Surface Chemistry — Part 2: Colloids, emulsions and coagulation: visual lesson, CBSE Class 12 Chemistry

Lesson

Colloids

Part 2 — mixtures between true solutions and suspensions.

In this lesson you will

Distinguish true solutions, colloids and suspensions by particle size and properties
Classify colloids by phases and by the interaction between dispersed phase and medium (lyophilic and lyophobic)
Explain the Tyndall effect, Brownian movement, charge on sols, electrophoresis and coagulation (Hardy–Schulze rule)
Describe types of emulsions and everyday uses of colloids

Concept

True solutions, colloids and suspensions

The three kinds of mixture differ in particle size, which controls their properties.

Table comparing true solutions, colloids and suspensions
Property	True solution	Colloid	Suspension
Particle size	< 1 nm	1–1000 nm	1000 nm
Appearance	Clear, homogeneous	Looks homogeneous; actually heterogeneous	Heterogeneous
Passes through filter paper?	Yes	Yes	No
Passes through parchment membrane?	Yes	No	No
Settles on standing?	No	No (only on centrifuging)	Yes
Scatters light (Tyndall)?	No	Yes	Yes, if not opaque
Example	Salt in water	Milk, starch sol, fog	Sand in water

Worked example

Worked example: solution, colloid or suspension?

Problem. Classify each mixture and give one test that supports your answer: (a) copper sulfate solution; (b) dilute milk; (c) chalk powder shaken in water; (d) a sol whose particles are $50\ \text{nm}$ across.

(a) True solution — clear, passes through parchment, no Tyndall effect.

(b) Colloid (emulsion) — beam visible from the side (Tyndall), does not settle.

(c) Suspension — chalk settles on standing and is held back by filter paper.

(d) Colloid — particle size lies between 1 nm and 1000 nm.

Table classifying four mixtures with a supporting test for each
Mixture	Type	Supporting test
Copper sulfate solution	True solution	No Tyndall effect
Dilute milk	Colloid	Tyndall effect; no settling
Chalk in water	Suspension	Settles; held by filter paper
50 nm particles	Colloid	Size between 1 and 1000 nm

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