Law of Definite Proportions
In a reaction where hydrogen gas is burned to produce water, two hydrogen atoms combine with one oxygen atom. That is, 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 "Law of definite proportions." This law can explain the fact that when a compound is produced, two or more kinds of atoms are created by combining simple integer ratios.