Pure Substances vs Mixtures
In 2006, Food Standards Australia New Zealand tested 100 commercial bottled water samples and found dissolved minerals in every single one — "pure" on the label simply means safe to drink, not chemically pure.
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Q1 · List three things in your kitchen you would call "pure" and three things you'd call "mixed". What test did you use?
Q2 · Saltwater looks completely clear and even, but it isn't a single substance. Why is it still called a mixture?
● Know
- The definition of a pure substance and a mixture
- The difference between homogeneous and heterogeneous mixtures
- That mixtures can be separated by physical means
● Understand
- Why "pure" in chemistry is stricter than "pure" in the shops
- How to use a particle diagram to tell pure from mixed
- Why saltwater is "homogeneous" but muesli is "heterogeneous"
● Can do
- Classify common items as pure, homogeneous mix or heterogeneous mix
- Draw simple particle diagrams of each
- Spot Australian everyday examples (sand, seawater, bushfire smoke)
- Pure substance
- Mixture
- Homogeneous
- Heterogeneous
- Solution
- Mixture that looks even all the way through
- Matter made of one kind of particle only
- Homogeneous mixture where something is dissolved
- Mixture where you can see separate parts (chunky)
- Two or more substances physically combined
Pour a glass of water and stir in a spoonful of salt until it disappears — the liquid now looks exactly the same, but you know two different things are in there. That glass is a mixture; pure water is not. In chemistry, a pure substance contains only one type of particle — nothing else mixed in. Stick it under a powerful microscope and every particle would look the same.
Examples of pure substances:
- Distilled water — only H₂O molecules, nothing else.
- Pure gold — only gold atoms (Au).
- Oxygen gas in a science lab cylinder — only O₂ molecules.
- Table sugar (sucrose) — only sucrose molecules.
What about the "100% pure" labels on shop products? They use "pure" loosely. "Pure spring water" actually contains dissolved minerals (calcium, magnesium, fluoride). "Pure orange juice" is mostly water, sugar, fibre, vitamins and acids. Chemists call all of those mixtures.
One test: a pure substance always has the same boiling and melting point, and looks exactly the same at every microscopic level. If those properties change from sample to sample, it's a mixture.
A mixture is two or more substances physically stirred together. Important features:
- Each part keeps its own properties — salt in saltwater still tastes salty.
- The parts can be separated using physical methods (filtering, evaporating, sieving). No chemical reaction needed.
- The amounts can vary — you can have weak tea or strong tea using the same ingredients.
Australian everyday mixtures:
| Mixture | What's in it | How you could separate it |
|---|---|---|
| Seawater | Water + salt + tiny amounts of minerals | Evaporate the water → salt left behind |
| Beach sand | Quartz grains + shell bits + bits of other rocks | Sieve it / use a magnet for iron-rich grains |
| Bushfire smoke | Air + soot + water vapour + gases | Filter the soot, condense the vapour |
| Eucalyptus oil (raw) | Oil + water from steam distillation | Let them separate by density and pour off the oil |
Mixtures come in two flavours. The names are long but the idea is simple.
- Homogeneous (homo- means same): the mixture looks the same all the way through. You can't see the different parts even with a magnifying glass. Saltwater, air, brass and clean apple juice are homogeneous.
- Heterogeneous (hetero- means different): you can clearly see different bits. Muesli, beach sand, pizza, a salad and Italian dressing are heterogeneous.
The quick test: scoop a teaspoon from anywhere in the mixture. If every scoop is identical → homogeneous. If different scoops are different → heterogeneous.
A mixture looks the same all the way through, like . A mixture has visible separate parts, like .
Wrong: "Saltwater isn't a mixture — it's just one liquid." Saltwater looks even, but it has two substances (salt + water). Evaporate the water and the salt stays behind. That's the proof.
Right: Saltwater is a homogeneous mixture. It looks even, but it has two substances that can be separated by evaporation.
Wrong: "Air is a pure substance — it's just one invisible gas." Air is a mixture of about 78% nitrogen, 21% oxygen, plus argon, carbon dioxide, water vapour and traces of others. It's actually a textbook homogeneous mixture.
Right: Air is a mixture of many gases — invisible, but definitely not pure.
Wrong: "Anything you can see the parts of must be a mixture." True — if you can see different parts it's heterogeneous. But many mixtures look perfectly even (saltwater, air), so "I can't see anything else" does NOT mean "pure".
Right: Looking even doesn't prove purity. Pure means one kind of particle, which often needs lab tests to confirm.
Use these three questions in order whenever you meet a new sample:
- One kind of particle? → Pure substance.
- Two or more substances physically combined? → Mixture.
- If it's a mixture, can you see the separate parts? → Yes = heterogeneous. No = homogeneous.
| Item | Category | Why |
|---|---|---|
| Distilled water | Pure | Only H₂O molecules |
| Pure copper wire | Pure | Only copper atoms |
| Tap water | Homogeneous mixture | Water + dissolved minerals — looks even |
| Air | Homogeneous mixture | N₂ + O₂ + others, evenly mixed |
| Italian salad dressing | Heterogeneous mixture | You can see oil and vinegar layers |
| Beach sand | Heterogeneous mixture | Different coloured grains visible |
| Muesli | Heterogeneous mixture | Oats, raisins, nuts — all visible |
A glass of clear apple juice and a glass of cloudy orange juice are sitting side by side. Predict which is homogeneous and which is heterogeneous, and explain in one sentence.
How close was your prediction?
Nice — you used the "can you see the parts?" test correctly.
Good — remember the visible-parts test for telling the two mixtures apart.
Earlier you were asked: Saltwater looks completely clear and even — why is it still a mixture?
Now that you've worked through the lesson, write a fuller answer. Use the words homogeneous, mixture and physical at least once each.
Q1. Explain the difference between a pure substance and a mixture. Give one example of each. (3 marks)
Q2. Sort the following items into pure substance, homogeneous mixture or heterogeneous mixture: distilled water, beach sand, air, vegetable soup. Give a one-line reason for each. (4 marks)
Q3. A friend says "anything you can't see lumps in must be a pure substance". Evaluate this claim using at least two examples to argue your point. (4 marks)
Answers
▾MCQ 1
B — A pure substance has only one kind of particle. A label, colour, or natural source doesn't make something pure in the chemistry sense.
MCQ 2
D — Muesli has visibly different parts (oats, raisins, nuts) — that's the test for heterogeneous. Distilled water and pure oxygen are pure substances; salt water is homogeneous because it looks even.
MCQ 3
A — Air is made of many gases evenly mixed at the molecular level, so it's the textbook homogeneous mixture (definitely not pure, definitely not heterogeneous).
MCQ 4
C — Evaporation drives off the water (as water vapour) and leaves the salt as a solid. Filtering wouldn't work — the dissolved salt would go straight through the filter paper.
MCQ 5
B — Tap water has water plus dissolved minerals and added chlorine — multiple substances, so it's a homogeneous mixture, not a pure substance. Looking clear isn't proof of purity.
Short Answer 1
Model answer: A pure substance contains only one type of particle (e.g. distilled water — only H₂O molecules — or pure gold — only gold atoms). A mixture contains two or more substances physically combined; each substance keeps its own properties and the parts can be separated by physical methods (e.g. saltwater — water plus dissolved salt — or muesli).
Short Answer 2
Model answer: Distilled water = pure substance (only H₂O molecules). Beach sand = heterogeneous mixture (you can see different coloured grains, shell bits etc.). Air = homogeneous mixture (gases evenly spread, looks the same everywhere). Vegetable soup = heterogeneous mixture (visible chunks of vegetables in liquid).
Short Answer 3
Model answer: The claim is wrong because many mixtures look perfectly even at the visible level but still contain more than one substance — these are called homogeneous mixtures. For example, saltwater looks like one clear liquid but evaporating it leaves salt crystals behind, proving it had two substances. Air looks like one invisible gas but it's a mix of nitrogen, oxygen, argon and others. Looking even is NOT the same as being pure — being pure means only one kind of particle, which usually needs a lab test (e.g. constant boiling point) to confirm.