Assertion (A) : Addition of water to an aqueous solution of HCl increases the pH. Reason (R) : Addition of water suppresses the ionisation of HCl.

Assertion (A) is true because adding water to an aqueous solution of HCl results in dilution, which decreases the concentration of H+ ions per unit volume. Since pH is the negative logarithm of the H+ ion concentration, a decrease in concentration leads to an increase in pH towards 7 [2]. However, Reason (R) is false. HCl is a strong acid that undergoes almost complete ionization in aqueous solutions. According to Ostwald's dilution law, dilution actually increases the degree of ionization for weak electrolytes, and it certainly does not suppress the ionization of a strong acid like HCl [1]. In fact, dilution increases the total volume, and while the concentration of ions decreases, the absolute number of dissociated ions remains the same or slightly increases in the case of weak acids; it is never suppressed.

Sources

1. [2] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > 2.3 HOW STRONG ARE ACID OR BASE SOLUTIONS? > p. 26
2. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Activity 2.10 > p. 24