Laws of chemical combinations

Laws of chemical combinations

By studying quantitative measurements of many reactions, it was observed that the reactions taking place between various substances are governed by certain laws. They are called as the ‘Laws of chemical combinations’. They are given below.

1. Law of conservation of mass

2. Law of constant proportion

3. Law of multiple proportions

4. Gay Lussac’s law of gaseous volumes

In this lesson, we will study about the first two laws. You will study about Law of multiple proportions and Gay Lussac’s Law of gaseous volumes in standard IX.

1. Law of conservation of mass

The law of conservation of mass which relates the mass of the reactants and products during the chemical change was stated by a French chemist Lavoisier in 1774. It states that during any chemical change, the total mass of the products is equal to the total mass of the reactants. In other words the law of conservation of mass means that mass can neither be created nor be destroyed during any chemical reaction. This law is also known as Law of indestructibility of mass.

Activity 5

Take some ice cubes in an air tight container and note the weight of the container with ice cubes. Wait for a while for the ice cubes to become water. It is a physical change ie., ice cubes melt and they are converted into liquid. Now weigh the container and compare the weight before and after the melting of ice cubes. It remains the same. Hence it is proved that during a physical change, the total mass of matter remains the same.

Activity 6

Prepare 5% of barium chloride (5g of BaCl₂ in 100 ml of water) and sodium sulphate solutions separately. Take some solution of sodium sulphate in a conical flask and some solution of barium chloride in a test tube. Hang the test tube in the conical flask. Weigh the flask with its contents. Now mix the two solutions by tilting and swirling the flask. Weigh the flask after the chemical reaction is occurred. Record your observation. It can be seen that the weight of the flask and the contents remainsthe same before and after the chemical change. Hence, it is proved that during a chemical change, the total mass of matter remains the same.

Consider the formation of ammonia (Haber’s process) from the reaction between nitrogen and hydrogen

N₂   + 3H₂  →    2NH₃

28g + 6g   →  34g

During Haber’s process the total mass of the reactant and the product are exactly same throughout the reaction.

Now, it is clear that mass is neither created nor destroyed during physical or chemical change. Thus, law of conservation of mass is proved.

2. Law of constant proportions

Law of constant proportions was proposed by the scientist Joseph Proust in 1779. He states that in a pure chemical compound the elements are always present in definite proportions by mass. He observed all the compounds with two or more elements and noticed that each of such compounds had the same elements in same proportions, irrespective of where the compound came from or who prepared it. For example, water obtained from different sources like rain, well, sea, and river will always consist of the same two elements hydrogen and oxygen, in the ratio 1:8 by mass. Similarly, the mode of preparation of compounds may be different but their composition will never change. It will be in a fixed ratio. Hence, this law is also known as ‘Law of definite proportions’.