Knowledge database: Basic laws of chemistry: Law of definite proportions

This law says that a chemical compound always contains the same chemical elements in an exact and constant mass ratio. Thus, the analysis of different samples of the same substance from different sources will always show that the ratio of certain elements that the substance contains, remains constant, regardless of where it originated from and how it was created.

For example, 11.19 grams of hydrogen will always react with 88.81 grams of oxygen, giving always exactly 100 grams of water. This is also valid vice versa, 100 grams of water will always decompose into 11.19 grams of hydrogen and 88.81 grams of oxygen. If there is a surplus of any of the elements in the synthesis of water, that element will simply be left unreacted after the reaction, along with 100 grams of synthesized water.

However, some deviations were noticed due to the fact that certain elements combine in different ratios. Thus iron can combine with oxygen in form of two different substances: Fe₂O₃ and Fe₃O₄, with different mass ratios, but with the same elements present. Compounds in which one can notice different ratios of the same elements, are called berthollides, while those of constant composition are known as daltonides.

Table salt always has the same and exact composition (sodium chloride). There is no difference between salt that comes from seas (image on the left) and mineral salt (image on the right).