Q. 51

State the law of conservation of mass. Give one example to illustrate this law.

Answer :

Law of conservation of mass was given by Antoine Lavoisier in 1744. According to this law, “Matter can neither be created nor destroyed in a chemical reaction”. This law states that mass of an isolated system will remain constant over time. It means that when mass is enclosed in a system and nothing is allowed in or out, its quantity will never change. That is mass will be conserved, and hence this is called Law of Conservation of Mass. This means total mass of products is always equal to the total mass of reactants.

In a chemical reaction, the substances that combine or react are known as reactants and the new substance/substances formed are called product or products. A chemical reaction can be represented in general as follows:


Reactant + Reactant Product


Example: When calcium oxide is dissolved in water, then calcium hydroxide is formed. The reaction involved in this can be written as:


Calcium oxide + Water Calcium hydroxide


In this reaction calcium oxide and water are reactants while calcium hydroxide is product.


In this reaction 74 g of calcium hydroxide is obtained when 56 g of calcium oxide reacts with 18 g of water, which is proved by experiment.


Calcium oxide (56 g) + Water (18 g) Calcium hydroxide (74 g)


{CaO = Ca(40) + O(16) = 56g; H2O = H2(2)+O(16) = 18g; Ca(OH)2 = Ca(40)+O(32)+H(2) = 74g}


Here the total mass of reactants, i.e. calcium oxide and water is equal to 74 g. And the mass of product, i.e. calcium hydroxide is also equal to 74g. This proves that the total mass of reactants is always equal to the total mass of product, which proves the Law of Conservation of Mass.


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