Page 21 - ICSE Chemistry 8

• On mixing 30 g ice with 50 g water, the resul ng mixture formed a er some dura on will be water.
The mass of water will be 30 g + 50 g = 80 g (Fig. 1.8).

30 g ice + 50 g water 80 g water

Fig. 1.8: Total mass before and after the change remains constant

• When wood is burnt in air, ash is formed. The mass of the burnt wood is not equal to the mass of
the residual ash formed. In this case, we compare the total masses before and a er the change
(Fig. 1.9).

+ O 2

+ CO 2 + CO + NO 2

Fig. 1.9: Conservation of mass in the burning of wood

Total mass of wood Mass of the residual ash
+ +
Mass of oxygen taken from = Mass of other gases such as CO , NO , CO
2
2
the air and water vapour
(Total mass of ma er before burning) (Total mass of ma er a er burning)

• On burning magnesium ribbon in air, magnesium oxide (a white solid) is formed. In this case, we
compare the total mass of the reactants and the products, which comes out to be equal.
2Mg + O 2 heat 2MgO
Magnesium Oxygen Magnesium oxide
48 g 32 g 80 g
The total mass during a chemical change or chemical reac on remains constant. In other words, mass
of reactant at the start of a chemical reac on will be equal to the mass of the product at the end of
the chemical reac on. There occurs a rearrangement of atoms in the reactants to form products. These
molecules break apart and new ones combine together to form the products, but the atoms in the products

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