When simple or complex substances interact with each other, they usually let us know about it.

   Drop a piece of zinc into a solution of sulphuric acid. Immediately gas bubbles begin to rise from it and after some time the metal disappears. The zinc dissolves in the acid, liberating hydrogen. You could see for yourself how it all happened.

   Or light a lump of sulphur. It burns with a bluish flame and you can smell the asphyxlating odour of sulphur dioxide, the chemical compound which forms when sulphur combines with oxygen.

   Moisten anhydrous copper sulphate CuSO₄, a white powder, with water and it immediately turns blue. The salt combines with the water to form crystals of blue vitriol CuSO₄· 5H₂O. Substances of this kind are called crystal hydrates.

Do you know what quenching of lime is? Water is poured on quicklime and the result is slaked lime Ca(OH)₂. Though the substance does not change colour, it can easily be seen that a reaction has occurred, because when lime is quenched a great deal of heat is liberated.

   The primary and invariable condition of all chemical reactions is that they are accompanied by the liberation or absorption of thermal energy. Sometimes so much heat is liberated that it can readily be felt. When the amount of heat evolved is small, special methods of measurement are used. 

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