While investigating the quantitative relations between substances in chemical reactions, scientists formulated the three basic laws of chemical combination. These are:

The law of conservation of mass: In 1774, the French chemist Antoine Lavoisier performed an experiment in which he heated a sealed glass container that held a sample of tin in air. He measured the mass of the substances before and after heating and found them to be the same. This and other similar experimental observations became the basis of the law of conservation of mass.

This law is also known as the law of indestructibility of matter and shown as:

Mass of reactants = Mass of products.

There is no loss or gain of substances during a chemical reaction, and mass is conserved.

For example, consider the decomposition of mercury(II) oxide. When 100 g of mercury(II) oxide decomposes by heat, 92.6 g of mercury and 7.4 g of oxygen are formed. Note that the total mass of mercury and oxygen after decomposition is 100 g:

The law of definite proportions: The law of definite proportions states that a compound always contains the same elements in the same proportion by mass. This means that all pure samples of a compound have the same composition regardless of the source of sample. This law is also known as the law of constant composition. For example, sample of water could be obtained from different sources, such as from a river, the ground, or the ocean. But whatever the original source, all forms of pure water contains 11.2% hydrogen and 88.8% oxygen by mass. These percentages represent a ratio of 1.0 to 8.0 (1:8), by mass, of hydrogen to oxygen.

This ratio is constant (fixed) for water. In other words, a compound with a different ratio of hydrogen and oxygen is not water.

Similarly, in forming the compound ZnO, 65.0 g of zinc combines with 16.0 g of oxygen. This is 80.2% zinc and 19.8% oxygen, by mass.

The law of multiple proportions: The law of multiple proportions states that when two elements combine to form more than one compound, the masses of one element combined with a fixed mass of the second element are in the ratio of small whole numbers. This law can be illustrated by the two oxides of carbon. The two oxides of carbon are carbon monoxide (CO) and carbon dioxide (CO₂). In CO₂, 1.0 g of carbon is combined with 2.67 g of oxygen; whereas in CO, 1.0 g of carbon is combined with 1.33 g of oxygen.