12.9 - Tests for Gases

Tests for the common gases

Hydrogen

• Hold a lit splint at the open end of a test tube of the gas.
• Hydrogen burns quickly with a squeaky pop.

Oxygen

• Insert a glowing splint into a test tube of the gas.
• If the gas is oxygen, the splint relights.

Carbon dioxide

• Bubble the gas through limewater (a colourless solution of calcium hydroxide).
• If the gas is carbon dioxide, the limewater turns cloudy (milky) as a white precipitate of calcium carbonate forms.

Chlorine

• Chlorine has a sharp, choking smell, but it is toxic, so an unknown gas is never smelled deliberately.
• Hold damp blue litmus paper in the gas.
• Chlorine bleaches the paper white (it may turn red briefly first because chlorine is acidic in solution).
• Damp universal indicator paper can be used instead: it turns red and is then bleached white.
• The paper must be damp so the gas can dissolve in the water.

The test for ammonia

• Hold damp red litmus paper in the gas.
• Ammonia dissolves in the water on the paper to form an alkaline solution, turning the paper blue.
• The paper must be damp so the ammonia can dissolve.

The test for sulfur dioxide

• Bubble the gas through acidified aqueous potassium manganate(VII) solution.
• The solution changes from purple to colourless.
• Sulfur dioxide reduces purple manganate(VII) ions (MnO₄^-) to manganese(II) ions (Mn²⁺), which are colourless in dilute solution.

Tests for water

A chemical test for water uses an anhydrous salt (one containing no water) that changes colour when water is added.

Anhydrous copper(II) sulfate

• Add the liquid to white anhydrous copper(II) sulfate.
• If water is present, the powder turns blue.

Anhydrous cobalt(II) chloride

• Add the liquid to blue anhydrous cobalt(II) chloride paper (or powder).
• If water is present, it turns pink.

Testing for purity

• These tests show only that water is present.
• They do not show that the water is pure: any aqueous solution will also give a positive result.