OxygenOxford AQA IGCSE Chemistry: Revision notes

Section 1

What is the test for oxygen gas?

To test whether a gas is oxygen, insert a glowing splint into the test tube or container holding the gas. If the gas is oxygen, the glowing splint relights, bursting back into flame. This is the standard confirmatory test for oxygen used throughout chemistry.

Key termsoxygenglowing splint

Section 2

Why does a glowing splint relight in oxygen?

Combustion needs a fuel, heat, and oxygen. A glowing splint still has enough heat energy stored in its smouldering tip to reignite, but in normal air the oxygen concentration (about 21%) is often too low to sustain that reignition once the flame has gone out. In a test tube of pure or concentrated oxygen, the much higher oxygen concentration provides enough oxygen for the smouldering wood to burst back into flame.

Think of it like this

Think of the glowing tip as a fire that is starved of oxygen rather than out of fuel - give it a rush of extra oxygen and it flares back up immediately.

Section 3

How is the test carried out in practice?

Light a wooden splint, then blow it out so only the tip glows red (no visible flame).
Quickly insert the glowing splint into the mouth of the test tube or container holding the gas sample.
Observe what happens to the splint.
If the splint relights and bursts into flame, the gas present is oxygen.

Common mistake

A common error is waiting too long before inserting the splint - if it has fully stopped glowing (gone cold and black), no test result will be seen even in pure oxygen.

Section 4

How does this compare with other common gas tests?

Oxygen is one of several gases identified with simple, quick tests using flames or colour changes.

Gas	Test	Positive result
Oxygen	Glowing splint	Splint relights
Hydrogen	Lit splint	Squeaky pop
Carbon dioxide	Bubble through limewater	Limewater turns cloudy white

These tests are distinguished by what triggers the observation: oxygen relights a glowing splint, hydrogen produces a sound with a lit splint, and carbon dioxide is tested with limewater rather than a splint at all.

Key termslimewater

Must Know

The test for oxygen gas: insert a glowing splint into the gas.
A positive result is that the glowing splint relights.
The splint must still be glowing (not fully out) for the test to work.
Oxygen supports combustion, which is why a higher concentration of it reignites the smouldering splint.
Do not confuse this test with the hydrogen test (squeaky pop with a lit splint) or the carbon dioxide test (limewater turns cloudy).

That's the notes covered.

Carry on to the next subtopic.