Chlorine gas is a very useful oxidant, which was first introduced as a toxic weapon by the German Army. Even today, it's still used as a weapon, most recently in the Iraq War by insurgents. But chlorine gas has more useful (and less lethal) applications, and if you want to learn how to make some at home, NurdRage has the answers.
Warning: Chlorine gas is toxic and this should only be performed by an experienced chemist in a fume hood.
The more modern method is to mix pool chlorinator with the appropriate amount of hydrochloric acid. Pool chlorinator usually consists of calcium hypochlorite, trichloroisocyanuric acid, or sodium dichloroisocyanuric acid. Four mole equivalents are needed for the calcium hypochlorite, and three mole equivalents are needed for the isocyanuric acid derivatives.
Be sure to use appropriate gas collection equipment to collect the chlorine gas.
Sometimes hydrochloric acid isn't available and this can be substituted with an equimolar mixture of sodium chloride salt and sodium bisulfate. Sodium bisulfate should also be available as a pH lowering chemical sold alongside the pool chlorinator, if hydrochloric acid isn't also sold.
Remember, you still need to use the correct stoichiometry of hydrochloric acid to pool chlorine.
To test for chlorine gas, a mixture of 0.1g of starch and 0.5g of potassium iodide in 20mL of water can be used. Filter strips dipped in the solution will turn blue or brown in the presence of chlorine.
NOTICE: There is a major equation error in the middle of the video, but it's now covered up with annotations of the correct equation.