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We have frequently used the terms stable and unstable to denote whether a given compound is with difficulty resolved into its elements, or into related compounds, or whether this change takes place easily and spontaneously. Thus, water and hydrogen chloride are stable compounds: they show no tendency to decompose spontaneously into their elements, or into other compounds of these. The oxides of chlorine, ozone and hydrogen peroxide, on the other hand, are all unstable substances, decomposing spontaneously, or when heated, or when brought in contact with other substances. There are also different degrees of stability, e.g., the stability of perchloric acid is greater than that of hypochlorous acid.
In general, a substance formed with considerable evolution of energy will be stable, whilst a compound formed with considerable absorption of energy will be unstable. The stability is roughly in proportion to the amount of energy evolved in formation; thus, perchloric acid in solution is more stable than either hypochlorous acid or chloric acid, although hypochlorous acid is less stable than the other two. Aqueous perchloric acid is formed with considerable evolution of heat, and is quite stable.
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