The number of colloidal systems from the following, which will have 'liquid' as the dispersion medium, is ___________.
Gem stones, paints, smoke, cheese, milk, hair cream, insecticide sprays, froth, soap lather

Solution

Chemistry ma’am · 3 steps

For each system, ask what the continuous phase is. That is the medium, whatever the scattered bits are.

1. Spot the medium. Every colloid has a dispersed phase (the tiny particles) and a dispersion medium (the continuous phase they sit in). Only the medium matters here.

   Dispersion medium	Systems from the list	Colloid type	Count
   Solid	gem stones, cheese	solid sol (solid in solid), gel (liquid in solid)	2
   Liquid	paints, milk, hair cream, froth, soap lather	sol (solid in liquid), emulsion (liquid in liquid), foam (gas in liquid)	5
   Gas	smoke, insecticide sprays	aerosol (solid or liquid in gas)	2

2. The tricky ones. Froth and soap lather are gas bubbles trapped in a liquid, so the liquid is the medium. An insecticide spray has liquid droplets floating in air, so its medium is gas, not liquid.
3. So. Paints, milk, hair cream, froth and soap lather have a liquid medium, which gives 5.

The common slip: 6. Counting insecticide spray as a liquid medium gives 6. The liquid there is only the dispersed droplets, and air is the medium.

The idea to keep. The dispersion medium is the continuous phase that surrounds everything else. Foams have liquid outside and gas inside, while sprays have air outside and liquid inside.

Answer from the NTA final answer key. Solution by PrepWiser.