(I once asked a class to hold their breath for as long as they could. After about 30 seconds, one student gasped, "Okay, I get it — I really need air." That moment taught them more about this chapter than any diagram could. Air is not "nothing." It is a carefully balanced mixture of gases, and you are alive because of it.) Take a slow, deep breath. Right now, without thinking about it, you've just pulled in about a litre of air. That air contains nitrogen, oxygen, argon, carbon dioxide, water vapour, and tiny traces of other gases — all mixed together in just the right proportions to keep you alive. Oxygen is the gas that keeps your cells burning fuel for energy. Carbon dioxide is the gas your body produces as waste, and it's also the gas that plants use to make food. When you light a candle, drive a car, or cook rice, you're watching combustion — a chemical reaction that depends entirely on oxygen. And when iron rusts, or an apple browns after you cut it, that's oxygen at work again. Air, oxygen, and combustion are the chemistry of everyday survival. Let's dig into it — you'll never take a breath for granted again.
Air is a mixture of gases — not a compound, not an element, but a mixture. That's why the proportions can vary slightly from place to place, and why we can separate the components by physical means (like fractional distillation of liquid air).
For centuries, people thought air was a single substance. It wasn't until the 1700s that chemists like Lavoisier showed it was a mixture. Today we know its composition quite precisely.
Here's the composition of dry air by volume. Learn these numbers — they appear in nearly every exam.
| Gas | Percentage by Volume | Importance |
|---|---|---|
| Nitrogen (N₂) | About 78% | Dilutes oxygen; used by plants to make proteins; used industrially to make ammonia and fertilisers. |
| Oxygen (O₂) | About 21% | Needed for respiration in living things; needed for combustion; used in hospitals and industry. |
| Argon (Ar) | About 0.9% | Inert gas; used in light bulbs and welding to prevent oxidation. |
| Carbon dioxide (CO₂) | About 0.04% | Used by plants for photosynthesis; a greenhouse gas; produced by respiration and combustion. |
| Other gases (neon, helium, krypton, etc.) | Trace amounts | Various specialised uses. |
| Water vapour | Variable (0–4%) | Affects weather; part of the water cycle. |
(Here's a way to remember the big two: "78–21 — the air we rely on." Nitrogen takes the biggest slice; oxygen comes second. Everything else is a tiny garnish.)
Oxygen is the gas that makes life possible for animals, including you. Every cell in your body uses oxygen to break down glucose and release energy — a process called respiration. Without oxygen, your cells would suffocate within minutes.
Oxygen is also essential for combustion — burning. No oxygen, no fire. That's why firefighters sometimes use foam or carbon dioxide to smother a fire: they're cutting off the oxygen supply.
Industrially, oxygen is used in:
In the laboratory, oxygen is usually prepared by catalytic decomposition of hydrogen peroxide using manganese(IV) oxide as a catalyst. The reaction is:
2H₂O₂ → 2H₂O + O₂
The manganese(IV) oxide speeds up the reaction but is not used up — that's what a catalyst does. The oxygen gas is collected over water (by downward displacement of water) because it is only slightly soluble in water.
Another common method is heating potassium permanganate (KMnO₄) or potassium chlorate (KClO₃). But the hydrogen peroxide method is the safest and most common in school labs.
Test for oxygen: Insert a glowing splint into the gas. If the splint relights (bursts into flame), the gas is oxygen. This is the classic test — learn it.
Here's a handy table of gas tests you should memorise:
| Gas | Test | Positive Result |
|---|---|---|
| Oxygen (O₂) | Insert a glowing splint | Splint relights (burns brighter) |
| Carbon dioxide (CO₂) | Bubble through limewater | Limewater turns milky/cloudy |
| Hydrogen (H₂) | Hold a lit splint at the mouth of the tube | A "pop" sound |
| Ammonia (NH₃) | Hold damp red litmus paper near the gas | Litmus turns blue |
| Chlorine (Cl₂) | Hold damp litmus paper near the gas | Litmus is bleached (turns white) |
| Sulphur dioxide (SO₂) | Hold damp potassium dichromate(VI) paper | Paper turns from orange to green |
(Whenever you do a gas test, say the test out loud as you do it. "Glowing splint into oxygen — relights." Your mouth helps your memory. It sounds silly, but it works.)
Combustion is the chemical reaction in which a substance reacts with oxygen, releasing heat and light. It's also called burning. Combustion is an exothermic reaction — it releases energy.
For combustion to happen, three things must be present at the same time. This is called the fire triangle:
| Condition | Explanation |
|---|---|
| Fuel | Something that can burn — wood, petrol, methane, paper. |
| Oxygen | Usually from the air. Without oxygen, nothing burns. |
| Heat | Enough to start the reaction. Once started, combustion produces its own heat and sustains itself. |
Remove any one of the three, and the fire goes out. That's exactly what firefighters do: they remove the fuel, or smother the fire to cut off oxygen, or cool it with water to remove heat.
When a fuel burns, the products depend on how much oxygen is available.
| Type | Oxygen Supply | Products | Flame |
|---|---|---|---|
| Complete combustion | Plenty of oxygen | Carbon dioxide + water | Blue, clean, hot |
| Incomplete combustion | Limited oxygen | Carbon monoxide + carbon (soot) + water | Yellow, smoky, cooler |
Why this matters: Carbon monoxide (CO) is a colourless, odourless, highly toxic gas. It binds to haemoglobin in your blood far more strongly than oxygen does, preventing your blood from carrying oxygen. This is why faulty gas heaters and blocked chimneys are so dangerous — they produce carbon monoxide, and people can die in their sleep without ever knowing.
(I tell my students: if you ever feel dizzy or sleepy in a room with a gas heater, get out and get fresh air immediately. Carbon monoxide gives no warning. It's the silent killer of this chapter. Please remember that.)
Rusting is the corrosion of iron when it reacts with oxygen and water. It's not combustion — there's no flame — but it's still an oxidation reaction. The product, rust (hydrated iron(III) oxide), is a reddish-brown flaky solid.
Conditions needed for rusting:
Remove either one, and rusting doesn't happen. That's why iron nails don't rust in dry air, and don't rust in boiled water (which has no dissolved oxygen) — but they rust quickly in ordinary tap water exposed to air.
Methods of preventing rusting:
| Method | How It Works | Examples |
|---|---|---|
| Painting | Creates a barrier preventing oxygen and water from reaching the iron. | Gates, bridges, car bodies |
| Oiling / greasing | Same principle — a barrier. | Tools, bicycle chains, machine parts |
| Galvanising (coating with zinc) | Zinc is more reactive than iron, so it corrodes first, protecting the iron underneath. This is called sacrificial protection. | Dustbins, roofing sheets, buckets |
| Electroplating (coating with tin, chromium, nickel) | Barrier coating of a less reactive metal. | Food cans (tin-plated), taps (chrome-plated) |
| Alloying (making stainless steel) | Mixing iron with chromium and nickel changes its structure and prevents rusting. | Cutlery, surgical instruments, kitchen sinks |
Carbon is the element of life. It moves constantly between the air, the land, the oceans, and living things. This movement is called the carbon cycle.
| Process | What Happens | Direction of Carbon |
|---|---|---|
| Photosynthesis | Plants take in CO₂ from the air and use sunlight to make glucose. | Air → plants |
| Respiration | Living things break down glucose using oxygen, releasing CO₂. | Living things → air |
| Combustion | Burning fossil fuels or wood releases CO₂. | Fuels → air |
| Decomposition | Bacteria and fungi break down dead matter, releasing CO₂. | Dead matter → air |
| Fossilisation | Dead organisms buried over millions of years become coal, oil, and gas. | Living things → fossil fuels |
| Dissolving in oceans | CO₂ dissolves in seawater, forming carbonic acid and carbonate rocks. | Air → oceans |
The carbon cycle keeps the level of CO₂ in the atmosphere roughly balanced. But when humans burn huge amounts of fossil fuels, we add CO₂ faster than plants and oceans can absorb it. That's the root of climate change — a chemistry problem with global consequences.
Question: Methane (CH₄) burns in a plentiful supply of oxygen. Write the word and balanced chemical equations for the reaction.
Solution:
• Word equation: Methane + oxygen → carbon dioxide + water
• Balanced equation: CH₄ + 2O₂ → CO₂ + 2H₂O
• Reasoning: With plenty of oxygen, combustion is complete. The carbon in methane becomes CO₂; the hydrogen becomes H₂O. Balance the oxygen atoms — 4 on the right, so 2O₂ on the left.
Question: A student collects a colourless gas from the decomposition of hydrogen peroxide. Describe how to test that it is oxygen.
Solution: Insert a glowing splint into the mouth of the test tube. If the splint relights (bursts into flame), the gas is oxygen. This works because oxygen supports combustion, so the glowing ember bursts back into flame.
Question: Explain why galvanising protects iron from rusting, even if the zinc coating is scratched.
Solution: Galvanising coats iron with zinc. Zinc is more reactive than iron, so it corrodes in preference to iron — this is called sacrificial protection. Even if the coating is scratched, the zinc continues to corrode first, protecting the iron underneath. This is why galvanised steel is so durable in outdoor conditions.
1. State the percentage of nitrogen and oxygen in dry air.
Nitrogen: about 78%. Oxygen: about 21%.
2. Describe the test for oxygen gas.
Insert a glowing splint into the gas. If the splint relights, the gas is oxygen.
3. What are the three conditions needed for combustion?
Fuel, oxygen, and heat — the three sides of the fire triangle.
4. What is the difference between complete and incomplete combustion?
Complete combustion occurs with plenty of oxygen and produces carbon dioxide and water. Incomplete combustion occurs with limited oxygen and produces carbon monoxide, soot (carbon), and water.
5. What two conditions are needed for iron to rust?
Oxygen and water (moisture).
6. Why is carbon monoxide dangerous?
Carbon monoxide is colourless, odourless, and highly toxic. It binds to haemoglobin in the blood more strongly than oxygen, preventing the blood from carrying oxygen. This can cause suffocation and death.
7. Name two processes in the carbon cycle that release carbon dioxide into the atmosphere.
Respiration and combustion (also decomposition). Any two accepted.
Question 1 [8 marks]
(a) Describe the composition of dry air by volume, naming the two main gases and their approximate percentages. [3]
(b) State one important use of oxygen and one important use of nitrogen. [2]
(c) Explain why the proportion of carbon dioxide in the atmosphere is increasing and why this is a concern. [3]
(a) Air is about 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide, with trace gases and variable water vapour. (3 marks: 1 for nitrogen, 1 for oxygen, 1 for any other correct detail.)
(b) Oxygen: used in hospitals for patients with breathing difficulties (or in steel-making, welding). Nitrogen: used to make ammonia and fertilisers (or as a coolant, or in food packaging). (2 marks: 1 for each correct use.)
(c) The proportion of CO₂ is increasing mainly because of the burning of fossil fuels and deforestation. This is a concern because CO₂ is a greenhouse gas — it traps heat in the atmosphere, contributing to global warming and climate change. (3 marks: 1 for cause, 1 for "greenhouse gas", 1 for consequence.)
Question 2 [10 marks]
(a) Describe the laboratory preparation of oxygen from hydrogen peroxide, including the equation and the role of manganese(IV) oxide. [4]
(b) Describe how you would test the gas collected to confirm it is oxygen. [2]
(c) Explain why oxygen is collected over water rather than by downward delivery. [2]
(d) State one industrial use of oxygen. [2]
(a) Hydrogen peroxide is decomposed by manganese(IV) oxide, which acts as a catalyst. Equation: 2H₂O₂ → 2H₂O + O₂. The manganese(IV) oxide speeds up the reaction but is not used up. (4 marks: 1 for hydrogen peroxide, 1 for catalyst, 1 for equation, 1 for "not used up".)
(b) Insert a glowing splint into the gas. If the splint relights, the gas is oxygen. (2 marks: 1 for "glowing splint", 1 for "relights".)
(c) Oxygen is only slightly soluble in water, so collecting it over water is safe and simple. It is also slightly denser than air but not enough to make downward delivery efficient — collecting over water avoids losing gas to the atmosphere. (2 marks: 1 for "slightly soluble", 1 for "avoids loss".)
(d) Any one of: used in hospitals for patients with breathing difficulties; used in steel-making; used in welding; used in spacecraft. (2 marks: 1 for use, 1 for clarity.)
Question 3 [10 marks]
(a) State the three conditions necessary for combustion. [3]
(b) Explain, using the fire triangle, how each of the following puts out a fire: (i) pouring water on it, (ii) covering it with a fire blanket, (iii) turning off the gas supply. [6]
(c) Name one product of incomplete combustion that is harmful and state why it is dangerous. [1]
(a) Fuel, oxygen, and heat. (3 marks: 1 for each.)
(b) (i) Water removes heat — it cools the fuel below its ignition temperature. (ii) A fire blanket cuts off the oxygen supply, smothering the fire. (iii) Turning off the gas removes the fuel — no more fuel means no more combustion. (6 marks: 2 per correct explanation — 1 for identifying the side of the triangle removed, 1 for explaining.)
(c) Carbon monoxide — it is colourless, odourless, and highly toxic; it binds to haemoglobin and prevents oxygen transport in the blood. (1 mark for naming it or explaining its danger.)
Challenge 1: A student says, "Rusting is just slow burning." Is this statement accurate? Explain your reasoning, referring to the conditions needed and the products formed in each process.
The statement is partially true but not entirely accurate. Both rusting and burning are oxidation reactions involving oxygen, and both release energy. However, rusting requires water as well as oxygen, while combustion does not. Rusting is a slow oxidation with no flame, while combustion is rapid and produces heat and light. The products differ too: rusting produces hydrated iron(III) oxide, whereas combustion of a fuel produces carbon dioxide and water (or carbon monoxide and soot in incomplete combustion). So while both are oxidation, rusting is not simply "slow burning" — it has different conditions and different products.
Challenge 2: Explain why carbon monoxide is more dangerous than carbon dioxide, even though both are produced during combustion.
Carbon dioxide is a normal product of respiration and is present in the atmosphere at low concentrations. It is not acutely toxic at normal levels. Carbon monoxide, however, is highly toxic. It binds to haemoglobin in the blood about 200–250 times more strongly than oxygen does, forming carboxyhaemoglobin. This prevents the blood from carrying oxygen around the body, leading to oxygen starvation of tissues. Because CO is colourless and odourless, people can be poisoned without realising it. This makes it far more dangerous than CO₂ in enclosed spaces.
Challenge 3: The carbon cycle keeps the level of carbon dioxide in the atmosphere balanced. Explain how human activities have disrupted this balance, and describe one consequence of this disruption.
Human activities — especially the burning of fossil fuels (coal, oil, and gas) and deforestation — have added large amounts of carbon dioxide to the atmosphere faster than natural processes (photosynthesis, ocean absorption) can remove it. Deforestation also reduces the number of trees available to absorb CO₂. As a result, atmospheric CO₂ levels have risen significantly since the industrial revolution. One consequence is the enhanced greenhouse effect: CO₂ traps heat in the atmosphere, leading to global warming, which in turn causes climate change — rising sea levels, more extreme weather, and disruption to ecosystems.
"Every breath you take is chemistry. Every flame you light is chemistry. Now go breathe — and notice."
