Chlorine compounds are frequently found dissolved in public pools.
Chlorine is one of the more familiar chemical elements, the second heaviest of the halogens. Halogens, in general, are extremely reactive. Because of the reactivity of the halogens, chlorine gas is the only pure form of chlorine that is stable.
Considerations
Chlorine gas is extremely toxic, care must be taken whenever one is generating it. Other chloride compounds -- such as sodium chloride (table salt) -- are far friendlier, but often less interesting, chemically speaking. A fume hood or other closed system is a requirement for safe generation of pure chlorine. If chloride is sufficient, however (that is, a substance containing a stable form of the chemical, rather than the pure element itself), far fewer considerations need be taken.
Significance
While pure chloride gas is an extremely reactive compound, there are many other compounds in which chlorine (more properly, chloride) is found, generally in its ionic form. Many of these compounds are easily formed with simple lab methods. Other chloride-containing compounds should be found in most well-stocked laboratories. The presence of sodium chloride, potassium chloride, hydrochloric acid and bleach (sodium hypochlorite), as well as least one or two chlorate compounds, make routine experiments far easier to implement.
Methods
The reaction of sodium hydroxide and hydrochloric acid is perhaps the simplest (and cheapest) reaction known to generate chlorine gas. Combining the two creates chlorine gas and sodium chloride (common table salt) in aqueous solution (Dihydrogen monoxide is a byproduct of this reaction.). Be sure to conduct this reaction under properly protective conditions as the chlorine gas generated is highly toxic.
