Law of Definite Proportions

Dalton studied the composition of several compounds and found that irrespective of the sample of a particular substance, the proportion by weight of its elementary constituents remained the same. Thus, water taken from any source is composed of hydrogen and oxygen. The proportion of hydrogen to oxygen (by weight) in water is always 1:8.

It was also observed by Dalton that the constituents can combine in several ways to form different substances.

John Dalton

These substances possess different properties. For example, hydrogen and oxygen combine in two different ways to produce water (H2O) and hydrogen peroxide (H2O2).

In water, 1 g of hydrogen combines with 8 g of oxygen. In hydrogen peroxide, 1 g of hydrogen combines with 16 g of oxygen. The ratio of oxygen combining with the same amount of hydrogen (i.e. 1 g) is 8 : 16 or 1 : 2 between water and hydrogen peroxide - a small integral ratio. Dalton called this the Law of Multiple Proportions. We can define it as - "When two elements X and Y combine to form more than one compound, the masses of X which combine with a fixed mass of Y are in simple ratio".

Let us consider another example, carbon combines with oxygen to form two oxides of carbon, namely, carbon monoxide (CO) and carbon dioxide (CO2).

In carbon monoxide, 12 g of carbon combines with 16 g of oxygen. In carbon dioxide, 12 g of carbon combines with 32 g of oxygen. The ratio of the masses of oxygen which combines with a fixed mass of carbon is 16:32, i.e. 1:2, which is a simple ratio.

Law of Definite Proportions states that the proportion in which two or more elements combine in forming a particular compound is always identical

Zinc + Sulphur Zinc sulphide
Zn + S ZnS

This proportion will remain the same no matter how zinc sulphide is formed.