Can the chemical composition of water be changed? will introducing chemicals change it's composition?

Composition? What do you mean? There will be an equilibration with ambient gas concentrations (Oxygen and Carbon Dioxide mainly). If Chlorination has been used, then Chlorine gas will slowly escape. If humans have been in the water then urine and other bodily substances will affect the composition of the water. The chlorination level, in particular, will be affected by reaction with Urine.