Main composition is calcium hypochlorite bleaching powder, calcium hypochlorite with CO2 and water vapor in the air to generate hypochlorous acid and calcium carbonate (the reaction of bleaching powder is in fact the work), if you don't use will be photolysis (long dark also reacts to) produce the oxygen and hydrogen chloride gas (or hydrochloric acid), then failed

The main components of bleaching powder are calcium hypochlorite and calcium chloride, the active component is calcium hypochlorite. The bleaching principle of bleaching powder is the reaction of calcium hypochlorite and acid to produce bleach material hypochlorite:

Ca(ClO)2＋2HCl(rare)＝CaCl2＋2HClO 

Ca(ClO)2＋CO2＋H2O＝CaCO3↓＋2HClO 

Hypochlorous acid is a weak acid that can oxidize pigments and thus achieve bleaching purposes.

The effective component of bleaching powder is calcium hypochlorite, and the hypochlorite acid generated by acid has bleaching effect

Failure is hypochlorite calcium acid acid generated hypochlorite, and decomposition.

Ca(ClO)2+CO2+H2O=CaCO3+HClO(Strong acid to weak acid)
2Hclo=2Hcl+O2↑

Without hypochlorite, of course, it's no longer effective.