Classifying matter and dispersed systems
Chemists sort matter in steps. Pure substances are elements (one kind of atom, e.g. copper) or compounds (fixed ratio, e.g. water). Compounds can be sorted again as oxides, acids, bases and salts, and one class can turn into another: a metal → its oxide → a base → a salt; a non-metal → its oxide → an acid → a salt.
Mixtures are sorted by how finely one part is spread in the other. This is a dispersed system:
- Dispersed phase: the substance broken into small bits.
- Dispersion medium: the substance it is spread through.
| True solution | Colloid | Suspension | |
|---|---|---|---|
| Particle size | < 1 nm | 1–100 nm | > 100 nm |
| Seen by eye / microscope? | No | No (ultramicroscope only) | Often yes |
| Settles on standing? | No | No | Yes |
| Passes filter paper? | Yes | Yes | No |
| Tyndall effect? | No | Yes | Yes (cloudy) |
| Example | salt water, sugar water | milk, starch in water, fog | muddy water, chalk in water |
1 nm (nanometre) = 10−9 m. A sugar molecule is about 1 nm; a hair is about 80 000 nm thick.
The Tyndall effect
Light passes straight through a true solution because its particles are far smaller than the wavelength of light (400–700 nm). Colloid particles are big enough to scatter light sideways, so from the side you see the beam's path. This is the Tyndall effect, the quickest test to tell a colloid from a solution: shine a laser pointer or torch through both glasses in a dark room.
Everyday cases: beams of sunlight in a forest or dusty room, a cinema projector beam, searchlights in fog, the blue look of diluted milk.
Types of colloids
Name colloids by the state of the dispersed phase and the medium:
| Dispersed phase | Medium | Name | Examples |
|---|---|---|---|
| Liquid | Gas | Aerosol | fog, mist, clouds, deodorant spray |
| Solid | Gas | Aerosol | smoke, dust in air |
| Gas | Liquid | Foam | shaving cream, soap lather, whipped cream |
| Liquid | Liquid | Emulsion | milk, mayonnaise, face cream |
| Solid | Liquid | Sol | paint, ink, starch or egg white in water |
| Liquid | Solid | Gel | jelly, cheese, butter, hair gel |
| Gas | Solid | Solid foam | sponge, pumice, foam rubber |
| Solid | Solid | Solid sol | ruby glass, some gemstones, alloys |
Gas in gas is never a colloid: gases always mix completely.
An emulsifier (soap, egg yolk) keeps two liquids that do not mix, like oil and water, as a stable emulsion.
Why colloids stay stable: Brownian motion and charge
Seen under an ultramicroscope, colloid particles move in a non-stop zig-zag. Water molecules hit them unevenly from all sides: this is Brownian motion. It keeps them from settling.
Most colloid particles also carry the same electric charge (they pick up ions on their surface). Like charges repel, so particles cannot come close and join.
Coagulation and its uses
Coagulation (flocculation) is the clumping and settling of colloid particles. Ways to cause it:
- Add an electrolyte (salt): its oppositely charged ions cancel the particles' charge. Ions with a larger charge (Al³⁺, SO₄²⁻) work much better than Na⁺ or Cl⁻.
- Heat: boiling an egg turns liquid egg white into solid.
- Mix two colloids of opposite charge.
- Electrophoresis: an electric field pulls particles to one electrode.
Uses: alum purifies drinking water; river mud coagulates where it meets salty sea water, building deltas; a Cottrell precipitator removes smoke particles from factory chimneys with high voltage; alum or ferric chloride stops bleeding from a small cut by coagulating blood; curd and paneer form when milk proteins coagulate.
Try it at home
- Take three clear glasses of water. Stir salt into the first, a little milk into the second, and a spoon of soil into the third.
- In a dark room, shine a phone torch through each from the side. Where can you see the beam?
- Leave them for 20 minutes. Which one has a layer at the bottom?
- Pour each through a coffee filter or tissue. Which ones pass through?
Predict first, then check with the 3D free play.
Key formulas and definitions
- 1 nm = 10⁻⁹ m
- Solution < 1 nm · Colloid 1–100 nm · Suspension > 100 nm
- Colloid = dispersed phase + dispersion medium
- Coagulating power rises with ion charge: Al³⁺ > Mg²⁺ > Na⁺ (Hardy–Schulze rule)
Worked examples
1. A mixture has particles of 50 nm. Name the type and say if it shows the Tyndall effect.
50 nm lies between 1 and 100 nm → colloid. Its particles scatter light → yes, Tyndall effect.
2. Name the dispersed phase and medium in (a) fog (b) shaving cream (c) butter.
(a) liquid water drops in gas (air) – aerosol. (b) gas (air) in liquid – foam. (c) liquid (water) in solid fat – gel.
3. Starch is shaken with water and filtered. The filtrate looks clear. How can you show it is not a true solution?
Shine a torch through it in the dark. The beam path is visible (Tyndall effect), so it is a colloid, even though it passed filter paper.
4. Why does a little alum make muddy water clear?
Clay particles are negatively charged and repel each other. Alum gives Al³⁺ ions which cancel the charge; the particles clump (coagulate) and settle.
5. Which will coagulate a negatively charged sol fastest: NaCl, MgCl₂ or AlCl₃?
The positive ion does the work. Al³⁺ has the highest charge, so AlCl₃ is fastest.
6. Convert 0.25 µm to nm and classify a mixture with particles this size.
1 µm = 1000 nm, so 0.25 µm = 250 nm > 100 nm → suspension; it settles on standing.
Common mistakes
- Thinking a colloid must look cloudy. Many colloids (dilute starch, gelatin) look clear; test with a beam.
- Saying colloids settle like suspensions. Brownian motion and charge keep them spread.
- Listing gas-in-gas as a colloid. Gases always form true solutions.
- Thinking filter paper separates a colloid. Colloid particles are smaller than its pores; special membranes are needed.