AQA GCSE Chemistry (8462) — Chemical analysis
Have you ever noticed the sharp, distinct smell at a public swimming pool? That scent is associated with chlorine, a highly reactive and toxic gas. In the laboratory, we cannot rely on smell or sight alone to identify gases; we need a reliable chemical test to confirm their presence safely.
Testing for chlorine requires a specific step-by-step method due to the hazards involved:
When the damp blue litmus paper is exposed to chlorine gas, you will see a rapid, two-stage colour change:
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Students often just write 'turns white' and lose marks — AQA 'describe' questions require the full sequence: state that the damp blue litmus paper turns red, then white.
The word 'damp' is a critical marking point; if you forget to write that the litmus paper is damp, you will not get the mark for the procedure.
Do not state that chlorine 'smells like a swimming pool' or is a 'pale green-yellow gas' when asked for a chemical test, as physical characteristics do not score marks for chemical identification tests.
Damp blue litmus paper
Blue litmus paper moistened with water to allow gases to dissolve, enabling a chemical reaction and colour change.
Bleaching
The chemical process by which a substance permanently removes the colour from a material.
Chlorine
A highly reactive, pale green-yellow gas that is toxic and has a sharp, choking smell.
Toxic
Poisonous or harmful to health, requiring specific safety precautions when handled in the laboratory.
Fume cupboard
A well-ventilated enclosure used in laboratories to safely contain and extract hazardous gases.
Put your knowledge into practice — try past paper questions for Chemistry
Damp blue litmus paper
Blue litmus paper moistened with water to allow gases to dissolve, enabling a chemical reaction and colour change.
Bleaching
The chemical process by which a substance permanently removes the colour from a material.
Chlorine
A highly reactive, pale green-yellow gas that is toxic and has a sharp, choking smell.
Toxic
Poisonous or harmful to health, requiring specific safety precautions when handled in the laboratory.
Fume cupboard
A well-ventilated enclosure used in laboratories to safely contain and extract hazardous gases.
