Laws of chemical combination

Do you know what are the laws of chemical combination? The below article provides you with the laws of chemical combination. There are five laws of chemical combination. To know about these laws, please read this article.


On the basis of estimation of results of various chemical reactions, we came to know that chemical reactions between two or more than two substances are governed by some laws. These are known as the laws of chemical combination. There are five laws of chemical combination. Four out of five laws are related to the weight of substances and fifth law is related to the volume of gases. These laws are as below:

  1. Law of conservation of mass
  2. Law of constant or fixed or definite proportions
  3. Law of multiple or simple proportions
  4. Gay Lussac's law of gaseous volume
  5. Avogadro's law

Law of conservation of mass

Law of conservation of mass is also known as the law of indestructibility of matter. The law of conservation of mass was first of all put forward by the Russian scientist Lomonosoff in the year 1756, which was later confirmed by scientists Lavoisier, Landolt, etc. According to the law of conservation of mass, the matter can neither be created nor destroyed as a result of a physical or chemical change. In other words, the total mass of the products formed is always equal to the mass of the reacting substances after the physical or a chemical change. The law of conservation of mass can be proved by the following experiments:
Experiment I: A phosphorus piece is placed in a flask containing sand. The mouth of the flask is covered tightly with cork and weighed. The flask is then heated. Phosphorus burns and flask gets filled with phosphorus pentoxide vapour. Flask is further colled and weighed again. It is observed that its weight remains unchanged.
Experiment II: Landolt took an H-shaped tube which is made of Jena glass. In one limb of the tube, he took a potassium chloride solution and in the other limb, the silver solution was taken. The tube was sealed and then weighed. Now the tube was shaken to mix the two solutions thoroughly. A white precipitate of silver chloride was formed.
Potassium chloride + Silver nitrate = Potassium nitrate + silver chloride
In the formation of these solutions, no heat was evolved or absorbed. He weighed the H-tube again, the mass was found to be the same.

Law of constant or fixed or definite proportions

Law of constant or fixed or definite proportions was first given by a French chemist, Louis Proust in 1799. He observed that a chemical compound is always found to have same composition irrespective of its source. According to the law of constant proportions, a chemical compound obtained from any source or prepared by any method has the same chemical composition of its constituents by weight. For example, water may be obtained from the following sources:
  1. River, well, sea, rainfall, etc, i.e. from natural sources.
  2. Reaction of hydrogen and oxygen in laboratories.
  3. By any other chemical method.
By chemical analysis, it was found that, in terms of of weight, the ratio of hydrogen and oxygen was always 1:8.
In the same way, carbon dioxide may also be obtained by different methods. For example:
  1. By burning of carbon in air.
  2. From the reaction of dilute hydrochloric acid with sodium carbonate.
  3. On heating calcium carbonate.
Chemical analysis of carbon dioxide, obtained by any method revealed that, in terms of weight, the ratio of carbon and oxygen was always 12:32.

Law of multiple or simple proportions

The law of multiple or simple proportions was propounded by John Dalton in 1803. According to the law of multiple proportions, when two elements combine together to form two or more than two compounds, then the weights of one of the elements which combine with a fixed weight of the other, bears a simple ratio to one another. For examples:
  1. Carbon combines with oxygen in two ways
    C + 1/2 O2 = CO (carbon monoxide)
    C + O2 = CO2 (carbon dioxide)
    In the above reactions, 16 and 32 g of oxygen respectively combines with a fixed amount i.e., 12 g of carbon. The ratio of weights of oxygen combine with 12 g carbon is 16:32 or 1:2.
  2. The elements nitrogen and oxygen combine to produce as many as five oxides of nitrogen viz nitrous oxide, nitric oxide, dinitrogen trioxide, dinitrogen tetroxide or peroxide and dinitrogen pentoxide. The weights of nitrogen and oxygen in these oxides, which combine with one another are:
    Nitrous oxide, N2 = 28:16 or 14:8
    Nitric oxide, NO = 14:16
    Dinitrogen trioxide, N2O3 = 28:48 or 14:24
    Dinitrogen tetroxide, N2O4 = 28:64 or 14:32
    Dinitrogen pentoxide, N2O5 = 28:80 or 14:40
    In this example, different weights of oxygen combine with the fixed weight of nitrogen. Different weights of oxygen, i.e., 8, 16, 24, 32 and 40 which combine with a fixed weight of nitrogen-14 are in a simple ratio of 1:2:3:4:5.

Verification of law of of multiple proportions by an experiment

Take two ceramic dishes, heat them and keep them in a desiccator after cooling and then weigh. Take cuprous oxide (about one gm)in one dish and cupric oxide in another. Keep them in a wide hard glass tube. Heat the hard glass tube and pass hydrogen gas through it. Stop heating the hard glass tube and passing hydrogen gas through it after half an hour. Now take out the dish and then cool it, keep it in a desiccator and then weigh. Copper will be left in each dish. Now determine the amount of oxygen combined with the copper in the two dishes. The weights will be in a simple ratio.

Gay Lussac's Law of gaseous volumes

This law was put forward by Gay Lussac in 1808 which states that gases combine with each other in the simple ratio of their volumes. If the product is a gas it also bears the simple ratio with the volume of reactants provided all volumes are measured under similar conditions. For example, one volume of hydrogen reacts with one volume of chlorine to form two volumes of hydrogen chloride gas:
H2 + Cl2 = 2HCl
Hence, the ratio of volumes of hydrogen, chlorine and hydrogen chloride is 1:1:2 which is a simple whole-number ratio.

Avogadro's law

This law was proposed by Avogadro in 1811. According to this law, at equal temperature and equal pressure, in an equal volume of gases, the number of molecules must be equal. For example at equal temperature and equal pressure in containers of two-litre each having H2, He, CO2 and SO2 there will be an equal number of molecules. In fact, Avogadro proposed the concept of the molecule for the first time.

These laws are basic concepts of chemistry and every student of chemistry must know these laws.


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