2.5 Colloids & Suspensions

This topic explains two types of heterogeneous mixtures — colloids and suspensions — and compares them with true solutions.

Definition

Colloid

These are heterogeneous mixtures in which the solute particles are larger than those present in true solutions but not large enough to be seen by naked eye.

A colloid is a mixture that has particles ranging between 1 and 1000 nanometers in diameter, yet are still able to remain evenly distributed throughout the solution.

These are also known as colloidal dispersions because the substances remain dispersed and do not settle to the bottom of the container.

Simple explanation: A colloid is in between a true solution and a suspension — the particles are bigger than in a solution but still small enough to stay mixed.

Suspension

A suspension is defined as a heterogeneous mixture in which the solid particles are spread throughout the liquid without dissolving in it.

It is a mixture of undissolved particles in a given medium. Particles are big enough (greater than 1000 nm) to be seen with naked eyes.

Simple explanation: A suspension has large particles that float in the liquid but do not dissolve. They settle down if left standing.

Main Idea

Colloids and suspensions are both heterogeneous mixtures, but they differ in particle size and behaviour:

Core Concept

Tyndall Effect

The particles in a colloid are big enough to scatter a beam of light. This is called the Tyndall effect.

We can see the path of the scattered light beam inside the colloidal solution.

Tyndall effect is the main characteristic which distinguishes colloids from solutions.

Hence, colloids are also called false solutions or colloidal solutions.

Simple explanation: If you shine a torch through a colloid (like milk or fog), you can see the beam of light. In a true solution (like salt water), you cannot.

Detailed Explanation

Colloids

Particle size: 1 to 1000 nm

• Particles are larger than in true solutions but not large enough to be seen by naked eye.
• Particles remain evenly distributed — do not settle to the bottom.
• Particles are big enough to scatter light (Tyndall effect).
• They are heterogeneous mixtures.

• starch
• albumin
• soap solutions
• blood
• milk
• ink
• jelly
• toothpaste

Simple explanation: Milk looks uniform, but it is actually a colloid of fat droplets in water. You can see the Tyndall effect if you shine light through diluted milk.

Suspensions

Particle size: greater than 1000 nm

• Solid particles are spread throughout the liquid without dissolving.
• Particles are big enough to be seen with naked eye.
• Particles settle down when left standing.
• They are heterogeneous mixtures.

• chalk in water (milky suspension)
• paints
• milk of magnesia (suspension of magnesium oxide in water)

Simple explanation: If you stir chalk into water, it looks cloudy. If you let it sit, the chalk settles at the bottom — that's a suspension.

Comparison Table

The book gives this detailed comparison:

Intermediate Level

Why Colloids Don't Settle

Colloid particles are small enough that Brownian motion keeps them suspended. They do not settle out on standing, unlike suspension particles.

Why Colloids Show Tyndall Effect

The particle size in a colloid (1–1000 nm) is similar to the wavelength of visible light. So when light passes through a colloid, the particles scatter the light, making the beam visible. This is the Tyndall effect.

Filtration Behaviour

• Solution: particles pass through all filters (including ultra-filter).
• Colloid: particles pass through ordinary filter paper but are trapped by ultra-filter.
• Suspension: particles are trapped even by ordinary filter paper.

This is another way to distinguish the three types.

Important Points

• - Colloid: heterogeneous mixture; particle size 1–1000 nm; particles not visible to naked eye but scatter light (Tyndall effect); particles remain dispersed.
• - Suspension: heterogeneous mixture; particle size > 1000 nm; particles visible to naked eye; particles settle down on standing.
• - Tyndall effect distinguishes colloids from solutions.
• - Colloids are also called false solutions or colloidal solutions.
• - Examples of colloids: starch, albumin, soap, blood, milk, ink, jelly, toothpaste.
• - Examples of suspensions: chalk in water, paints, milk of magnesia.

Remember

• - Colloid: 1–1000 nm; not visible; Tyndall effect; does not settle.
• - Suspension: > 1000 nm; visible; settles down.
• - Tyndall effect = scattering of light by colloidal particles.
• - Colloids = false solutions.
• - Examples of colloids: milk, blood, ink, jelly, toothpaste.
• - Examples of suspensions: chalk in water, paints, milk of magnesia.

Common Mistakes

• 1. Thinking colloids are true solutions. Colloids are heterogeneous, not homogeneous.
• 2. Thinking colloids settle down. They do not settle (at least not for a long time).
• 3. Confusing colloids with suspensions. Suspensions have larger particles and settle.
• 4. Thinking all cloudy mixtures are suspensions. Milk is a colloid, not a suspension.
• 5. Forgetting the Tyndall effect. It is the key test to identify a colloid.

Quick Revision

• Colloid: heterogeneous mixture; particles 1–1000 nm; not visible to naked eye; scatter light (Tyndall effect); do not settle; pass through normal filter paper but not ultra-filter.
• Suspension: heterogeneous mixture; particles > 1000 nm; visible to naked eye; settle down on standing; cannot pass through ordinary filter paper.
• Solution: homogeneous; particles < 1 nm; not visible; no Tyndall effect; does not settle.
• Tyndall effect is the main test for colloids.
• Colloids are also called false solutions.
• Examples: colloids — milk, blood, ink, jelly, toothpaste; suspensions — chalk in water, paints, milk of magnesia; solutions — sea water.

60-Second Revision

• Colloid: 1–1000 nm; Tyndall effect; no settling.
• Suspension: > 1000 nm; visible; settles.
• Solution: < 1 nm; no Tyndall effect; no settling.
• Tyndall effect distinguishes colloids from solutions.
• Examples: milk (colloid), muddy water (suspension).
• Warning: colloids are heterogeneous, not homogeneous.