Question

The acidic behaviour of acids is due to the presence of hydrogen($H^{+}$) ions in them. They produce hydrogen ion in the presence of water. Water is a polar solvent and this propertiy of water helps in weakening the bond between the ions and makes them soluble. Hence, acids and bases produce ions in aqueous solutions.

1. How does the concentration of hydronium ions [H₃O⁺] changes when the solution of an acid is diluted with water?
2. You have two solutions, A and B. The pH of solution A is 6 and the pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?
3. What would you observe on adding dil HCl acid to
   1. Sodium bicarbonate placed in a test tube.
   2. Zinc metal in a test tube.
   3. Sodium hydroxide in a test tube.

Answer 1

1. When an acid is diluted, the concentration of hydronium ions (H₃O⁺) per unit volume decreases. This means that the strength of the acid decreases.
2. A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution. Therefore, the solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.

(a) CO₂ gas will evolve accompanied by brisk effervescence.
${\mathrm{NaHCO}}_{3\left(s\right)}+{\mathrm{HCl}}_{\left(\mathrm{aq}\right)}\to {\mathrm{NaCl}}_{\left(\mathrm{aq}\right)}+{{\mathrm{CO}}_2}_{\left(g\right)}+H_2{O}_{\left(\mathrm{aq}\right)}$

(b) H₂ gas will evolve accompanied by brisk effervescence.

Zn_(s) +2HCl_(aq) à ${\mathrm{ZnCl}}_2$_(aq) + H₂O_(g)

(c) Sodium chloride salt and water is produced.

NaOH_(aq) + HCl_(aq) à NaCl_(aq) +H₂O_(l)