Potassium
Potassium reacts in many chemical reactions similar to sodium and other alkali metals. It tarnishes in air to produce a whitish oxidized layer on the surface. This is why it is stored in oil. It also reacts very fast with water, which is another reason for its storage in oil. The hydrogen produced during its reaction with water can burst into flames when a large amount of potassium is added to water. Potassium hydroxide is also produced. Potassium also burns in air easily, to make the peroxide or the superoxide.