Answer the question on basis of passage given below:

Lyophobic sols like those of metals (Au, Ag, etc) are unstable and are easily precipitated by addition of electrolytes. However, it is observed that the addition of certain lyophilic colloids like gums, soaps, gelatine, etc. to lyophobic colloids (like a metal sol) render lyophobic colloids difficult to coagulate by the addition of electrolytes. The process is known as 'protection' and the lyophilic colloids are termed as Protective colloids. It is believed that the protective action of the lyophilic colloids is due to the covering up of the particles of the lyophobic colloid by those of the lyophilic colloid. Evidently, smaller the gold number of a protective colloid, the greater is its protective action.

Which of the following does not form a lyophilic colloid?

Metallic sulphides added to water

The correct answer is option 4. Metallic sulphides added to water.

A lyophilic colloid is a type of colloidal system where the dispersed phase (the substance that forms the colloidal particles) has a strong affinity for the dispersion medium (the solvent). In other words, the particles of the dispersed phase readily form stable colloidal solutions in the solvent.

Let us break down the options given:

1. Rubber dissolved in benzene: Rubber is a lyophilic substance, meaning it has an affinity for certain organic solvents like benzene. When rubber is dissolved in benzene, it forms a stable colloidal solution. The rubber particles disperse evenly throughout the benzene due to their strong attraction to the benzene molecules.

2. White portion of the egg dissolved into water: The white portion of the egg, also known as albumin, is a lyophilic substance. When dissolved in water, albumin forms a stable colloidal solution. The albumin molecules disperse evenly throughout the water, forming a colloidal system.

3. Starch in water: Starch is another example of a lyophilic substance. When starch is mixed with water, it forms a stable colloidal solution. The starch molecules disperse uniformly in the water, creating a colloidal system.

4. Metallic sulphides added to water: Metallic sulphides typically do not have a strong affinity for water (they are not lyophilic). Therefore, when metallic sulphides are added to water, they do not readily form stable colloidal solutions. Instead, they may precipitate out of the solution or form large aggregates, leading to a heterogeneous mixture rather than a true colloidal system.

In summary, metallic sulphides added to water do not form a lyophilic colloid because metallic sulphides are not attracted to water and do not readily disperse in it to form stable colloidal solutions.