Pure substance or mixture?

Classroom resources

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Students will be able to

• 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.

Timing

Engage and Explore 8 min · Explain 10 min · Even and uneven mixtures 5 min · Evaluate 3 min · Wider adds about 15 min

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

1. Engage: ask who chose the juice. The shopping meaning of 'pure' is the main competitor.
2. Explore: ask students to count kinds of particles aloud for each sample.
3. 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

Answers to the checks

1. Engage: which one is pure?: Which one of these is a pure substance, as a scientist means it?
   Distilled water: it is the only one with a single kind of particle. Juice, seawater and air are all mixtures.
2. Explore: zoom into five materials: Zoom into all five materials.
   
3. Classify: Pure substance or mixture?
   Distilled water → Pure substance; Salt water → Mixture; Sandy water → Mixture; Air → Mixture; Copper wire → Pure substance
   Count the kinds of particles, not how the material looks.
4. Salt water: Salt water looks exactly the same all the way through. Why is it still a mixture?
   It contains two kinds of particles, even though they are evenly spread
   Evenly spread or not, two kinds of particles make a mixture.
5. "Pure" juice: So is "100% pure" orange juice a pure substance?
   No: it contains water, sugars, acids and many other kinds of particles
   "Pure" on the label only means nothing was added.
6. Mix it up: Looking back at the particle model: if you could zoom into a drop of pure water, what would you see?
   Only water particles, with empty space between them, always moving
   Pure means one kind of particle. Those particles still have empty space between them and never stop moving.
7. Elaborate: even and uneven mixtures: 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 A → Heterogeneous mixture; Diagram B → Pure substance; Diagram C → Homogeneous mixture
   A and C are both mixtures; the difference is how evenly the particles are spread.
8. Elaborate: around the house: Pure substance or mixture? Think about the particles, not the label or the look.
   Table sugar from the packet → Pure substance; Milk → Mixture; A gold ring stamped 24 carat → Pure substance; Concrete → Mixture; A cup of tea → Mixture; A diamond → Pure substance; Tap water → Mixture
9. Dissolve: Add salt to the water and watch where its particles go.
   
10. Solvent: In salt water, what is the solvent?
    Water
    The solvent does the dissolving: water.
11. Solutes: In a soft drink, what are the solutes?
    Sugar and carbon dioxide gas
    A gas can be a solute too: that's the fizz.
12. Stainless steel: Stainless steel looks like a single shiny metal. It is mostly iron, with chromium and nickel particles spread evenly through it. Pure substance or mixture?
    Mixture
    Alloys are mixtures, even though they look like one metal.
13. Seawater: Seawater can contain more or less salt depending on where it is collected. Is seawater with extra salt still the same mixture?
    Yes: the same mixture with different amounts
    Mixtures can vary in how much of each part they contain.
14. The definition: Which statement is true of every pure substance?
    It always has the same make-up, with one kind of particle
    That is what makes the definition useful: a pure substance's make-up can't vary.

Solutions

One possible solution for each task. Other correct programs are fine.

Elaborate: even and uneven mixtures

Open task: any program that does what the step asks.

Elaborate: solvents and solutes › Dissolve

Open task: any program that does what the step asks.

Elaborate: solvents and solutes › Solvent

Open task: any program that does what the step asks.

Elaborate: solvents and solutes › Solutes

Open task: any program that does what the step asks.

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?

Coach prompt (optional)

For a student who has done the lesson and is still stuck. They paste it into an AI tool your school allows; it asks questions about this lesson's one idea and doesn't give answers. Students see it on the course recap page only through a link ending in ?coach=1: recap with coach prompts.

You are a patient science coach for a Year 7 student in Australia. Help me understand ONE idea only, from the lesson "Pure substance or mixture?".

The idea: Pure substances vs mixtures at the particle level: I can distinguish pure substances from mixtures using particle diagrams and classify familiar materials correctly.

What understanding looks like:
- Correctly distinguishes pure substance from mixture using particle diagrams
- Classifies 5 familiar materials correctly with justification
- Does not confuse homogeneous mixtures (e.g. solutions) with pure substances

Mistakes students often make here:
- A student says salt water is pure because it is clear.
- A student says the salt has disappeared.
- A student says juice is pure because the label says so.

How to coach me (rules from the Waypoints Kit coach):
- Ask me one short question at a time (2 to 4 sentences) and wait for my answer. Never give me the answer before you have heard my thinking, and don't hide the answer inside your question.
- Start in Discover: ask me to explain the idea in my own words, or to predict what happens in a simple example. I can switch any time: "teach it back" (you play a beginner and I explain), "spot the flaw" (you show me an answer with one common mistake and I find it), or "check" (I explain, and you tell me what is solid and what to revisit).
- If I am stuck or say "just tell me", ask what I think first, then give a small hint or a simpler example. Explain only after I have had a real go.
- Stay in this idea: don't teach later ideas or add extra content. If I ask about something else, say it is for another lesson and bring me back.
- After you help, ask me one reflective question, such as how I know my answer is right or what I would check.
- Praise my effort and method, not my ability.
- You can be wrong: if I rely on something you said, remind me to check it against my notes or the lesson. If you don't know, say so.
- Use simple words.
- Stop when I can explain the idea in my own words and get one new example right. Then tell me to go back to the lesson "Pure substance or mixture?".

Exit ticket

Is salt water a pure substance or a mixture? Explain using particles.

A mixture: it contains two kinds of particles, water and salt, even though they are evenly spread.

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.

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.