Tests for gases

Different chemical reactions can be used to test for common gases.

Hydrogen

Hydrogen is produced by many different chemical reactions:

Hydrogen is less dense than air, so it is usually collected by upward delivery.

Pure hydrogen burns with a colourless or pale blue flame. When the test for hydrogen is carried out, the hydrogen needs to be mixed with a little air.

Test for hydrogenResult
Place a lighted splint in a test tube of the gasGas burns with a squeaky 'pop'

Oxygen

Oxygen is usually made by the decomposition of hydrogen peroxide. It is also commonly seen as one of the products from electrolysis of salt solutions.

Test for oxygenResult
Place a glowing splint in a test tube of the gasThe splint relights

Chlorine

Chlorine is usually seen as one of the products from electrolysis of chloride salt solutions. Litmus paper is used to test for chlorine. There are two types of litmus paper:

  • red litmus, usually used to test for alkalis
  • blue litmus, usually used to test for acids

The test for chlorine can use either type of litmus paper, but blue litmus paper is used most commonly. The litmus paper must be damp - the water dissolves some of the chlorine so that it can react with the indicator on the litmus paper.

Test for chlorineResult
Place damp blue litmus paper in a test tube of the gasThe blue litmus paper is bleached

This test shows that chlorine is a powerful bleach.

An alternative test for chlorine is to place damp starch-iodide paper into a test tube of chlorine. A positive result for chlorine is indicated by the paper turning from white to blue-black.

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