In the treatment of domestic water supplies, chlorine is added to the water to form chloric(i) acid, hclo.

cl2(aq) + h2o(i) → h+(aq) + cl–(aq) + hclo(aq)

the hclo reacts further to give the chlorate(i) ion.(clo-)

hclo(aq) + h2o(i) → h3o+(aq) + clo–(aq)

both hclo and clo–kill bacteria by oxidation.

what is the change in oxidation number of chlorine in forming the chlorate(i) ion from the aqueous chlorine?

show working : )

Chlorine goes from Cl₂ to ClO⁻.

Oxidation state of Cl in Cl₂ is zero whereas in ClO⁻ it is +1. Hence change in oxidation state of Chlorine is 1.

Calculation of oxidation number of Cl in ClO⁻ or HClO⁻ :
Let x be oxidation number of Cl in ClO⁻ the. Now since the net charge on ClO⁻ is -1, sum of oxidation number of all must be equal to -1. 

Therefore,
x + (-2) = -1 [ oxidation number of O is -2]
∴ x = 2-1 = +1
Therefore oxidation number of Cl in ClO⁻ is +1

Abbreviated as mg-25.