Question

A piece of pumice stone is used for scrubbing dirt from hands and feet. Pumice is a colloidal solution of a:
A. Liquid in a solid
B. Solid in a liquid
C. Solid in a solid
D. Gas in a solid

Answer 1

Hint: Unlike a true solution, a colloidal solution has two distinct phases. One is called the dispersed phase while the other is called the dispersion medium. The dispersed phase is basically spread throughout in the dispersion medium.

Complete solution:
Colloids are basically solutions in which the particle size of the solute varies from to. They are heterogeneous in nature, which means their composition is not uniform throughout. Colloidal solutions appear to be homogeneous. They can not be separated by simple physical means like filtration or sedimentation, however, they can be separated through ultimate means like ultracentrifugation in which the colloidal particles settle down due to high force of centrifugation.
There are total 8 types of colloidal solutions possible which can be made by the combination of different physical states of both dispersed phase and the dispersion media as follows:

	Dispersed Phase	Dispersed Medium	Name	Example
1.	Solid	Solid	Solid Sol	Gem Stones
2.	Solid	Liquid	Sol	Muddy Water
3.	Solid	Gas	Aerosol	Smoke dust
4.	Liquid	Solid	Gel	Cheese
5.	Liquid	Liquid	Emulsion	Milk
6.	Liquid	Gas	Aerosol	Fog
7.	Gas	Solid	Solid Foam	Pumice Stone
8.	Gas	Liquid	Foam	Froth

So, from the above table, it is pretty much clear that pumice stone is a colloidal solution in which a gaseous dispersed phase is dispersed throughout a solid dispersion medium.

So the correct option is D. Gas in a solid

Note:
Students must keep in mind that a gas never dissolves heterogeneously in a gas. That is why, a gas in gas colloid is never possible because gas in gas is always a true solution.