--- title: "Pure substance or mixture?" subtitle: "Year 7 Chemistry · Pure substances and mixtures · Lesson 6" --- # Pure substance or mixture? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** 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. :::teacher-note **In one sentence:** Students distinguish pure substances from mixtures at the particle level and classify familiar materials with a particle reason (AC9S7U06, node 1, a hinge node). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **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 My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Distilled water: it is the only one with a single kind of particle. Juice, seawater and air are all mixtures. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: pure means one kind of particle :::keep-together ### Classify **Pure substance or mixture?** Groups: Pure substance, Mixture - Distilled water: {{field-15}} - Salt water: {{field-15}} - Sandy water: {{field-15}} - Air: {{field-15}} - Copper wire: {{field-15}} ::: :::teacher-note **Answer:** 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. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** It contains two kinds of particles, even though they are evenly spread Evenly spread or not, two kinds of particles make a mixture. ::: :::keep-together ### "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 ::: :::teacher-note **Answer:** No: it contains water, sugars, acids and many other kinds of particles "Pure" on the label only means nothing was added. ::: ## Mix it up :::keep-together **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 ::: :::teacher-note **Answer:** 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. ::: :::typical ## Elaborate: even and uneven mixtures On paper: draw your own particle diagram for each type and label the key difference. :::keep-together **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: {{field-15}} - Diagram B: {{field-15}} - Diagram C: {{field-15}} ::: :::teacher-note **Answer:** 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. ::: ::: :::wider ## Elaborate: around the house :::keep-together **Pure substance or mixture? Think about the particles, not the label or the look.** Groups: Pure substance, Mixture - Table sugar from the packet: {{field-15}} - Milk: {{field-15}} - A gold ring stamped 24 carat: {{field-15}} - Concrete: {{field-15}} - A cup of tea: {{field-15}} - A diamond: {{field-15}} - Tap water: {{field-15}} ::: :::teacher-note **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 ::: ::: :::wider ## Elaborate: solvents and solutes A **solution** is a homogeneous mixture. The **solute** dissolves; the **solvent** does the dissolving. :::keep-together ### Solvent **In salt water, what is the solvent?** ☐ Water ☐ Salt ::: :::teacher-note **Answer:** Water The solvent does the dissolving: water. ::: :::keep-together ### Solutes **In a soft drink, what are the solutes?** ☐ Sugar and carbon dioxide gas ☐ Water ::: :::teacher-note **Answer:** Sugar and carbon dioxide gas A gas can be a solute too: that's the fizz. ::: ::: :::wider ## Elaborate: borderline cases :::keep-together ### 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 ::: :::teacher-note **Answer:** Mixture Alloys are mixtures, even though they look like one metal. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** Yes: the same mixture with different amounts Mixtures can vary in how much of each part they contain. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** 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. ::: ::: ## 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. ## Exit ticket :::keep-together **Is salt water a pure substance or a mixture? Explain using particles.** {{lines-3}} ::: :::teacher-note **Answer:** A mixture: it contains two kinds of particles, water and salt, even though they are evenly spread. :::