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# Pure substance or mixture?

Year 7 Chemistry · Pure substances and mixtures

<!-- Speaker note: 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 (Engage and Explore 8 min · Explain 10 min · Even and uneven mixtures 5 min · Evaluate 3 min · Wider adds about 15 min). -->

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## Today

**We are learning to** 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.

- 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.

<!-- Speaker note: 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". -->

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## Why it matters

> 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.

<!-- Speaker note: Before you can separate a mixture, you need to know whether you have one. That is a question about **particles**, not appearance. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**Which one of these is a pure substance, as a scientist means it?**

- Clear, filtered seawater
- Distilled water
- Fresh mountain air
- "100% pure" orange juice

<!-- Speaker note: What happens: Distilled water: it is the only one with a single kind of particle. Juice, seawater and air are all mixtures. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: ask who chose the juice. The shopping meaning of 'pure' is the main competitor. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry%2Fpure-mixtures%2Fstep-2&code=widget%3A+zoom%0Asamples%3A+distilled%2C+salt%2C+sand%2C+air%2C+copper%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry/pure-mixtures/?full=step-2#step-2)

<!-- Speaker note: Each colour or size is a different kind of particle. Count the kinds in each material. -->

<!-- Speaker note: Teacher: ask students to count kinds of particles aloud for each sample. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Distilled water: it is the only one with a single kind of particle. Juice, seawater and air are all mixtures.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Classify

**Pure substance or mixture?**

Groups: Pure substance · Mixture

Distilled water · Salt water · Sandy water · Air · Copper wire

* Answer: Distilled water → Pure substance; Salt water → Mixture; Sandy water → Mixture; Air → Mixture; Copper wire → Pure substance

<!-- Speaker note: Count the kinds of particles, not how the material looks. -->

<!-- Speaker note: Teacher: salt water is the key case. If they get it, they have the definition. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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
- Because it is slightly cloudy
- Because people added the salt
- It isn't: the salt has disappeared, so it is pure water

* Answer: It contains two kinds of particles, even though they are evenly spread

<!-- Speaker note: Evenly spread or not, two kinds of particles make a mixture. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## "Pure" juice

**So is "100% pure" orange juice a pure substance?**

- No: it contains water, sugars, acids and many other kinds of particles
- Yes, nothing has been added

* Answer: No: it contains water, sugars, acids and many other kinds of particles

<!-- Speaker note: "Pure" on the label only means nothing was added. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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
- Water particles with a little air between them
- Smooth water, all the way down
- Water particles and salt particles evenly mixed

* Answer: Only water particles, with empty space between them, always moving

<!-- Speaker note: Pure means one kind of particle. Those particles still have empty space between them and never stop moving. What this step is for: A pure substance is defined by its particles, so the particle picture of one substance comes back before the idea of mixtures. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: even and uneven mixtures

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry%2Fpure-mixtures%2Fstep-5&code=widget%3A+windows%0Aset%3A+abc%0Acaption%3A+Three+particle+diagrams.+Each+colour+or+size+is+a+different+kind+of+particle.%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry/pure-mixtures/?full=step-5#step-5)

<!-- Speaker note: On paper: draw your own particle diagram for each type and label the key difference. What this step is for: Homogeneous mixtures are where students most often mistake a mixture for a pure substance. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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.**

Groups: Pure substance · Homogeneous mixture · Heterogeneous mixture

Diagram A · Diagram B · Diagram C

* Answer: Diagram A → Heterogeneous mixture; Diagram B → Pure substance; Diagram C → Homogeneous mixture

<!-- Speaker note: A and C are both mixtures; the difference is how evenly the particles are spread. What this step is for: Homogeneous mixtures are where students most often mistake a mixture for a pure substance. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: around the house

**Pure substance or mixture? Think about the particles, not the label or the look.**

Groups: Pure substance · Mixture

Table sugar from the packet · Milk · A gold ring stamped 24 carat · Concrete · A cup of tea · A diamond · Tap water

* Answer: 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

<!-- Speaker note: What this step is for: Everyday materials, labels and alloys force the particle definition over appearance. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: solvents and solutes

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry%2Fpure-mixtures%2Fstep-7&code=widget%3A+dissolve%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry/pure-mixtures/?full=step-7#step-7)

<!-- Speaker note: A **solution** is a homogeneous mixture. The **solute** dissolves; the **solvent** does the dissolving. What this step is for: Watching the solute's particles spread between the solvent's shows why a solution is a mixture even though it looks like one substance. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Solvent

**In salt water, what is the solvent?**

- Water
- Salt

* Answer: Water

<!-- Speaker note: The solvent does the dissolving: water. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Solutes

**In a soft drink, what are the solutes?**

- Sugar and carbon dioxide gas
- Water

* Answer: Sugar and carbon dioxide gas

<!-- Speaker note: A gas can be a solute too: that's the fizz. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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
- Pure substance

* Answer: Mixture

<!-- Speaker note: Alloys are mixtures, even though they look like one metal. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Seawater

**Seawater can contain more or less salt depending on where it is collected. Is seawater with extra salt still the same mixture?**

- No: it becomes a new pure substance
- Yes: the same mixture with different amounts

* Answer: Yes: the same mixture with different amounts

<!-- Speaker note: Mixtures can vary in how much of each part they contain. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The definition

**Which statement is true of every pure substance?**

- It looks shiny and even
- It always has the same make-up, with one kind of particle
- It occurs naturally

* Answer: It always has the same make-up, with one kind of particle

<!-- Speaker note: That is what makes the definition useful: a pure substance's make-up can't vary. -->

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## 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.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this 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. -->