Testing for Oxygen and Carbon Dioxide

Section 1 of 8Test for oxygen

1

Collect the gas in a test tube.

2

Light a wooden splint, then blow out the flame so the end is glowing.

3

Place the glowing splint inside the test tube of gas.

4

Positive result: the glowing splint relights, which shows the gas is oxygen.

Section 2 of 8Test for carbon dioxide

1

Prepare limewater, which is an aqueous solution of calcium hydroxide.

2

Bubble the gas through the limewater.

3

Positive result: the limewater turns milky or cloudy.

4

The cloudiness is a fine white precipitate of calcium carbonate: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).

Section 3 of 8Results at a glance

Gas	Test	Positive result
Oxygen, O2	Glowing splint placed in the gas	Splint relights
Carbon dioxide, CO2	Gas bubbled through limewater	Limewater turns cloudy or milky

Section 4 of 8Why each test works

1Oxygen supports burning

• Oxygen supports burning Pure oxygen makes substances burn much more strongly than the oxygen in air, which is only about one-fifth of the mixture.

2Glowing splint reignites

• Glowing splint reignites The extra oxygen at the glowing tip lets the wood burn with a flame again, so the splint relights.

3Carbon dioxide reacts with limewater

• Carbon dioxide reacts with limewater Carbon dioxide reacts with dissolved calcium hydroxide to make calcium carbonate, which is insoluble in water.

4Solid scatters light

• Solid scatters light The tiny particles of calcium carbonate are suspended in the liquid, so it looks milky or cloudy.

Section 5 of 8Carrying out the tests safely

Collecting the gas

• Collecting the gas Gas can be collected in an upturned test tube or a gas syringe, and a bung helps to stop it escaping before you test it.
• Test oxygen straight away because the gas mixes with air quickly.

Limewater care

• Limewater care Limewater is an alkali and mild irritant, so wear eye protection.
• Use a delivery tube that dips well below the surface so the gas has to bubble through the solution.

Excess carbon dioxide

• Excess carbon dioxide If carbon dioxide continues to be bubbled through for a long time the cloudiness can disappear again,
  • so look carefully at the first change.

Section 6 of 8Exam wording and common errors

A flame going out proves nothing

• A flame going out proves nothing A burning splint is extinguished by any gas that cannot support combustion, including nitrogen.
• It only shows oxygen is lacking, so never give it as the test for carbon dioxide.

Relighting wording

• Relighting wording Sometimes the splint just glows brighter, which is still a positive result.
• In an exam, state that oxygen relights a glowing splint.

Describing limewater

• Describing limewater Say that it turns cloudy or milky, not that it goes white or that the gas turns it cloudy without naming limewater.
• Make sure you say the gas is bubbled through the solution.

Section 7 of 8Key terms

Term	Meaning
Limewater	A solution of calcium hydroxide used to test for carbon dioxide.
Glowing splint	A wooden splint that has been lit and blown out so the end still glows.
Precipitate	A fine insoluble solid formed in a solution.
Calcium carbonate	The white insoluble solid that makes limewater cloudy.

Section 8 of 8Exam technique

1

Using a burning splint that goes out as the test for carbon dioxide - other gases such as nitrogen do the same.

2

Using a lit, flaming splint for oxygen instead of a glowing one.

3

Saying limewater turns clear when carbon dioxide is added - it turns cloudy or milky.

4

Not naming the reagent - say the gas is bubbled through limewater, not just 'a test solution'.