Which one of the following is not a colloid?
- A.Milk
- B.Mud
- C.Butter
- D.Boric acid
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Explanation
The correct answer is D, boric acid. Boric acid dissolves in water to give a true solution, in which the dispersed particles are individual molecules small enough to pass through a filter paper and even a parchment membrane, and the mixture shows no Tyndall effect. A true solution is therefore not a colloid.
Option A is wrong because milk is a colloid, an emulsion of liquid fat droplets dispersed in water. Option B is wrong because mud is a colloidal dispersion of fine soil particles in water, which is why it stays turbid for a long time before settling. Option C is wrong because butter is also a colloid, with water dispersed in fat, that is a gel-like emulsion. The rule to apply is that only a true solution is excluded from the colloidal state.