CBSE Class 10 Science 2019 · Paper 31/2/1 · Question 7(a)

(a) Dissolving an acid in water is a highly exothermic process. When the acid is added slowly to water with constant stirring, the heat released is absorbed by the large amount of water, so the mixture stays safe. If water is added to the concentrated acid, the heat boils the small amount of water: the mixture may splash out and cause burns, and the container may crack from the excessive local heating. (b) Hydrogen chloride forms hydrogen ions (${\text{H}}^{+}$, present as ${\text{H}}_3{\text{O}}^{+}$) only in the presence of water. Dry HCl gas and dry litmus paper contain no water, so no ${\text{H}}^{+}$ ions are formed and the litmus does not change colour.