Identification of Gases

In the lab, simple tests help you recognise common gases. You use small samples, litmus paper, limewater, and wooden splints. Observations are quick and clear when done correctly.

Key gas tests

• Ammonia (NH3): Turns damp red litmus paper blue. Ammonia is alkaline.
• Carbon dioxide (CO2): Turns limewater milky (white). This forms solid calcium carbonate.
• Chlorine (Cl2): Bleaches damp litmus paper (turns it white). Chlorine is a pale green, toxic gas.
• Hydrogen (H2): Burns with a ‘pop’ when tested with a lighted splint.
• Oxygen (O2): Relights a glowing splint. Oxygen supports combustion.
• Sulfur dioxide (SO2): Turns acidified aqueous potassium manganate(VII) from purple to colourless. SO2 is a reducing gas.

Why use damp litmus?

Gases must dissolve in a thin film of water to affect the dye. “Damp” means slightly moist, not dripping.

Helpful reactions

Limewater test for carbon dioxide:

$$\mathrm{CO_2 + Ca(OH)_2 \rightarrow CaCO_3\downarrow + H_2O}$$

Safe testing

• Use small amounts of gas at the mouth of the test tube.
• Do not smell gases directly; waft air gently toward your nose if asked.
• Keep chlorine and sulfur dioxide away from your face; they are harmful.