Which term describes the ability of a substance to dissociate in water?

• A. Ionization constant / K value
• B. pKa value
• C. Buffer capacity
• D. Equilibrium constant

Answer: (A)

The ability of a substance to dissociate in water is described by the ionization constant. This constant is the equilibrium constant for the ionization reaction, quantifying how much of the substance exists as ions versus its molecular form in solution. For an acid, HA, dissociating in water as HA + H2O ⇌ H3O+ + A−, the ionization constant Ka = [H3O+][A−]/[HA] measures how strongly it ionizes—the larger the Ka, the more the substance dissociates. The pKa is simply the negative logarithm of Ka, providing a convenient way to compare strengths, but it is a transformed representation, not the direct measure of dissociation tendency. Buffer capacity relates to how well a solution resists pH change, not to the intrinsic tendency to dissociate. Equilibrium constant is a general term for any reaction, whereas the ionization constant is specifically the equilibrium constant for ionization in water.