CBSE Class 10 Science 2019 · Paper 31/3/1 · Question 16(a)

(a) A double displacement reaction is a reaction in which two compounds exchange their ions to form two new compounds. Example: when solutions of sodium sulphate and barium chloride are mixed, a white precipitate of barium sulphate forms, ${\text{Na}}_2{\text{SO}}_4\text{(aq)}+{\text{BaCl}}_2\text{(aq)}\to {\text{BaSO}}_4\text{(s)}+2\text{NaCl(aq)}$: the ${\text{Ba}}^{2+}$ ions pair with the ${\text{SO}}_4^{2-}$ ions and the ${\text{Na}}^{+}$ ions with the ${\text{Cl}}^{-}$ ions. (b) (i) Combination reaction: a reaction in which two or more substances (elements or compounds) combine to form a single product. (ii) $\text{CaO(s)}+{\text{H}}_2\text{O(l)}\to {\text{Ca(OH)}}_2\text{(aq)}+\text{heat}$; the product is calcium hydroxide (slaked lime). (iii) The quick lime reacts vigorously with the water, often with a hissing sound, and a large amount of heat is released, so the beaker becomes hot.