Test for oxygen: The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.

The idea

Oxygen is identified by its effect on a glowing splint. Light a splint, blow it out so it is still glowing, then insert the glowing splint into a test tube containing the gas. In oxygen, the splint relights because oxygen supports combustion. The test works because the gas is an oxidising agent and allows the wood to burn more vigorously. Do not use a burning splint, because that tests for hydrogen and gives a pop. Do not confuse 'relights' with 'burns more brightly'; the expected observation is that the glowing splint relights. The test is assessed by describing the correct procedure and the positive result, and by distinguishing oxygen from hydrogen, which gives a pop with a burning splint.

What earns the marks

• A glowing splint is inserted into a test tube containing the gas.
• The splint relights in oxygen.
• The splint must be glowing, not burning, before it is inserted.
• Oxygen supports combustion, which is why the glowing splint relights.
• The test is positive only for oxygen; hydrogen gives a pop with a burning splint instead.

Common mistakes and how to put them right

• Using a burning splint instead of a glowing splint.

  Put it right: A burning splint tests for hydrogen; for oxygen the splint must be glowing.

• Saying the splint 'burns brightly' rather than 'relights'.

  Put it right: The expected observation is that it relights.

• Inserting the splint without first making it glow.

  Put it right: A cold splint will not relight; light it and blow it out until it glows.

Exam tips

• State both the procedure and the result: 'Insert a glowing splint into the test tube; the splint relights in oxygen.'.
• Use the word 'glowing' rather than 'burning' when describing the splint for the oxygen test.
• If asked to compare tests, contrast the glowing splint relighting for oxygen with the burning splint pop for hydrogen.