reactivity
Reactivity is the ability of matter to combine chemically with other substances. Some kinds of matter are extremely reactive; others are extremely unreactive. For example, potassium is very reactive, even with water. When a pea-sized piece of potassium is added to a small amount of water, it reacts explosively. You can observe this reaction in the video at the URL below. Caution: Dont try this at home! In contrast, noble gases such as helium almost never react with any other substances.