Question

Student paper 2 solutions, a solution of alum and a solution of fine sand in water . Distinguish between the two on the basis of transparency and stability.

Answer 1

• A true solution- common salt, sugar and alum with water
• A suspension- soil, chalk powder and fine sand in water
• A colloid-starch in water and egg albumin in water

Dissolving solutes(salts,soil etc) in liquids may give rise to a homogeneous (solution) or heterogeneous material (a suspension or a colloid).The distinction lies in the size of the dispersed particles. In suspension, the particle size is of the order of 10^-5 cm or larger. The particles can be seen distinctly in the dispersion medium (solvent) by the naked eye or under an ordinary microscope. The suspensions are not very stable. The suspended particles may settle down after sometime, e.g.,muddy water or smoke in the air.

Colloids have particles smaller than 10^-5 cm (generally 10^-7 to 10^-5 cm). The dispersed particles show very little tendency to separate. Colloid particles cannot be seen by the naked eye or under an ordinary microscope. Colloids are stable. Gum, milk and blood are typical colloids.

In a solution, the particles are of molecular dimensions, smaller than 10^-7 cm. Solutions are transparent and homogeneous and very stable.

Alum completely dissolves in water.ie,it form a homogeneous solution with water so it is stable and transparent.
But sand in water forms suspension,which may settle after some time and not stable also not transparent.(Think of sand mixed with water which give a solution something like a muddy colour which is not transparent.)