Identification of common gases

The test for hydrogen uses a lit flame to produce a distinctive sound.

• •Hold a burning splint near the open end of a test tube containing the gas.
• •If hydrogen is present, it burns rapidly with a characteristic 'squeaky pop' sound.
• •The squeaky pop sound is caused by hydrogen reacting rapidly with oxygen to form water.

The test for oxygen uses an embers-only splint to demonstrate combustion.

• •Light a wooden splint and blow it out so it is only glowing at the tip.
• •Insert the glowing splint into a test tube containing the gas sample.
• •If oxygen is present, the glowing splint relights into a flame.

The test for carbon dioxide uses calcium hydroxide solution to form a precipitate.

• •Bubble the gas sample through or shake it with an aqueous solution of limewater.
• •If carbon dioxide is present, the limewater turns cloudy (or milky).
• •The cloudy appearance is due to the formation of a fine white precipitate of insoluble calcium carbonate.

The test for chlorine relies on its strong bleaching properties.

• •Place a piece of damp litmus paper into the test tube containing the gas.
• •If chlorine is present, the litmus paper is bleached and turns white.
• •The litmus paper may turn red briefly before bleaching because chlorine forms an acidic solution with water.