Which of the following colloids cannot be formed by direct mixing?

Lyophobic colloids

The correct answer is option 2. Lyophobic colloids.

Out of the listed options, the colloid that cannot be formed by direct mixing is: Lyophobic colloids.

1. Lyophilic colloids (solvents loving): These colloids consist of particles with a strong affinity for the dispersion medium (usually water). Examples include starch solutions and gelatin solutions. Due to this affinity, they can be directly formed by mixing the substance with the liquid.

2. Lyophobic colloids (solvents hating): These colloids consist of particles that don't have a natural affinity for the dispersion medium. Examples include metal sols and some sulphide sols. Direct mixing often results in aggregation or precipitation because the particles tend to clump together. Special methods like peptization or reduction are needed to prepare lyophobic colloids.

3. Macromolecular colloids: These colloids consist of large molecules that can disperse throughout a liquid medium. Examples include starch solutions and protein solutions. Due to their large size, they can form stable dispersions through direct mixing.

4. Associated colloids: These colloids form when individual molecules come together to create larger aggregates that disperse in a medium. Soap solutions are a common example. In some cases, associated colloids can be formed by directly mixing the components that allow the association to occur.

Therefore, while other colloid types can be formed through direct mixing, lyophobic colloids typically require additional methods due to the repulsion between the particles and the dispersion medium.