In physical chemistry and physics, fog is an example of which type of colloidal system?

Introduction / Context:
Colloids are mixtures where tiny particles of one substance are dispersed in another substance. Different everyday phenomena such as fog, smoke, foam and emulsions are all examples of colloidal systems. This question tests whether you can correctly identify the type of dispersion that fog represents, combining basic ideas from atmospheric science and colloid chemistry.

Given Data / Assumptions:

• We are considering ordinary fog that appears near the ground, especially in cool and humid conditions.
• Fog consists of water droplets suspended near the earth's surface.
• The continuous phase is the surrounding air.
• We must classify fog in terms of which state is dispersed and which state forms the medium.

Concept / Approach:
Fog is essentially a cloud formed at ground level. It consists of tiny liquid water droplets suspended in air. In colloid terminology, the dispersed phase is liquid and the dispersion medium (continuous phase) is gas, which is mainly air. Such a system, where liquid is dispersed in a gas, is called a liquid aerosol. The question uses more general phrases like “liquid dispersed in gas,” so we match fog to that description. The other options involve solids or gases in liquids or solids and do not fit the actual composition of fog.

Step-by-Step Solution:
Step 1: Identify the visible component of fog: tiny droplets of liquid water. Step 2: Identify the medium in which these droplets are suspended: the air, which is a gas. Step 3: Therefore, the dispersed phase is liquid, and the dispersion medium is gas. Step 4: In colloid classification, this combination is described as a liquid dispersed in a gas (aerosol). Step 5: Match this classification with the options and choose “Liquid dispersed in gas.”

Verification / Alternative check:
Standard tables of colloid types list fog and mist under aerosols, specifically as examples of liquid in gas systems. Smoke, by contrast, is listed as solid particles dispersed in gas. Milk is a liquid in liquid emulsion, and foam is gas in liquid. Since fog is visually similar to clouds and arises from condensation of water vapour into droplets, it clearly belongs with the liquid in gas category. This confirms that the correct classification is liquid dispersed in gas.

Why Other Options Are Wrong:
Solid dispersed in liquid: This describes suspensions such as muddy water or colloidal sols, not fog.

Liquid dispersed in solid: This is typical of gels or certain solid emulsions, for example, jelly, not atmospheric fog.
Gas dispersed in liquid: This corresponds to foams, such as whipped cream or shaving foam, again very different from fog.

Common Pitfalls:
Because fog is often associated with humidity and water, some students may focus only on the liquid part and forget that air is the continuous medium. Others may confuse fog with smoke, which is solid particles in gas. A clear mental picture helps: fog consists of tiny liquid water droplets floating in air, so think “liquid in gas.” Remembering related examples like mist and spray, which are also liquid aerosols, makes this classification easier to recall.

Final Answer:
Fog is an example of a colloidal system where liquid is dispersed in gas.