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💬 Think about it
- In a suspension, big particles float about and settle when left alone.
- In a solution, the particles are so small they simply disappear.
- But milk does neither. It looks uniform, yet it is not a true solution. What exactly is it?
What is a colloid?
- A colloid is a heterogeneous mixture that looks completely homogeneous, but whose particles are a little larger than those in a solution and smaller than those in a suspension.
- The commonest everyday examples are milk , tomato sauce and ice cream .
- Look carefully at milk — its composition seems perfectly uniform, as if it were a true solution. But its story is different.
Definition
A colloid is a heterogeneous mixture whose particles are between 1 nm and 1000 nm in size — too small to be seen and too small to settle, yet large enough to scatter light.
How does a colloid behave?
A colloid
A suspension
Left undisturbed, the particles
never settle
The particles settle at the bottom
The mixture is completely
stable
The mixture is unstable
Particles pass
through
ordinary filter paper
Particles are held back by filter paper
Particles cannot be seen with the naked eye
Particles are clearly visible
- So a colloid cannot be separated by ordinary filtration at all.
Is blood a suspension, or a solution?
- Your book asks this through a very interesting question. Blood can be separated by centrifugation, and blood also coagulates — so is it a suspension?
- But we cannot see blood cells with the naked eye — so is it a solution?
- The answer is that blood is neither a true solution nor an ordinary suspension. Blood is a colloid .
- Blood cells are not small enough to disappear into the water the way a true solute does, and they do not settle under gravity the way a suspension does.
- They stay uniformly spread through the blood plasma.
How do the three kinds of mixture compare by particle size?
| Mixture | Particle size | Visible to the naked eye? |
|---|---|---|
| True solution | Smallest — less than 1 nm in diameter | No |
| Colloid | Medium — between 1 nm and 1000 nm | No |
| Suspension | Largest — more than 1000 nm in diameter | Yes |
Because of this size difference, the particles of solutions and colloids are invisible to us, while the mud particles of a suspension are clearly visible.
📝 Summary - What we have learnt
- A colloid looks homogeneous but is really heterogeneous; milk, tomato sauce and ice cream are colloids.
- Its particles never settle and pass through filter paper , so it is stable and cannot be filtered.
- Blood is a colloid — the cells stay evenly spread through the plasma.
- Particle size: solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
Recap — Section 5.4 — Separating Heterogeneous Mixtures
✅ Test Yourself
-
Milk looks perfectly uniform, like a true solution. Is it one?
View Answer
No — milk is a colloid. Its particles are too small to settle or be seen, but large enough to scatter light, unlike a true solution. -
Why does camphor mixed with sand end up as a deposit on a funnel wall instead of melting into a liquid?
View Answer
Camphor sublimes — it turns directly from solid to vapour without becoming a liquid. That vapour then touches the cool funnel wall and deposits back as a solid. -
What is the outward force called that pushes heavier particles to the bottom of a spinning centrifuge tube?
View Answer
Centrifugal force — the same force you feel pulling you outward when you spin around holding a partner's hands.
Important Definitions
- Immiscible liquids
- Liquids that do not mix and form two separate layers, such as oil and water.
- Sublimation
- A solid changing directly into vapour without passing through the liquid state.
- Suspension
- A heterogeneous mixture in which undissolved solid particles stay suspended in a medium.
- Centrifugation
- Spinning a mixture at high speed so the denser particles are driven to the bottom of the tube.
- Coagulation
- Fine suspended particles joining together into larger clumps so that they can settle.
- Colloid
- A heterogeneous mixture with particles of 1 to 1000 nm that never settle and cannot be filtered.
Questions at the End of the Chapter — Related to This Section
📋 NCERT Question 5 — Cooking oil and water
A pack of cooking oil is mixed with water — will layers form, which one stays on top, and how would you separate them?
View Answer →
📋 NCERT Question 7 — Choosing a method for each mixture
Match each mixture in a table to the separation method that suits it, and explain the reason.
View Answer →
📋 NCERT Question 10 — Blood is a colloid
Blood is neither a true solution nor an ordinary suspension — identify its dispersed phase and dispersion medium.
View Answer →
📋 NCERT Question 11 — Sand, common salt and naphthalene
A mixture of sand, common salt and naphthalene is separated in steps — put the steps in the correct order.
View Answer →
