Show that water illustrates the law of constant proportions.
The law of constant proportion is also known as the law of definite proportion. This law, which was introduced by Proust, stated that ‘in a compound, the elements are always present in definite proportions by mass’.
For example, pure water obtained from any source (well, river, lake, or sea) and from any country (India, Russia, America, etc.) will always contain two hydrogen atoms and one oxygen atom. Hydrogen and oxygen respectively combine together in the ratio of 1:8 by mass to form water. The ratio by the number of atoms for water will always be H : O = 2 : 1. Thus, 18 g of water contains 2 g of hydrogen and 16 g of oxygen, or the ratio of masses of hydrogen to oxygen will always be 1:8.
(a) The elements present in each sample are hydrogen and oxygen only.
(b) The ratio of weight of hydrogen to the weight of oxygen is 1 8.
Hence, water illustrates the law of constant proportions.