CBSE Class 10 Science 2019 · Paper 31/5/1 · Question 5(a)

Acetic acid (${\text{CH}}_3\text{COOH}$) is the weak acid. It ionises only partly in water, so its solution contains a much lower concentration of hydronium ions (${\text{H}}_3{\text{O}}^{+}$) than hydrochloric acid of the same concentration, which ionises completely. Example: zinc granules react vigorously with dilute hydrochloric acid, giving brisk effervescence of hydrogen, but only slowly with dilute acetic acid of the same concentration. Other correct examples also earn credit, such as universal indicator showing a lower pH for hydrochloric acid than for acetic acid of the same concentration.