Colloids — Fog in the atmosphere is an example of which type of colloidal system (dispersed phase in dispersion medium)?

Introduction:
Everyday phenomena can be understood through colloid science. Fog is a familiar atmospheric colloid. Correctly classifying the dispersed phase and the dispersion medium helps build intuition that transfers to process engineering topics like sprays, mists, and filtration.

Given Data / Assumptions:

• Fog consists of microscopic water droplets suspended in air.
• Droplet sizes are typically on the order of micrometers or less.
• System is stable enough to persist over minutes to hours under calm conditions.

Concept / Approach:
Colloids are classified by the physical state of the dispersed phase and the medium. When liquid droplets are dispersed in a gas (air), the result is called an aerosol. Variants include natural fog/mist and engineered sprays. In contrast, smoke refers to solids in a gas; foam refers to a gas dispersed in a liquid; and emulsion denotes liquid in liquid.

Step-by-Step Solution:
Identify the dispersed phase: liquid water droplets.Identify the medium: air (gas).Match the pair to the standard colloid class: liquid in gas → aerosol → fog/mist.

Verification / Alternative check:
Meteorology texts specifically define fog as a suspension of tiny water droplets (and occasionally ice crystals) in air near the Earth’s surface, mapping exactly to liquid-in-gas aerosols.

Why Other Options Are Wrong:

• Solid in gas: smoke/particulate haze, not fog.
• Solid in liquid: sols like paint or clays in water.
• Gas in liquid: foams (e.g., shaving foam), not fog.
• Liquid in liquid: emulsions such as milk or mayonnaise.

Common Pitfalls:
Confusing fog with vapor; vapor is a true gas phase of water, whereas fog is liquid droplets dispersed in air.

Final Answer:
Liquid dispersed in gas (aerosol)