Why do suspensions settle down on standing whereas colloids do not, and why do we need different methods for separating mixtures (Class 9)?

What you are being asked

You need to explain two ideas from “mixtures” in Class 9: (1) why suspension particles settle but colloid particles do not, and (2) why separation depends on the properties of the mixture.

(a) Why suspensions settle but colloids do not

Suspension

• Particles are relatively large (often visible) and not truly dissolved.
• Because the particles are bigger and heavier, gravity pulls them down when the mixture is left undisturbed.
• So a sediment forms at the bottom, and the mixture becomes non-uniform.

Colloid

• Particles are much smaller than in a suspension (not visible to the naked eye).
• They remain dispersed because:
• Random motion of particles helps keep them spread out, and
• Colloidal particles are often stabilized (they do not easily clump and fall).
• So colloids generally do not settle on standing.

(b) Why different mixtures need different separation methods

A separation method works only if it targets a property in which the components differ. Common properties used are:

• Particle size: filtration separates insoluble solids from liquids; it works well for suspensions but not for true solutions.
• Solubility: evaporation and crystallization separate dissolved solids from liquids.
• Boiling point: distillation separates liquids (or a liquid and dissolved substance) based on different boiling points.
• Density: sedimentation and decantation work when one component is much denser.
• Magnetism: magnets separate magnetic substances like iron from non-magnetic materials.

Because mixtures can be solid-solid, solid-liquid, liquid-liquid, and can have very different properties, no single method can separate every mixture.

Quick examples

• Sand + water (suspension): sedimentation, decantation, filtration.
• Salt + water (solution): evaporation or crystallization.
• Alcohol + water (miscible liquids): distillation.