Why does tap water sometimes smell and taste like chlorine?

Effective disinfection of tap water is absolutely necessary to prevent the recurrence of potentially fatal infectious diseases. Chlorine is injected into the water at very low levels (safe for your health) to prevent it being contaminated by pathogenic viruses or bacteria between the production site and your tap. 

A litre of tap water can contain a maximum of a quarter of a milligram of chlorine. 

As chlorine evaporates in the open air (and not in the pipes), its taste disappears very quickly if the water is left to stand in an open jug.