If I understand well your question, you would like to know why the bleach solution loses activity even without use. If was it, I believe the answer below would explain .
The bleach solution based on sodium hypochlorite is obtained by absorption of chlorine in sodium hydroxide solution. This reaction is at equilibrium as follows:
2 NaOH + Cl2 NaOCl + H2O
As the Cl2 is a gas if there is an exchange with the environment, chlorine is slowly released into the atmosphere, so after some time in contact with air, the bleach solution even without being used as such, loses its oxidizing action.
Thankyou very much that's exactly what I wanted to know, but is there an equation that shows how oxygen reacts with hypochlorite? I believe oxygen oxides hypochlorite so what would the equation be?
In fact Hypochlorite do not react with oxygen, it can decompose to produce oxygen, see this and other ways in which hypochlorite can decompose. Decomposition will occur due to the following two reactions:
Transformation into chlorate:
Release of oxygen:
In a good quality sodium hypochlorite solution, the chlorate decomposition pathway accounts for about 90 % of the total decomposition.
Increasing temperature, hypochlorite concentration and ionic strength (salt content) will increase the reaction rate of both reactions to about the same extent. UV-light(sunlight) also catalyzes both decomposition reactions.