2cGases in the atmosphere

Dry air is a mixture, and four gases account for almost all of it.

Reading those figures

Nitrogen is the most abundant gas, not oxygen. Many people assume air is mostly oxygen because that is the part we use.

Nitrogen and oxygen together make up about 99% of dry air. Everything else — argon, carbon dioxide and the rest — shares the remaining 1%.

The word dry matters. Water vapour is present in real air but its amount varies enormously with weather and location, so it is excluded from the standard figures.

Carbon dioxide: small percentage, large effect

Carbon dioxide is only about 0.04% of the atmosphere, and argon is more than twenty times as abundant. Yet carbon dioxide is the one that matters environmentally.

The reason is what each gas does. Argon is chemically inert and absorbs nothing. Carbon dioxide is a greenhouse gas.

How a greenhouse gas works

1. Energy from the Sun passes into the atmosphere and warms the Earth's surface.
2. The Earth radiates energy back out.
3. Carbon dioxide absorbs some of that outgoing energy, preventing it escaping into space.
4. More energy is retained, so average temperatures rise.

The common misunderstanding is that carbon dioxide blocks sunlight coming in. It does not — it absorbs the energy radiated back out.

A tiny percentage of a strongly absorbing gas can change the balance. Presence matters more than abundance.

The 21% figure is not something to take on trust — it can be measured with simple apparatus, and the method is examined regularly.

The idea

Use a substance that reacts with oxygen and nothing else. Whatever volume disappears was the oxygen.

Two substances are commonly used: damp iron wool (which rusts) or heated copper (which forms copper(II) oxide).

The method with copper

1. Measure a known volume of air, held between two gas syringes.
2. Pass the air back and forth over heated copper, so all of it meets the metal.
3. The copper turns from brown to black as copper(II) oxide forms.
4. Continue until the volume stops changing.
5. Measure the final volume.

The calculation

percentage of oxygen = (decrease in volume ÷ starting volume) × 100

If 100 cm³ falls to 79 cm³, the decrease is 21 cm³, so the air was 21% oxygen.

Use the decrease, not the volume remaining. Using 79 would give the percentage of everything that is not oxygen.

Why the volume falls

The oxygen is not destroyed — it becomes part of a solid oxide. Having left the gas phase, it no longer contributes to the volume.

Two conditions that carry marks

With iron wool, the metal must be damp. Rusting needs oxygen and water. Dry iron wool will not react and the experiment produces nothing.

Continue to constant volume. This is the evidence that all the oxygen has reacted. Stopping early gives a result that is too low, and "the experiment was not left long enough" is the standard explanation for an under-estimate.

Why a sealed rigid tube does not work

The gas must stay at atmospheric pressure for volume to be the thing that changes. In a sealed rigid container, consuming the oxygen lowers the pressure instead, and there is nothing to measure.

That is why these experiments use gas syringes, or a tube standing open over water so the water can rise as oxygen is used.

Burning elements in oxygen

Burning an element in oxygen produces its oxide.

Sulfur burns to sulfur dioxide, SO₂, not the trioxide. That is the product formed in air.

The oxides tell you the element type

Dissolve each oxide in water and test the pH:

• Magnesium oxide gives an alkaline solution → magnesium is a metal.
• Sulfur dioxide gives an acidic solution → sulfur is a non-metal.

This is the same metal/non-metal test from the Periodic Table topic, now applied to substances you have just made. It also explains why sulfur dioxide contributes to acid rain.

Thermal decomposition

Thermal decomposition is breaking a compound down using heat.

It has one distinguishing feature: only one reactant. Nothing from the air is involved, unlike combustion.

For metal carbonates:

metal carbonate → metal oxide + carbon dioxide

Heating green copper(II) carbonate turns it black, as copper(II) oxide forms, and carbon dioxide is released.

CuCO₃ → CuO + CO₂

That equation is already balanced as written — always check rather than assume, but here nothing needs adding.

Why the mass falls

The solid in the tube weighs less after heating, because carbon dioxide gas has escaped.

Mass is not lost. It leaves the apparatus as a gas — a different statement, and the one that earns the mark.

Compare with magnesium burning, where the mass increases because oxygen from the air is added to the solid. One reaction takes a gas in; the other lets a gas out.

Testing the gas

Bubble it through limewater: it turns milky if the gas is carbon dioxide.

The milkiness is a precipitate of insoluble calcium carbonate. "It goes cloudy" describes the observation; naming the insoluble product explains it.

Air is about one-fifth oxygen. The experiment removes the oxygen and measures how much the volume falls.

Method 1 — passing air over heated copper

Apparatus

Two gas syringes joined by a silica tube packed with copper turnings, Bunsen.

Method

1. Measure a known volume of air into one syringe — say 100 cm³.
2. Heat the copper.
3. Push the air slowly backwards and forwards over the hot copper.
4. Let the apparatus cool, then read the total volume.
5. Repeat until the volume stops falling.

Result

The copper turns from pink-brown to black as copper(II) oxide forms:

2Cu + O₂ → 2CuO

percentage of oxygen = (decrease in volume ÷ starting volume) × 100

From 100 cm³ down to about 79 cm³ — roughly 21%.

Why each step

Pass the air back and forth repeatedly. One pass leaves oxygen unreacted, and the volume decrease comes out too small. Repeating until the reading is steady is what shows the reaction is complete.

Use excess copper. If the copper runs out, oxygen is left behind and the result is again too low.

Cool before reading. Hot gas has expanded. Measuring while hot makes the remaining volume look larger and the oxygen percentage smaller.

Method 2 — burning a non-metal over water

Phosphorus burning in a sealed bell jar over water uses up the oxygen; water rises to fill about a fifth of the space.

Why the candle version gives the wrong answer

A candle under a jar over water is the version most people meet first, and it typically suggests much less than 20%.

The flame goes out before all the oxygen is used — a candle cannot burn in low oxygen concentrations. The reaction stops early, so the volume change is too small.

Burning a candle also produces carbon dioxide, a gas, which partly replaces the oxygen consumed and further reduces the apparent change.

Copper avoids both problems: it keeps reacting while any oxygen remains, and the product is a solid, so every molecule of oxygen removed shows up as a fall in volume.

Where marks are lost

• Reading the volume while the apparatus is still hot.
• Giving the final volume instead of the decrease.
• Explaining the candle result as "the oxygen ran out" — the whole point is that it did not.