What clear, colorless chemical might be found in drinking water?

In drinking water the clear, colorless chemical chlorine might be found. The chlorine is added to drinking water to disinfect it and kill germs. The process is called Water Chlorination.  Chlorine is a highly efficient disinfectant. Its purpose in drinking water is to kill bacteria, viruses, and protozoans, that commonly grow in water supply reservoirs, on the walls of water mains and in storage tanks. 

D. most models represent a real world object in a simpler way