Pure Substances vs Mixtures
You'll be able to classify common items as pure, homogeneous mix or heterogeneous mix.
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.
Printable Worksheets
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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.
Distilled water is pure because every particle in the bottle is an H₂O molecule and there is nothing else in there at all. Pure gold is pure for the same reason with a different particle: every atom in the bar is a gold atom. The oxygen in a science lab cylinder is only O₂ molecules, and table sugar out of a chemical supplier is only sucrose molecules. Notice that being pure has nothing to do with being an element. Gold is an element and water is a compound, and both are pure, because the test is not how simple the particle is but whether every particle in the sample is the same as every other one.
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 settles it, and you can run it in a school lab. A pure substance has a fixed, sharp melting point and a fixed, sharp boiling point, the same value every time, from every sample. Pure water boils at 100 °C, and it stays at 100 °C the whole time it is boiling. Put salt water on the same hotplate and it starts to boil somewhere above 100 °C and the temperature keeps climbing as it boils, so there is no single number you could write down as "the boiling point of salt water".
Here is why that follows. In pure water every particle has the same neighbours and feels the same pull, so they all break free at the same temperature and the thermometer parks on one number. In salt water the dissolved salt particles are in the way, holding the water particles back, so it takes more heat to get them out, and as water leaves the remaining solution gets saltier and needs more heat still. A range instead of a number is the fingerprint of a mixture, which is why a chemist checking a sample for purity reaches for a thermometer rather than for their eyes.
Text description
- Pure element, e.g. pure gold: all the atoms are identical.
- Pure compound, e.g. water (H₂O): every particle is identical; in water each one is a molecule of one oxygen atom bonded to two hydrogen atoms.
- Mixture, e.g. air or salt water: a random mix of different particles of different sizes.
A mixture is two or more substances physically stirred together, and all three of the things that make a mixture a mixture follow from that one word, physically. Because nothing has joined chemically, each part carries on behaving the way it always did: the salt in saltwater still tastes salty, because those salt particles are unchanged and are simply spread out among the water. Because nothing has joined chemically, the parts can be pulled back apart by physical methods such as filtration, evaporating or sieving, with no chemical reaction needed. And because nothing has joined chemically, there is no rule fixing how much of each part you use, which is why weak tea and strong tea are both tea, made from exactly the same two ingredients in different amounts.
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.
Text description
- Pure substance: one kind of particle.
- Homogeneous mixture: two kinds of particle, spread evenly.
- Heterogeneous mixture: two kinds of particle, in clumps.
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.