At room temperature and normal atmospheric temperature, chlorine is a yellow-green gas that has a density heavier than air. Lowering the temperature to minus 29 degrees Fahrenheit or increasing the pressure converts it to a yellow liquid. Chlorine is highly reactive and does not exist as a pure compound in nature.
Chlorine gas consists of molecules of chlorine, which are two chlorine atoms chemically bonded to one another. When exposed to any element other than noble gases, chlorine reacts to form a chloride. If the other element is a metal, the chloride is generally crystalline in nature.
The human nose is able to detect chlorine at levels above 3.5 parts per million. When the concentration in air is above 1,000 parts per million, the gas causes death to humans in a matter of seconds. It is a respiratory irritant for mucous membranes. Chlorine causes burns on the skin. The military used chlorine as a war gas in 1915.
Sodium chloride or table salt is the most abundant source of chlorine. Laboratories often use chlorine in the form of hydrochloric acid. Consumers use chlorine in the form of laundry bleach. In industry, it bleaches pulp in paper production. Municipal water suppliers use chlorine to disinfect drinking water.