Chlorine (symbol Cl) is a chemical element which can be found in the earth and the oceans. It is a greenish yellow gas at room temperature, and it is 2.5 times heavier than air. It turns to a liquid state at -34ºC (-29ºF), and it becomes a yellowish crystalline solid at -103ºC (-153ºF). Chlorine is highly chemically active and, as such, is typically found in nature bonded to other elements like sodium, potassium and magnesium.