Chlorine is a greenish-yellow gas with a sharp, harsh smell that's used to kill germs in swimming pools and drinking water. You've probably smelled it at a public pool: that distinct “pool smell” comes from chlorine reacting with substances in the water.

Chlorine is incredibly useful because it destroys bacteria, viruses, and other microorganisms that could make people sick. Cities add small amounts of chlorine to tap water to keep it safe to drink. Pool owners add it to pool water to prevent swimmers from spreading illnesses to each other. Before chlorine treatment became common in the early 1900s, diseases spread through contaminated water killed thousands of people every year.

As a pure element, chlorine is dangerously toxic, which is why it must be handled carefully and used in precise amounts. In nature, chlorine doesn't exist by itself: it bonds with other elements to form compounds. Table salt, for instance, is sodium chloride, a compound of sodium and chlorine that's safe to eat.