Chemical stability refers to the relative ease with which a compound or mixture of compounds undergoes a chemical change. For example, consider two different mixtures of gases: the first is a mixture of equal parts of helium and oxygen; the second is a mixture of two parts hydrogen to one part oxygen. These samples are very similar in color, density and volume. However, the first sample is chemically very stable and will not undergo chemical change even on strong heating. The second sample (hydrogen and oxygen) is chemically unstable, and explodes violently if a spark or lighted match is applied. The reaction of these two gases to produce water is strongly exothermic, and once the initial activation energy is supplied the reaction proceeds rapidly to completion.

2 H2   +   O2     2 H2O   +   Energy