxide + is decomposition, not burning; magnesium burning in air is combustion.
Basic (metal) vs acidic (non-metal) oxides
Match the oxide''s acidity to the element type. Metals form basic oxides (magnesium oxide); most non-metals form acidic oxides (sulfur dioxide, carbon dioxide). Candidates mix these up or guess. Test with damp universal indicator: basic oxides turn it blue/purple, acidic oxides turn it red/orange.
Sulfur burns with a BLUE flame
Sulfur burns with a blue flame — a commonly missed observation because few candidates have seen it. Do not write "yellow" (the colour of solid sulfur) or leave it blank. Pair it with the product: , an acidic oxide. Learn combustion observations explicitly: magnesium = bright white light, sulfur = blue flame.
Reacting mass: equation, then moles
For a reacting-mass question: (1) write the balanced equation for the mole ratio; (2) convert the mass to moles with ; (3) apply the ratio to get moles of the product; (4) convert back to a mass. Show each substitution — method marks survive an arithmetic slip.
A 2-mark calc needs two visible steps
A 2-mark calculation needs two clear steps — show both. For the percentage of oxygen: first find the volume used (start − end), then divide by the starting volume and × 100. Underline the data first so you do not confuse "volume used" with "volume remaining".
Gas tests: give method AND result
Every gas test needs both a method and a result. Oxygen: insert a glowing splint → it relights. Carbon dioxide: bubble through limewater → it turns milky/cloudy. Giving only the result, or only the method, drops a mark.
Composition of dry air (by volume):
| Gas | Approximate % by volume |
|---|---|
| Nitrogen () | 78% |
| Oxygen () | 21% |
| Argon (noble gas) | ~0.9% |
| Carbon dioxide () | ~0.04% |
Combustion — a reaction in which a substance reacts with oxygen and releases energy (heat). Both elements are needed for the definition: (1) reacts with oxygen, (2) releases energy.
Thermal decomposition — the breaking down of a single compound into two or more simpler substances using heat. Both elements: (1) one compound splits into simpler products, (2) heat causes it.
Greenhouse gas — a gas (such as , methane or water vapour) that absorbs outgoing thermal (infrared) radiation from the Earth's surface, keeping the atmosphere warmer (the greenhouse effect).
Oxide acidity rule: metals form basic oxides; most non-metals form acidic oxides.
Key relationship , where = amount (mol), = mass (g), = relative formula mass.
State the approximate percentages by volume of the two most abundant gases in dry air.
Magnesium burns in oxygen to form magnesium oxide.
Write the balanced symbol equation for this reaction.
