The law of multiple proportions states that if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers.

Examples of this law include →i) Hydrogen combines with oxygen to form two compounds, water and hydrogen peroxide.

2H₂ + O₂→ 2H₂O

H₂ + O₂→ H₂O₂

In this example, the ratio of masses of oxygen which combine with the fixed mass of hydrogen is 16:32 or 1:2, which is a simple whole number ratio.

→ii) Carbon and oxygen combine to form carbon dioxide and carbon monoxide.

C + O₂→ CO₂

2C + O₂→ 2CO

In this example, the ratio of masses of oxygen which combine with the fixed mass of carbon is 16:32 or 1:2, which is a simple whole number ratio.

The law and the examples of the law show that in the formation of a compound, there are constituents which combine in a fixed proportion. The constituents may be atoms. Thus, this law confirms the existence of atoms which combine to form molecules and compounds.