Law of Definite Proportions

In a reaction where hydrogen gas is burned to produce water, two hydrogen atoms combine with one oxygen atom. Hydrogen gas and oxygen gas always react in a ratio of 2:1 to produce water.

In this way, when two or more substances react to form new compounds, there is a constant quantitative relationship between the reactants. This is called the 'Law of definite proportions.' This law can explain that two or more kinds of atoms are created by combining simple integer ratios when a compound is produced.