Question

Explain the following observation:

Ferric hydroxide sol coagulates on addition of a solution of potassium sulphate.

Answer 1

Ferric hydroxide sol is positively charged, and potassium sulphate (K₂SO₄) provides \[\ce{SO^{2-}4}\] ions, which are negatively charged. These \[\ce{SO^{2-}4}\] ions neutralise the positive charges on the sol particles, reducing repulsion. As a result, the particles aggregate and coagulate. This is in accordance with the Hardy–Schulze rule, which states that ions with higher valency are more effective in causing coagulation.