Separation of Mixtures¶
Syllabus mapping
CH-11-01 — Properties and Structure of Matter. Covers: using the physical properties of the parts of homogeneous and heterogeneous mixtures to explain how they can be separated; Aboriginal methods of separating mixtures — extracting dyes from plants and using woven baskets for filtration (explicit, examinable content integration point); a laboratory experiment to find the percentage composition of a mixture by mass.
Wording above is paraphrased for teaching use — check the official syllabus before treating any line as verbatim NESA text.
You need to know: the difference between a pure substance and a mixture, the difference between homogeneous and heterogeneous mixtures, which separation technique to use for which mixture (and why it works), and how to calculate the percentage composition of a mixture by mass.
Assumed knowledge check¶
Before this page makes sense, you should be comfortable with (all from Years 7–10 Science):
- The particle model — solids, liquids and gases are made of particles that are always moving
- What "dissolve", "solute", "solvent" and "solution" mean
- Melting point and boiling point
Core¶
Pure substances vs mixtures¶
A pure substance has only one kind of particle in it, and a fixed set of properties — pure water always boils at 100 °C at sea level.
- Elements are pure substances made of only one kind of atom (e.g. iron, oxygen).
- Compounds are pure substances made of two or more elements chemically bonded together in a fixed ratio (e.g. water, \(\ce{H2O}\); table salt, \(\ce{NaCl}\)).
A mixture is two or more substances mixed together but not chemically bonded. Each part of a mixture keeps its own properties — that's the key idea this whole page is built on.
Homogeneous vs heterogeneous mixtures¶
| Homogeneous | Heterogeneous | |
|---|---|---|
| What it looks like | The same all the way through — you can't see separate parts | Not the same all the way through — you can often see separate parts |
| Examples | Salt water, air, vinegar, brass (an alloy) | Sand and water, muddy water, oil and water, granite |
| Also called | A solution (when something is dissolved) | — |
A quick test: if you can't see the different parts even with a magnifying glass, it's homogeneous.
The big idea: use a difference in a physical property¶
Because each part of a mixture keeps its own properties, you can pull the parts apart by finding a physical property that's different between them — size, boiling point, density, solubility, or magnetism. No chemical reaction needed.
| The parts differ in… | Technique | How it works | Example |
|---|---|---|---|
| Particle size (one part doesn't dissolve) | Filtration | Liquid and dissolved particles pass through the filter paper; undissolved solid gets stuck | Sand from water |
| Particle size (two solids) | Sieving | Small pieces fall through the mesh, big pieces stay | Gravel from sand |
| Boiling point (a solid dissolved in a liquid) | Evaporation / crystallisation | Heat drives off the liquid, the dissolved solid is left behind | Salt from salt water |
| Boiling point (you want to keep the liquid) | Distillation | The liquid boils, the vapour is cooled back into a liquid in a condenser and collected | Pure water from sea water |
| Boiling point (two liquids that mix) | Fractional distillation | Liquids with lower boiling points rise further up a column before condensing | Ethanol from water; separating crude oil |
| Density (two liquids that don't mix) | Separating funnel or decanting | The denser liquid sinks to the bottom and is drained (or poured) off | Oil and water |
| Density (fine solid in a liquid) | Centrifuging | Spinning very fast forces denser particles to the bottom | Blood cells from blood plasma |
| How strongly each part sticks to paper vs dissolves in a solvent | Chromatography | Parts that dissolve better travel further up the paper | The different dyes in a black marker |
| Magnetism | Magnetic separation | A magnet pulls out the magnetic part | Iron filings from sulfur powder |
Words you'll need: the solid caught in filter paper is the residue; the liquid that passes through is the filtrate; the liquid collected from distillation is the distillate.
Worked example — choosing a sequence
You're given a mixture of sand, salt and iron filings. Describe how to get all three back separately.
The trick is to deal with one difference at a time:
- Magnetism first. Run a magnet (wrapped in plastic wrap, so the filings come off easily) through the dry mixture. Iron filings stick to the magnet; sand and salt don't.
- Add water and stir. Salt dissolves; sand doesn't.
- Filter. Sand stays on the filter paper as the residue. Salt water passes through as the filtrate.
- Evaporate the filtrate. Water leaves; salt crystals are left behind.
Why magnetism first? If you add water first, the iron filings get wet, mixed in with the sand on the filter paper, and start to rust.
Percentage composition by mass¶
Once you've separated a mixture, you can work out how much of each part was in it:
Worked example
A student separates a \(12.0\ \text{g}\) sample of a sand–salt mixture. After filtering and drying, the sand has a mass of \(8.4\ \text{g}\). Find the percentage of sand and of salt in the mixture.
Step 1 — sand:
Step 2 — salt: the rest of the mixture must be salt.
Check: \(8.4 + 3.6 = 12.0\ \text{g}\) ✓ (30% of 12.0 g is 3.6 g)
Aboriginal and Torres Strait Islander content — placeholder, not yet written
NESA lists Aboriginal methods of separating mixtures — extracting dyes from plants and using woven baskets for filtration — as explicit, examinable syllabus content, not an optional cultural add-on. The chemistry links are real and straightforward (extracting a dye is a solubility-based separation; a woven basket works as a filter because of particle size), but the specific cultural practices, the plants and materials involved, and whose Knowledge this is need proper research — ideally with input from your school's Aboriginal Education team, local Community, or a community-verified source — rather than a generic paraphrase.
Deliberately left unwritten here rather than filled with unverified content.
Practical¶
Suggested prac: percentage composition of a sand–salt mixture by mass. Weigh a sample of the mixture, add water and stir to dissolve the salt, filter to collect the sand, then dry and weigh the sand. Evaporate the filtrate to recover the salt and weigh it too. Calculate the percentage of each part and check that the two masses add back up to the starting mass.
Things that commonly go wrong (and are worth discussing in your evaluation):
- Sand not fully dry when weighed → sand mass too high, salt percentage too low.
- Some salt solution soaked into the filter paper → recovered salt mass too low.
- Salt spitting out of the evaporating basin when heated too strongly → recovered salt mass too low. Heat gently near the end.
See Precision and Uncertainty and Evaluating Data for how to write these up properly.
Advanced
Why simple distillation isn't enough for two liquids. If two liquids have boiling points that are close together (e.g. ethanol at 78 °C and water at 100 °C), the vapour from simple distillation contains a lot of both. A fractionating column gives the vapour many chances to condense and re-evaporate on the way up — each time, it gets richer in the lower-boiling liquid. That's why fractional distillation is used for crude oil, where dozens of substances have boiling points close together.
Multi-step separations. Exam questions often give a mixture of three or four substances and ask for a full method. Write a table of each component's properties (soluble in water? magnetic? boiling point? state?) before choosing the order — then remove one component at a time, and check that each step doesn't ruin a later one (as with wetting the iron filings in the worked example above).
Error analysis with percentage composition. If the recovered masses add up to more than the starting mass, something extra is being weighed (usually water still in the sample). If they add up to less, something was lost. Being able to say which way an error pushes your result — and why — is exactly what "evaluate" questions want.
Extension
Beyond the syllabus for this course — won't appear in the HSC, included for interest / depth study inspiration.
📎 Depth study idea
In paper chromatography, each substance travels a characteristic fraction of the distance the solvent travels. That fraction is called the retention factor:
Under the same conditions (same paper, same solvent), a substance always has the same \(R_f\) value — so it can be used to identify unknown substances. A depth study could compare the dyes in different brands of food colouring or lollies, test how changing the solvent changes \(R_f\) values, or identify which pen wrote a "ransom note".
Mining connection. Australia's mining industry relies on large-scale separations — for example, froth flotation separates valuable metal ores from waste rock by making the ore particles stick to air bubbles that float to the top. It's the same big idea as this page (exploit a difference in a physical property), on an industrial scale.
Video/visual resources¶
- 🖼️ Diagram needed — labelled set-ups for filtration (funnel, filter paper, residue, filtrate), simple distillation (flask, thermometer at the side-arm, condenser with water in at the bottom and out at the top, distillate), and a separating funnel with two layers. Consistent glassware style across all three. TODO — placeholder until sourced/drawn.
- 🎥 Khan Academy — TODO: source a separating mixtures explainer covering filtration, distillation and chromatography.
- 🎥 TODO: source a short video of fractional distillation (lab or crude oil refinery) — the column is much easier to understand moving than in a still image.
Check yourself¶
-
Classify each as a pure substance, a homogeneous mixture or a heterogeneous mixture: (a) air, (b) carbon dioxide, (c) muddy water, (d) seawater.
Answer
(a) Homogeneous mixture — several gases, evenly mixed.
(b) Pure substance (a compound).
(c) Heterogeneous mixture — you can see the soil particles, and they settle out.
(d) Homogeneous mixture — the salts are dissolved, so it looks the same all the way through.
-
Name the best technique to separate: (a) oil from water, (b) pure water from ink, (c) the coloured dyes in a green lolly coating.
Answer
(a) Separating funnel (or decanting) — they don't mix and have different densities.
(b) Distillation — the water boils off and is condensed; the ink dyes stay behind.
(c) Chromatography — the dyes travel different distances up the paper.
-
A \(25.0\ \text{g}\) mixture of sand and copper(II) sulfate is separated. The dry sand has a mass of \(17.5\ \text{g}\). Calculate the percentage by mass of copper(II) sulfate.
Answer
Mass of copper(II) sulfate \(= 25.0 - 17.5 = 7.5\ \text{g}\)
\(\%\ = \dfrac{7.5}{25.0} \times 100 = 30\%\)
-
In the sand–salt prac, a student weighs the sand while it is still damp. Will their calculated percentage of salt be too high or too low? Explain.
Answer
Too low. The damp sand weighs more than it should, so the sand percentage comes out too high. If the salt percentage is worked out as \(100\% - \%\ \text{sand}\), it comes out too low.