We know that matter is made up of small particles which may be atoms or molecules. Atoms and molecules are two different species. Molecules are formed by combining the atoms chemically. Now the question arises how the elements combine together to form compounds? Antoine L. Lavoisier laid down the foundation of chemical sciences by establishing important laws of chemical combination:

� Law of conservation of mass
� Law of constant proportion

In this blog, we will learn about the laws of chemical combination

Law of conservation of mass

In the year 1774, Antoine L. Lavoisier stated that- �Mass can neither be created nor be destroyed in a chemical reaction� In other words, in any chemical reaction, the total mass of reactants is equal to the total mass of the products. This is called the law of conservation of mass.

To understand this let us do an activity:-

� Take a solution of Barium Chloride and a solution of sodium sulfate
� Take a little amount of sodium sulfate solution in a conical flask and some solution of Barium chloride in an ignition tube.
� Hang the ignition tube in the flask as shown here and fix that with a cork such that the two solutions do not mix.
� Weigh the flask with its contents carefully.
� Now tilt and swirl the flask so that the two solutions mix.
� Now weigh the flask again.

What do you observe? One, when the two solutions are mixed, some white ppt is formed indicating that a chemical reaction has taken place. Second, the weight of the system remains the same. As the apparatus is the same in both the weighing and there is no change in the two weights taken before the reaction and after the reaction, this means that the total mass of the reactants and the products is the same, so it justifies the law of conservation of mass.

Law of constant proportion

This law deals with the composition of the elements present in a given compound: It was put forward by J.L Proust in 1799.

Law of Constant Proportion states that a chemical compound is always made up of the same elements combined together in the same fixed proportion by mass

E.g. Water obtained from any resource like a river, rain, tap, etc is always made up of the same elements i.e. hydrogen and Oxygen combined together in a fixed proportion by mass i.e. 1:8.

If one decomposes 9 g of water from any source by passing electricity and collect the gas formed, we always get 1 g of hydrogen and 8 g of oxygen.

Similarly, in ammonia, nitrogen and hydrogen are always present in the ratio of 14:3 by mass.

Read More: Postulates Of Dalton�s Atomic Theory - Science Class 9