Test for chlorine: identification and procedure

Chemical analysis • Identification of common gases

Flashcards

Test your knowledge with interactive flashcards

Which chemical reaction with water explains the initial red colour change?

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Cl2 + H2O ⇌ HCl + HClO forming acidic species

Key concepts

What you'll likely be quizzed about

Nature and appearance of chlorine

Chlorine is a toxic, pale greenish-yellow gas at room temperature that irritates the eyes, throat and lungs. Chlorine dissolves slightly in water and reacts with water to form acidic and oxidising species that cause characteristic reactions with indicators and dyes. The faint green colour may be hard to see at low concentrations. Visual observations therefore rely on reactions with moist indicators rather than colour alone.

Step-by-step test using damp blue litmus paper

A strip of blue litmus paper is dampened and held at the mouth of the container or near the gas source without directly inhaling the gas. Chlorine gas first turns the damp blue litmus paper red and then bleaches the red colour to white. The two distinct observations-red then white-provide diagnostic evidence. Moisture on the paper is essential for the initial acid-forming step; dry paper does not show the same sequence.

Chemical cause: hydrolysis and oxidation

Chlorine reacts with water to form hydrochloric acid (HCl) and hypochlorous acid (HClO) via Cl2 + H2O ⇌ HCl + HClO. The formation of acidic species causes the damp blue litmus paper to turn red. Hypochlorous acid (and related chlorine oxidants) oxidises the coloured dye in litmus, causing bleaching to white. The oxidising action explains the second, irreversible bleaching observation.

Limitations and interfering factors

Low chlorine concentration produces a slow or faint effect; weak exposure may show only slight discolouration rather than clear bleaching. Excess moisture, contaminated paper, or pre-existing strong acids on the paper can obscure the initial red colour. Other strong oxidising gases or bleaching agents can also destroy dyes and give a bleaching effect. Careful interpretation of both the red then white sequence and context reduces false positives.

Safety and practical considerations

Chlorine gas is toxic and irritant. Experiments occur in a fume cupboard or well-ventilated area and exposure is avoided. Handling uses minimal quantities and indirect testing (holding damp litmus at a distance) to reduce inhalation risk. Direct breathing from equipment or enclosed spaces is hazardous. Appropriate eye protection and gloves are standard precautions when preparing or testing for chlorine.

Key notes

Important points to keep in mind

Damp blue litmus paper turns red then bleaches white in the presence of chlorine.

Moisture is essential: dry paper does not show the initial red change.

Hydrolysis (Cl2 + H2O ⇌ HCl + HClO) causes the acid effect; HClO causes bleaching.

Low chlorine concentrations give slow or faint changes.

Other oxidising gases can bleach dyes and cause false positives.

Work in a fume cupboard or well-ventilated area because chlorine is toxic.

Avoid direct breathing from the test apparatus; hold indicator paper at a safe distance.

Visible greenish-yellow colour supports identification but is unreliable at low concentrations.

Contaminated or pre-acidified paper can obscure the initial colour change.

Record both the sequence and timing of observations for accurate identification.