Question

Fog is an example for colloidal system of

• A. Liquid in gas
• B. Gas in liquid
• C. Solid in gas
• D. Gas in solid

Answer 1

Answer:

A. Liquid in gas

Understanding Colloidal Systems

• A colloidal system, or colloid, is a heterogeneous mixture in which the particles of one substance (the dispersed phase) are distributed uniformly throughout another substance (the dispersion medium).
• The size of the particles in a colloidal system typically ranges from 1 nanometer (nm) to 1000 nm. This intermediate size distinguishes them from true solutions (particles < 1 nm) and suspensions (particles > 1000 nm).
• Colloidal particles do not settle out on standing, unlike suspensions, and cannot be easily separated by simple filtration.

Classification of Colloids Based on Physical States

• Colloids are classified into eight types depending on the physical state (solid, liquid, or gas) of the dispersed phase and the dispersion medium. Note that gas in gas forms a homogeneous mixture and is therefore not a colloid.
• One important classification for competitive exams is Aerosols, where the dispersion medium is a gas.
• If the dispersed phase is a liquid and the dispersion medium is a gas, the colloid is specifically called a liquid aerosol.
• If the dispersed phase is a solid and the dispersion medium is a gas, the colloid is called a solid aerosol.

Fog: A Prime Example of Liquid Aerosol

• Fog consists of tiny water droplets (liquid) suspended and uniformly dispersed in the air (gas).
• These water droplets are of colloidal dimensions, which means they are small enough to remain suspended but large enough to scatter light, reducing visibility.
• Therefore, fog is an excellent example of a colloidal system where the dispersed phase is liquid and the dispersion medium is gas. Clouds and mist are similar examples.

Key Characteristics and Related Facts for Competitive Exams

• Tyndall Effect: Colloidal particles are large enough to scatter light, making the path of a light beam visible through the colloid. This phenomenon is known as the Tyndall effect (e.g., a beam of sunlight visible in a dusty room or through fog).
• Brownian Movement: Colloidal particles exhibit a continuous, random, zig-zag motion due to constant bombardment by molecules of the dispersion medium. This movement helps keep the particles suspended.
• Electrophoresis: Colloidal particles often carry an electrical charge and will move towards an electrode of opposite charge when an electric field is applied.
• Coagulation/Flocculation: Colloidal particles can be made to precipitate or settle down by adding electrolytes, which neutralize their charge, leading to their aggregation.
• Other Common Colloidal Systems and Their Types:
  • Emulsion: Liquid in Liquid (e.g., milk, mayonnaise).
  • Sol: Solid in Liquid (e.g., paint, blood, starch solution).
  • Gel: Liquid in Solid (e.g., jelly, cheese, butter).
  • Foam: Gas in Liquid (e.g., whipped cream, soap lather).
  • Solid Foam: Gas in Solid (e.g., pumice stone, foam rubber).
  • Solid Sol: Solid in Solid (e.g., colored glass, alloys like ruby glass).