Which row of the table describes what happens when a solution of a weak acid is diluted? (Assume constant temperature.) | K_a | Extent of acid ionisation | |-------|--------------------------| | A | Decreases | Increases | | B | Decreases | Decreases | | C | Remains the same | Increases | | D | Remains the same | Decreases | - HSC - SSCE Chemistry - Question 6 - 2021 - Paper 1

When a weak acid is diluted, the extent of ionisation generally increases. This is due to Le Chatelier's principle, which states that if a system at equilibrium is disturbed, the system will shift in a direction that counteracts that disturbance. Dilution reduces the concentration of the acid, which shifts the equilibrium to produce more ions. Thus, the dissociation constant $K_a$ remains the same because it is determined by the nature of the acid and the temperature, but the extent of ionisation increases. Therefore, the correct answer is C: 'Remains the same' for $K_a$ and 'Increases' for the extent of acid ionisation.