CBSE Class 10 Science 2020 · Paper 31/C/3 · Question 16

(a) Take water in a glass beaker, kept cool in a trough of cold water, and add the concentrated sulphuric acid to it slowly, a few drops at a time, stirring continuously with a glass rod. Never add water to the concentrated acid: the mixing is highly exothermic, and the heat could make the mixture splash out and cause burns, or crack the glass. (b) Dilution decreases the concentration of ${\text{H}}_3{\text{O}}^{+}$ ions per unit volume, so for the acid the ratio of ${\text{H}}_3{\text{O}}^{+}$ to ${\text{OH}}^{-}$ ions decreases (it moves towards $1$ as the pH rises towards $7$). (c) The pH decreases. For pH $0.5$ to $2.0$ (strongly acidic) the pH paper turns red (shades of red).