Question

(a) Fresh milk has a pH of 6. When it changes into curd (yogurt), will its pH value increase or decrease? Why?
(b) Name the gas evolved when dilute HCl reacts with sodium hydrogencarbonate. How is it recognised?
(c) On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue-green. Predict the new compound formed which imparts a blue-green colour to the solution.

Answer 1

(a) When fresh milk changes into curd, its pH value decreases because of an increase in the acidity of the milk. Bacteria usually change fresh milk to curd by producing lactic acid.

(b) When sodium hydrogen carbonate reacts with dilute hydrochloric acid, it gives sodium chloride, carbon dioxide, and water. When the gas liberated is passed through lime water, lime water turns milky.

${\mathrm{NaHCO}}_3+\mathrm{HCl}\to \mathrm{NaCl}+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$

(c) When dilute hydrochloric acid is added to copper oxide powder, a blue-green coloured compound called copper chloride will be obtained.

$\mathrm{CuO}+2\mathrm{HCl}\to \underset{\mathrm{blue}\mathrm{green}}{{\mathrm{CuCl}}_2}+{\mathrm{H}}_2\mathrm{O}$