How do you make chlorine?

Chlorine occurs naturally but not in its elemental (gas) form (as Cl₂). It is produced by starting with three raw materials - salt, water and electricity. An electric current is sent through brine, a solution of salt in water, in a process called electrolysis, which has been known for over 125 years. Chlorine production plants are high-tech electrochemical facilities built with state-of-the-art technology for maximum safety and environmental protection using different  production processes - nowadays usually diaphragm or membrane.

As well as getting chlorine gas from electrolysis, you also get caustic soda (sodium hydroxide) or caustic potash (potassium hydroxide) as a liquid and hydrogen. All are extremely useful chemicals that are used in hundreds of vital end products.