Which of the following is an example of a solid sol, that is, a colloidal system in which a solid is dispersed in a solid medium?

Introduction / Context:
Colloids are mixtures in which one substance is dispersed in another in the form of very small particles. Depending on the physical states of dispersed phase and dispersion medium, colloids are classified into types such as sols, gels, foams and emulsions. This question asks specifically for a solid sol, which is a colloid where both the dispersed phase and the dispersion medium are solids. Identifying correct examples helps strengthen understanding of colloid classification in general chemistry.

Given Data / Assumptions:

• A solid sol is defined as a colloidal system of solid particles dispersed in a solid medium.
• Each option represents a familiar colloidal or related system.
• We assume standard textbook definitions of sols, foams, and other colloids.
• The task is to match the example that fits the solid in solid category.

Concept / Approach:
To identify a solid sol, recall that in such a system both phases are solid, for example, coloured glass or gemstones where tiny solid particles impart colour to a solid matrix. Milk of magnesia is a sol where solid is dispersed in a liquid. Foam is a gas dispersed in a liquid or solid. Rubber can be treated as a type of solid polymer, often classified as a gel or elastic solid, not a classic solid sol in elementary classification. Fog is a liquid in gas system. Therefore coloured gemstones provide a typical example of solid solid colloids.

Step-by-Step Solution:

Verification / Alternative check:
Standard tables of colloid types in school chemistry list several examples. For solid sols, common examples include coloured glass, ruby glass, and certain coloured gemstones where the pigment is solid and the medium is also solid. For sols (solid in liquid), examples include paints and muddy water. For foams, examples include froth and whipped cream. For aerosols, fog is often listed. This comparison confirms that coloured gemstones belong to the solid sol category.

Why Other Options Are Wrong:

• Milk of magnesia: A suspension or sol with solid in liquid, not solid in solid.
• Foam: Gas in liquid or gas in solid, not solid in solid.
• Rubber: Usually regarded as a polymeric solid or gel, not a typical solid sol example in introductory texts.
• Fog: Liquid droplets dispersed in a gas, classified as an aerosol, not a solid sol.

Common Pitfalls:
Students sometimes try to memorise lists without understanding the underlying definitions, which leads to confusion when examples are slightly unfamiliar. Another common mistake is to focus only on the common name and not on the physical states of the dispersed phase and medium. A better approach is to identify what phase is dispersed and what phase acts as the medium, then classify the colloid accordingly. Doing this makes it clear why coloured gemstones are solid colloids of solid in solid.

Final Answer:
The correct example of a solid sol is Coloured gemstones.