Higher and foundation tiers
Carbon dioxide is a colourless odourless gas. It is produced mainly in the science lab by the reaction of metal carbonates with acids. To test for carbon dioxide gas uses a limewater (calcium hydroxide) solution. Carbon dioxide gas will turn a limewater solution milky or chalky in colour. Limewater is an alkaline solution and carbon dioxide is an acidic gas. The carbon dioxide gas react with the limewater solution to produce a cloudy suspension of calcium carbonate which is responsible for the milky or chalky colour.
Oxygen gas is needed for burning or combustion. In air if a burning splint is blown out but the end still glows it will not relight, however if a glowing splint is pushed into a test-tube of pure oxygen gas it immediately bursts back into flame.
Mixtures of hydrogen and oxygen gas are explosive. This explosion is used to test for the presence of hydrogen gas. If a burning splint is placed just above the mouth of a test-tube containing hydrogen gas then a squeaky pop will be heard as the hydrogen gas explodes in air.
Chlorine gas will bleach damp litmus paper white. The litmus paper may turn red at first but this will quickly fade. Chlorine dissolves in the water on the damp litmus paper to form an acid, this explains why it initially turns red.