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• Arsenic sulphide (As₂S₃) is an example of a lyophobic colloid.

• The sol is prepared in the lab first by the reaction of As₂O₃ and boiling water to produce arsenous acid, As(OH)₃.

• Next, arsenous acid is treated with hydrogen sulphide gas (H₂S) to produce the required arsenic sulphide.

• As₂S₃ is produced as a light yellow sol.

• The reactions involved are as follows:

OR

When catalysts exist in different phase than that of the reactants, it is said to be heterogeneous catalyst. These catalysts performing catalysis, is termed as heterogeneous catalysis.

Heterogeneous catalysis is also called surface catalysis. This is because generally, the catalyst is solid while the reactants are gases, due to which the reaction starts at the surface of the solid catalyst.

Eg.

• In the Contact process reaction (oxidation reaction of SO₂ to SO₃ using O₂), solid V₂O₅ is used as a catalyst, whereas the reactants, SO₂ and SO₃ are gases.

• In Haber’s process, manufacture of ammonia gas is done by mixing nitrogen gas and hydrogen gas in presence of finely divided iron (which is a solid) as a catalyst at 400-450°C, and at 200 atm pressure.

• Hydrogenation of oils to form vegetable ghee using Ni as catalyst.