SPM Chemistry

Reaction of Halogens with Water

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Chlorine + Water

Cl2 + 2H2O → HCl + HOCl

Observation:

  1. Chlorine gas dissolves in water to form a pale yellow solution.
  2. The solution turn blue litmus paper red before it is bleached.

Discussion:

  1. The solution formed is acidic and contains bleaching agent.
  2. Hypochlorous(I) acid is a strong bleaching agent. It decolourises the colour of litmus paper.

    Bromine + Water

    Br2 + 2H2O → HBr + HOBr

    Observation:

    1. Bromine liquid dissolves slowly in water to form a yellowish-brown solution.
    2. The solution turn blue litmus paper red before it is bleached slowly.

    Discussion:

    Hypobromous(I) acid is a weak bleaching agent.

    Iodine + Water

    I2 + 2H2O → HI + HOI

    Observation:

    1. Only a little iodine dissolves in water to form a yellowish solution.
    2. The solution turns blue litmus paper red but does not bleach it.

    Discussion:

    Hypoiodous(I) acid has very weak bleaching characteristic.

    Important Note

    1. Chlorine, bromine and iodine are soluble in water to form an acidic solution.
    2. The solubility decreases down the group.
    3. Aqueous chlorine and bromine are a bleaching agent.
    4. Aqueous iodine does not act as a bleaching agent.
    5. Chlorine gas is produced through the following reaction:

    2KMnO4 + 16HCl → 2KCl + 2MnCl2 + 5Cl2 + 8H2O