Test for Chlorine

Identifying Chlorine Gas

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.

The Procedure

Testing for chlorine requires a specific step-by-step method due to the hazards involved:

Perform the test in a fume cupboard or well-ventilated area, because chlorine is a toxic and poisonous gas.
Take a piece of damp blue litmus paper and hold it securely using tweezers.
Place the paper into a sample of the gas (for example, directly at the mouth of a test tube).

The Observations

When the damp blue litmus paper is exposed to chlorine gas, you will see a rapid, two-stage colour change:

First, the blue litmus paper turns red briefly.
Then, the paper undergoes bleaching and turns completely white.
A key negative feature to remember is that a dry piece of litmus paper does NOT work for this test; without moisture, no colour change will occur.

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Exam Tips

Common Mistake
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.
2
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.
3
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.

Key Terms(5)

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.

Previous Subtopic: Test for Carbon DioxideTest for Carbon Dioxide Next Topic: Ion IdentificationFlame Tests

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Key Terms(5)

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.

Test for Chlorine

Identifying Chlorine Gas

The Procedure

Testing for chlorine requires a specific step-by-step method due to the hazards involved:

Perform the test in a fume cupboard or well-ventilated area, because chlorine is a toxic and poisonous gas.
Take a piece of damp blue litmus paper and hold it securely using tweezers.
Place the paper into a sample of the gas (for example, directly at the mouth of a test tube).

The Observations

When the damp blue litmus paper is exposed to chlorine gas, you will see a rapid, two-stage colour change:

First, the blue litmus paper turns red briefly.
Then, the paper undergoes bleaching and turns completely white.
A key negative feature to remember is that a dry piece of litmus paper does NOT work for this test; without moisture, no colour change will occur.

Sign up to continue reading

Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
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.
2
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.
3
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.

Key Terms(5)

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.

Previous Subtopic: Test for Carbon DioxideTest for Carbon Dioxide Next Topic: Ion IdentificationFlame Tests

Back to Test for Chlorine

Put your knowledge into practice — try past paper questions for Chemistry

Key Terms(5)

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.