Testing for oxygen in atmosphere and gas samples Revision Notes | GCSE Edexcel Higher Chemistry

1. Why Test for Oxygen?

Oxygen is everywhere around us, but how do you prove it's there? You can't see it, smell it, or taste it. Yet oxygen is crucial for combustion - without it, nothing burns. This invisible gas has a signature behaviour that gives it away every time.

In chemistry practicals, you'll often produce gases and need to identify them. Oxygen might be released when you decompose hydrogen peroxide, or when plants photosynthesise. The glowing splint test is your go-to method for confirming oxygen's presence.

2. The Glowing Splint Test

Here's what happens step by step. You light a wooden splint, then blow it out so it's no longer flaming but still glowing red-hot at the tip. When you insert this glowing splint into a test tube containing oxygen, something dramatic happens - the splint bursts back into flame.

**Trap**: Many students think any hot object will relight in oxygen. Actually, the splint needs to be glowing (around 250°C) to provide the activation energy for combustion. A cold splint won't work.

Worked example: Performing the Oxygen Test

Test an unknown gas to see if it contains oxygen

1. Hold a wooden splint in a Bunsen flame until it catches fire — The splint needs to be burning first
2. Blow gently to extinguish the flame, leaving a glowing tip — The tip should glow red-hot but not be flaming
3. Quickly insert the glowing end into the gas sample — Work quickly before the splint cools down
4. Watch for the splint to relight with a flame — Relighting confirms oxygen is present

Answer: If oxygen is present, the glowing splint will relight. If not, it will just go out.

3. Why Does It Work?

The science behind this test is all about combustion. When wood burns, it needs three things: fuel (the wood), heat (from the glowing tip), and oxygen. Remove any one of these and combustion stops.

In normal air (21% oxygen), a glowing splint will eventually go out because there isn't enough oxygen concentration to sustain combustion. But in pure oxygen or oxygen-rich mixtures, there's plenty of oxygen molecules available. The glowing tip provides enough heat energy to start the combustion reaction again.

The key is oxygen concentration. Air contains about 21% oxygen, which isn't quite enough to relight a glowing splint. But if your gas sample contains 30% or more oxygen, you'll see that dramatic relighting effect.

4. Common Sources of Oxygen in Practicals

You'll encounter oxygen in several classic chemistry experiments. Knowing where it comes from helps you predict when to use this test.

The most common source is decomposing hydrogen peroxide with manganese dioxide catalyst: $2H_2O_2 \rightarrow 2H_2O + O_2$. The oxygen gas bubbles off and can be collected in a test tube for testing.

Electrolysis of water also produces oxygen at the positive electrode (anode): $4OH^- \rightarrow 2H_2O + O_2 + 4e^-$. You'll see bubbles forming that you can test with a glowing splint.

5. Exam Technique and Common Mistakes

Exam questions about gas tests are straightforward if you know the exact wording examiners expect. For oxygen, you must mention both the glowing splint AND the relighting.

**Common exam mistake**: Writing 'the splint lights up' instead of 'the splint relights'. The examiner wants to see you understand the splint was already glowing before being inserted into the gas.

The glowing splint test is reliable, quick, and requires no special equipment. It's been used by chemists for centuries because oxygen's role in combustion makes it uniquely detectable this way. Master this test and you'll never struggle to identify oxygen in your practicals.

Exam precision note: In Testing for Oxygen Gas - Test for oxygen - Identification of common gases, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

The oxygen test is a qualitative test. A glowing splint relights because oxygen supports combustion. In exam answers, the key point is the observation: the splint relights. Simply saying that oxygen burns is not precise enough because the splint is already glowing before the test is done.