# Separating Mixtures

> Filtration, evaporation, crystallisation, distillation, chromatography and more, and how to pick the right method.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/separating-mixtures/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: Water from the sea
A group of hikers got lost on a rocky coast with no fresh water. Their leader, a chemistry teacher, did something clever.

She dug a hole, put a cup in the middle, and filled the space around it with sea water. She stretched plastic wrap over the hole and put a small stone in the centre, right above the cup. As the sun heated the sea water, pure water evaporated, condensed on the cool plastic, ran down to the lowest point under the stone, and dripped into the cup. The salt stayed behind.

By evening they had a cup of drinkable water. She had built a solar still, using the same idea that every chemistry lab uses to separate mixtures: use a **difference in properties** to pull the parts apart.

**In short:** Mixtures are separated by physical methods that use a difference in the properties of their parts. Filtration removes an insoluble solid, evaporation and crystallisation recover a dissolved solid, distillation separates liquids by boiling point, and chromatography separates dissolved dyes.

## The key idea

In a mixture, each substance keeps its own properties. So you can separate them by using a property that is **different** for each one:

- Particle **size** (sieving, filtering)
- **Boiling point** (distillation, evaporation)
- **Density** (decanting, separating funnel, centrifuge)
- **Magnetism** (magnetic separation)
- How well they **dissolve** or how they move through paper (chromatography)

All of these are **physical** methods. No new substances are made.

## Simple methods

| Method | How it works | Separates | Example |
|---|---|---|---|
| **Hand picking** | Pick out the pieces | Large solids you can see | Taking peas out of rice |
| **Sieving** | Small bits fall through the mesh | Solids of different sizes | Sand from gravel |
| **Magnetic separation** | A magnet pulls out magnetic metals | Iron, nickel, cobalt from other solids | Iron filings from sand, recycling steel cans |
| **Decanting** | Carefully pour off the liquid | A liquid from a heavy solid that has settled | Water off sand |
| **Separating funnel** | Run off the bottom (denser) layer through a tap | Two liquids that do not mix | Oil and water |
| **Centrifuge** | Spin very fast so dense parts move to the bottom | Solids that settle slowly | Blood cells from plasma |

## Filtration

**Filtration** separates an **insoluble solid** from a **liquid**. The mixture is poured through filter paper in a funnel.

- The solid that stays in the paper is the **residue**.
- The liquid that passes through is the **filtrate**.

Sand and water can be filtered. Salt water **cannot**, because the dissolved salt particles are tiny and pass straight through the paper.

*Diagram: Filtration: a funnel with filter paper holding sand as the residue, sitting on a flask that collects clear filtrate*
*The filter paper traps the sand. The water runs through.*

## Evaporation and crystallisation

To get a **dissolved solid** back out of a solution, remove the solvent.

- **Evaporation**: heat the solution until all the water has gone, leaving the solid. Quick, but crystals may be small or the solid may break down.
- **Crystallisation**: heat gently to remove **some** of the water, then leave it to cool. Crystals form slowly as the solution can no longer hold all the solute. This gives larger, purer crystals. Sea salt and sugar are made this way.

## Distillation

**Distillation** separates a liquid from a solution, or two liquids with **different boiling points**. It **keeps** the liquid, unlike evaporation.

1. Heat the mixture. The substance with the lower boiling point **evaporates** first.
2. The vapour passes into a **condenser**, a tube cooled by cold water flowing around it.
3. The vapour **condenses** back into a liquid and drips into a beaker. This pure liquid is the **distillate**.

Distillation turns sea water into drinking water and separates alcohol from water.

## Chromatography

**Paper chromatography** separates **dissolved substances**, such as the dyes in ink.

1. Draw a pencil line near the bottom of the paper (pencil, because ink would run).
2. Put a dot of the ink on the line.
3. Stand the paper in a small amount of solvent, below the line.
4. The solvent soaks up the paper and carries the dyes with it. Dyes that dissolve better and stick less to the paper travel **further**.

A pure substance gives **one spot**. A mixture gives **several**. Police use chromatography to match inks, and food scientists use it to check food colourings.

### Key idea: Pick the method by asking two questions
1. **What are the parts?** (solid and liquid, two liquids, dissolved solid, dyes)
2. **Which do I want to keep?** (the solid, the liquid, or both)

| Mixture | Keep | Method |
|---|---|---|
| Sand and water | Sand | Filtration |
| Salt water | Salt | Evaporation or crystallisation |
| Salt water | Water | Distillation |
| Oil and water | Both | Separating funnel |
| Dyes in ink | See them | Chromatography |
| Iron and sulfur powders | Both | Magnet |

### Worked example: Sand, salt and water
How do you get dry sand and dry salt from a mixture of sand, salt and water?

1. **Filter**: sand stays in the paper (residue). Salt water passes through (filtrate).
2. Rinse and dry the sand.
3. **Evaporate** or **crystallise** the filtrate to get the salt.

### Watch out: Filtering salt water does nothing
Dissolved substances go straight through filter paper. To get salt out of water you must evaporate or distil.

**Mind map**

```text
Separating mixtures
  Big idea
    Use a property that differs
    Physical methods only
  Size
    Sieving
    Filtration: residue, filtrate
  Density
    Decanting
    Separating funnel
    Centrifuge
  Boiling point
    Evaporation
    Crystallisation
    Distillation
  Other
    Magnet
    Chromatography
```

### Year 11: Fractional distillation and Rf values
**Fractional distillation** separates a mixture of several liquids with **close boiling points**. A tall **fractionating column** sits above the flask. It is hot at the bottom and cooler at the top, so each liquid condenses at a different height. Crude oil is separated into petrol, diesel, kerosene and other fractions this way.

In chromatography, each substance has an **Rf value**:

$$R_f = \frac{\text{distance moved by the spot}}{\text{distance moved by the solvent}}$$

If the solvent moves 10.0 cm and a spot moves 4.0 cm, $R_f = \frac{4.0}{10.0} = 0.40$. Rf is always between 0 and 1. Comparing Rf values with known substances (in the same solvent) identifies the parts of a mixture.

### Year 12: Precipitation as a separation
**Precipitation** is a chemical way to remove an ion from a solution. You add a chemical that reacts with the unwanted ion to form an **insoluble solid** (a precipitate), then filter it out.

To remove lead ions from waste water, add sodium iodide. Lead iodide is insoluble and forms a yellow solid:

$$\mathrm{Pb^{2+}(aq)} + \mathrm{2I^-(aq)} \rightarrow \mathrm{PbI_2(s)}$$

Water treatment plants use similar reactions, and analytical chemists use them to measure how much of an ion is present by weighing the dried precipitate (**gravimetric analysis**).

## Quick summary

- Mixtures are separated by physical methods that use a **difference in properties**.
- **Filtration**: insoluble solid from a liquid. Residue stays, filtrate passes through.
- **Evaporation** and **crystallisation**: get a dissolved solid back.
- **Distillation**: get the liquid back, or separate liquids by boiling point.
- **Separating funnel**: two liquids that do not mix. **Centrifuge**: spin to separate by density.
- **Chromatography**: separate dissolved dyes. A pure substance gives one spot.
- **Magnets** pull out iron, nickel and cobalt.

## Quiz

Try these questions from the chapter quiz.

1. Which method separates sand from water?
   - A. Filtration
   - B. Distillation
   - C. Chromatography
   - D. Using a magnet
   - Answer: A. Filtration. Sand is insoluble, so filter paper traps it.

2. In filtration, the liquid that passes through the filter paper is the:
   - A. Residue
   - B. Filtrate
   - C. Solvent front
   - D. Distillate
   - Answer: B. Filtrate. The filtrate goes through. The residue stays behind.

3. Which method gets pure water from sea water?
   - A. Filtration
   - B. Crystallisation
   - C. Distillation
   - D. Sieving
   - Answer: C. Distillation. Distillation evaporates the water and condenses it, leaving the salt.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/separating-mixtures/#quiz