When chlorine is passed into water and subsequently exposed to sunlight, the gas evolved is?

Explanation

When chlorine gas (Cl₂) is dissolved in water, it reacts to form a mixture of two acids. The reaction is:

Cl₂ + H₂O → HCl + HOCl

HCl is hydrochloric acid and HOCl is hypochlorous acid (also called chloric (I) acid). The HOCl is unstable, which means it breaks down easily.

When this chlorine water is exposed to sunlight, the ultraviolet light from the sun causes the HOCl to decompose. The decomposition reaction is:

2HOCl → 2HCl + O₂

The sunlight provides enough energy to break down the hypochlorous acid into hydrochloric acid and oxygen gas (O₂). The oxygen gas is released as bubbles from the solution.

So the overall effect is that sunlight causes chlorine water to lose its oxidizing power over time as the HOCl breaks down. This is why chlorine water should be stored in dark or amber bottles to prevent decomposition. The gas evolved when exposed to sunlight is oxygen (O₂).