* Salicylic acid is a weak acid: It exists in equilibrium with its conjugate base, salicylate ion (C₇H₅O₃⁻).
* HCl is a strong acid: It completely ionizes in water, providing a high concentration of H⁺ ions.
* Le Chatelier's Principle: When HCl is added, the high concentration of H⁺ ions shifts the equilibrium of salicylic acid ionization to the left. This means more salicylic acid molecules remain in their undissociated form, which is less soluble in water.
Here's a simplified explanation:
1. Salicylic acid is partially soluble in water because a small amount of it ionizes, forming the salicylate ion (C₇H₅O₃⁻). This ion is more polar and thus more soluble in water than the undissociated salicylic acid molecule.
2. When HCl is added, the H⁺ ions from HCl react with the salicylate ions, forming undissociated salicylic acid. This reduces the concentration of salicylate ions in solution.
3. To re-establish equilibrium, more salicylic acid molecules will shift from the dissolved state to the undissociated form. This leads to a decrease in the overall solubility of salicylic acid.
In short, the addition of HCl suppresses the ionization of salicylic acid, causing more of it to remain in its less soluble, undissociated form.
