A pure substance contains only one kind of particle; a mixture contains two or more. Classify familiar materials by their particles, not by how they look.
In the supermarket, "100% pure" orange juice means nothing has been added. In science, pure means something much stricter, and you can only check it by looking at the particles.
Before you can separate a mixture, you need to know whether you have one. That is a question about particles, not appearance.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Practise: A few on your own, to check you have it.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: which one is pure?
ExploreTry it on the board and compare what happens with your prediction.
Explore: zoom into five materials
Each colour or size is a different kind of particle. Count the kinds in each material.
Try it yourselfNothing is saved: start again as often as you like.
Zoom into all five materials.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: pure means one kind of particle
In your book: explain the difference between a pure substance and a mixture using particles. Use salt water as one example.
Pure substance or mixture?
Distilled waterOnly one kind of particle.
Salt waterTwo kinds: water and salt.
Sandy waterTwo kinds: water and sand.
AirMostly nitrogen and oxygen.
Copper wireOnly copper particles.
PractiseA few on your own, to check you have it.
Mix it up
ElaborateUse the same idea in a new context.
Elaborate: even and uneven mixtures
On paper: draw your own particle diagram for each type and label the key difference.
Try it yourselfNothing is saved: start again as often as you like.
A **homogeneous** mixture is evenly mixed: every part is the same. A **heterogeneous** mixture is uneven: you can find different parts. Name each diagram.
Diagram ATwo kinds, unevenly spread: the big particles sit in a clump.
Diagram BOnly one kind of particle.
Diagram CTwo kinds, evenly spread through each other.
Elaborate WiderUse the same idea in a new context.
Elaborate: around the house
Pure substance or mixture? Think about the particles, not the label or the look.
Table sugar from the packetOne kind of particle: sucrose.
MilkWater, fats, proteins, sugars and more.
A gold ring stamped 24 carat24-carat gold is pure gold; lower carats are mixed with other metals.
ConcreteCement, sand, gravel and water.
A cup of teaWater plus many dissolved substances from the tea leaves.
A diamondOnly carbon.
Tap waterWater with small amounts of dissolved minerals and chlorine: clean, but not pure.
Elaborate WiderUse the same idea in a new context.
Elaborate: solvents and solutes
A solution is a homogeneous mixture. The solute dissolves; the solvent does the dissolving.
Try it yourselfNothing is saved: start again as often as you like.
Add salt to the water and watch where its particles go.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: borderline cases
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Which did you think was pure, and what made you think so? Write one sentence: "At first I thought… Now I think… because…"
Revision summary
A pure substance contains only one kind of particle. A mixture contains two or more.
You can't tell by looking: salt water and air look the same all the way through, but both are mixtures.
"Pure" on a food label means nothing added. It does not mean a pure substance.
For teachers
Students distinguish pure substances from mixtures at the particle level and classify familiar materials with a particle reason (AC9S7U06, node 1, a hinge node). About 25 minutes.
I can tell a pure substance from a mixture using a particle diagram.
I can classify familiar materials and give a particle reason.
I can explain why a solution like salt water is a mixture even though it looks the same all through.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider.
Hinge: technique selection in nodes 3 to 5 needs students to know what kind of mixture they have. The success criterion to watch is "does not confuse homogeneous mixtures with pure substances".
Running the lesson
Engage: ask who chose the juice. The shopping meaning of 'pure' is the main competitor.
Explore: ask students to count kinds of particles aloud for each sample.
Explain: salt water is the key case. If they get it, they have the definition.
The hard part (on purpose)
Judging by kinds of particle, not by how clear or 'natural' it looks. When a student says salt water is pure, ask how many kinds of particle are in the sample.
When students get stuck
A student says salt water is pure because it is clear. Ask: How many kinds of particles did you count in the salt water sample? Fix: Two kinds means a mixture, however even it looks.
A student says the salt has disappeared. Ask: If you boiled the water away, what would be left? Fix: The salt. Its particles are spread out between the water particles, not gone.
A student says juice is pure because the label says so. Ask: Is the label using the science meaning or the shopping meaning? Fix: Science: one kind of particle. Shopping: nothing added.
Stretch and support
Stretch: Is distilled water a pure substance even though it is made of hydrogen and oxygen? (Yes: one kind of particle. Compounds come in Year 8.)
Support: Give a two-question test to use every time: How many kinds of particles? Are they evenly spread?
Where it leads
Next: node 2, how differences in properties let us separate mixtures.
In class: dissolve salt in water, then evaporate a sample on a watch glass to show the salt was there all along.
What each step is for
Engage: which one is pure?: The shopping meaning of 'pure' and the idea that clear means pure are both exposed by this choice.
Explore: zoom into five materials: Counting kinds of particles across liquids, gases and solids replaces judging by appearance.
Explain: pure means one kind of particle: Classifying with a particle reason, then confronting salt water and juice, separates the science meaning from appearance and advertising.
Mix it up: A pure substance is defined by its particles, so the particle picture of one substance comes back before the idea of mixtures.
Elaborate: even and uneven mixtures: Homogeneous mixtures are where students most often mistake a mixture for a pure substance.
Elaborate: around the house: Everyday materials, labels and alloys force the particle definition over appearance.
Elaborate: solvents and solutes: Watching the solute's particles spread between the solvent's shows why a solution is a mixture even though it looks like one substance.
Elaborate: borderline cases: Alloys and variable mixtures test whether the definition is understood or just memorised.
Now you can
Tell a pure substance from a mixture using a particle diagram.
Classify familiar materials and give a particle reason.
Explain why a solution like salt water is a mixture even though it looks the same all through.