Suspensions and Colloids

Introduction

Salt water is clear, muddy water is cloudy and settles, and milk is cloudy but never settles no matter how long you leave it. These three liquids show that there is more than one way to spread one substance through another. Between true solutions and mixtures that separate into layers lie suspensions and colloids . Recognising them helps explain why some mixtures can be filtered and others cannot, and why a torch beam shows up in fog.

Core explanation

The key difference between these mixtures is the size of the dispersed particles .

Solutions. The solute is spread out as single molecules or ions, typically less than about 1 nanometre across (1 nm = one millionth of a millimetre). Solutions are transparent, never settle, and pass straight through filter paper.

Suspensions. A suspension contains visible or near-visible particles of an insoluble solid spread through a liquid, typically larger than about 1 micrometre (one thousandth of a millimetre). Muddy water, chalk shaken in water and many medicines labelled "shake well before use" are suspensions. Suspensions:

- look cloudy or opaque; - settle out on standing, because gravity pulls the large particles down; - can be separated by filtration , because the particles are too big to pass through filter paper.

Colloids. A colloid has particles in between these sizes, roughly 1 nm to 1 micrometre. They are too small to settle or be trapped by filter paper but large enough to scatter light. Colloids:

- often look cloudy or milky; - do not settle on standing; - pass through ordinary filter paper; - show the Tyndall effect : a beam of light passing through is visible from the side, because the particles scatter it.

Types of colloid. Colloids can form between different states:

Type Dispersed in Example --- --- --- Emulsion (liquid in liquid) liquid milk, mayonnaise Foam (gas in liquid) liquid whipped cream, shaving foam Aerosol (liquid or solid in gas) gas fog, mist, smoke Gel (liquid in solid network) solid jelly, hair gel

Stabilising emulsions. Oil and water are immiscible, so shaken together they soon separate. An emulsifier such as egg yolk in mayonnaise keeps tiny oil droplets spread through water, making a stable colloid.

Step-by-step reasoning

To classify an unknown cloudy or clear mixture:

1. Leave it to stand. If particles settle, it is a suspension. 2. If nothing settles, pass it through filter paper; solids trapped suggest a suspension. 3. Shine a narrow beam of light through it. If the beam is visible from the side, it is a colloid. 4. If it is clear, does not scatter light and nothing is trapped, it is a solution.

Visual explanation

Picture three glass jars lit from the side by a laser pointer. In salt water the beam is invisible inside the liquid. In dilute milk the beam shows as a bright line straight through. In muddy water the light is blocked by swirling particles, and a layer of mud forms at the bottom after an hour.

Real-world analogy

Think of particles as people of different sizes in a crowd crossing a net. Tiny children (solution particles) slip straight through; medium-sized teenagers (colloid particles) also get through, but you can spot them waving; adults (suspension particles) get caught in the net and eventually sit down on the ground.

Real-world example

Fog on a road is a colloid of tiny water droplets in air. Car headlight beams become visible as bright cones in fog because the droplets scatter the light — the Tyndall effect. This is why fog lights are mounted low and angled down, reducing glare scattered back into the driver's eyes.

Why?

Why do colloid particles not settle? They are so small that the constant bombardment by moving solvent particles keeps them jostling in random directions. This random motion is stronger than the gentle pull of gravity on such tiny particles, so they stay spread out.

Common misconception

"Milk is a solution because it does not separate." Milk does not settle, but it is a colloid, not a solution: it contains tiny fat droplets and protein particles that scatter light, which is why milk is white and opaque rather than clear.

Worked example

Question: A liquid looks cloudy. After a day nothing has settled, and it passes through filter paper unchanged. A torch beam is clearly visible inside it. Classify the mixture.

Reasoning: Not settling and passing through filter paper rule out a suspension. Scattering of the light beam rules out a true solution.

Answer: It is a colloid.

Quick check

1. Which type of mixture separates on standing and can be filtered? Answer: A suspension.

Exam focus

Compare the three mixtures by particle size, appearance, settling, filtration and light scattering. Learn a named example of each: salt water (solution), milk or fog (colloid), muddy water (suspension). The Tyndall effect is the quickest test for a colloid.

Advanced insight

Water treatment works deliberately turn colloids into suspensions. Adding a coagulant such as aluminium sulfate makes tiny clay particles clump together into larger flocs, which then settle and can be filtered out. The same principle — making particles bigger — is used to clean waste water from many industries.

Summary

Mixtures can be classified by particle size. Solutions have single molecules or ions, are clear and cannot be filtered. Colloids have intermediate particles that do not settle, pass through filter paper and scatter light (the Tyndall effect). Suspensions have large particles that settle and can be filtered. Milk, fog and jelly are colloids; muddy water is a suspension.

Practice questions

1. State two differences between a suspension and a solution. Answer: A suspension is cloudy and its particles settle on standing, whereas a solution is transparent and never settles; suspensions can also be filtered, solutions cannot. 2. What is the Tyndall effect and which type of mixture shows it? Answer: The scattering of a light beam so it becomes visible from the side; it is shown by colloids. 3. Mayonnaise is an emulsion. Explain what this means. Answer: It is a colloid of tiny droplets of one liquid (oil) spread through another immiscible liquid (water), kept stable by an emulsifier. 4. Why does a medicine label say "shake well before use"? Answer: The medicine is a suspension whose particles settle on standing, so shaking spreads them evenly and each dose contains the correct amount.