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Laws of Chemical Combination

(i) Law of conservation of mass: In all physical and chemical changes, the total mass of the reactants is equal to that of the products. In other words, matter can neither be created nor be destroyed.



(ii) Law of constant composition or definite proportion: A chemical compound is-always found to be made up of the same elements combined together in the same fixed proportion by weight.

For example, pure water obtained from whatever source or any country will always be made up of only hydrogen and oxygen elements combined together in the same fixed ratio of 1:8 by weight.

(iii) Law of multiple proportions: When two elements combine to form two or more chemical compounds then the weights of one of elements which combine with a fixed weight of the other bear a simple ratio to one another.

e.g. the weights of nitrogen and oxygen forming compound N₂O, NO, N₂O₂, N₂O₄, N₂O₅, if we fix the weight of nitrogen as 14 then weight of oxygen bears a simple ratio 1 : 2: 3 : 4: 5 to one another.

(iv) Law of reciprocal proportions: The ratio of the weights of two elements A and B which combine separately with a fixed weight of the third element C is either the same or some simple multiple of the ratio of the weights in which A and 8 combine directly with each other. For example, CO₂, SO₂ and CS₂, H₂S, H₂O and SO2.

(v) Gay Lussac's law of gaseous volumes: When gases react together they always do so in volumes which bears a simple ratio to one another and to the volumes of the products, if these are also gases, provided all measurements of volumes are done under similar conditions of temperature and pressure. For example

N₂ + 3H₂ -> 2NH₃

1 vol. 3 vol. 2 vol

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