Explain:-

Ferric Hydroxide sol coagulates on addition of a solution of Potassium Sulphate

Dear student, 
Stability of colloidal solution is basically due to the presence of charged colloids. 
When an electrolyte is added to the sol, the colloidal particles take up ions carrying opposite charges from the electrolyte. As the opposite charges combine they get neutralised and coagulation of particles take place .In the above case Fe³⁺ ions of ferric hydroxide are attracted by the negatively charged SO₄^2- sol particles and their charge gets neutralised. This leads to coagulation.
2Fe³⁺ + 3SO₄^2- → Fe₂(SO₄) ₃
Regards.