LAW OF CONSERVATION OF MASS:

For any chemical change total mass of active reactants are always equal to the mass of the product formed. It is a derivation of Dalton’s atomic theory ‘atoms neither created nor destroyed’.

Total masses of reactants = Total masses of products + Masses of unreacted reactants

ILLUSTRATIVE EXAMPLE (1): 5.2 g of CaCO₃ when heated produced 1.99 g of Carbon dioxide and the residue (CaO) left behind weighs 3.2g. Show that these results illustrate the law of conservation of mass.

SOLUTION: Weight of CaCO₃ taken = 5.2 g

            Total weight of the products (CaO +CO₃) = 3.20+ 1.99 = 5.19 g

            Difference between the wt. of the reactant and the total wt. of the products

            = 5.20 – 5.19 =0.01 g.

            This small difference may be due to experimental error.

            Thus law of conservation of mass holds good within experimental errors.

Limitation of Law of conservation of mass: Nuclear reactions do not follow the law of conservation of mass because some of the mass of reactants is converted into energy according to Einstein equation E=mc²     where c is the velocity of light.