Chlorine, a dense, greenish-yellow gas, was discovered in 1774. It has a choking smell and is very poisonous. It occurs as the chloride ion, Cl^-, in sea water and salt lakes, from which sodium chloride is recovered by solar evaporation.

Commercially, chlorine is prepared by the electrolysis of aqueous sodium chloride obtained from seawater or salt deposits. Another process for the production of chlorine is the Chloralkali Process.

Common usage of chlorine includes:

• the production of polyvinyl chloride, PVC.
• air-conditioner refrigerants, lubricants, plastic foams, insecticides, and aerosol propellants. These are organic compounds containing fluorine and chlorine, and are called chlorofluorocarbons (CFCs).
• household bleaching agents which contain chlorine and hypochlorites (OCl^-). Sodium hypochlorite or calcium hypochlorite are known as "bleaching powders.
• the control of the quality of the water in swimming pools. Both algae and bacterial growth must be controlled, and the pH should be kept near 7. The algae and bacteria are normally controlled by the use of hypochlorous acid (HOCl). In large public pools, "chlorination" is achieved by dissolving chlorine (Cl₂) gas in the water as it is circulated through the filter system. In domestic pools, the bleaching powder, calcium hypochlorite is used.

It should be noted that manufacturers and suppliers of chemicals refer to solid hypochlorites and other chlorinating agents as "chlorine". Chlorine is a greenish-yellow gas, not a white powder.

Take a look at the Chlorine Chemistry Web site.