CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 16

(a) A balanced chemical equation is one in which the number of atoms of each element is the same on both sides, the reactants and the products. Example: $2{\text{H}}_2+{\text{O}}_2\to 2{\text{H}}_2\text{O}$ ($4$ H and $2$ O atoms on each side). Chemical equations must be balanced because of the law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction. The atoms are only rearranged, so the total mass of the products must equal the total mass of the reactants, which is true only when every atom of the reactants appears in the products. A balanced equation also shows the correct ratio in which the substances react. (b) (i) Change in colour (the blue copper sulphate solution turns pale green): $\text{Fe(s)}+{\text{CuSO}}_4\text{(aq)}\to {\text{FeSO}}_4\text{(aq)}+\text{Cu(s)}$ (ii) Change in temperature (the mixture becomes hot): $\text{CaO(s)}+{\text{H}}_2\text{O(l)}\to {\text{Ca(OH)}}_2\text{(aq)}+\text{heat}$ (iii) Formation of precipitate (a white precipitate of barium sulphate): ${\text{Na}}_2{\text{SO}}_4\text{(aq)}+{\text{BaCl}}_2\text{(aq)}\to {\text{BaSO}}_4\text{(s)}+2\text{NaCl(aq)}$ (Any other correct balanced equation that shows each change also earns credit.)