--- title: "Mixed, not joined?" subtitle: "Year 8 Chemistry · Elements and compounds · Lesson 4" --- # Mixed, not joined? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** You breathe in oxygen and nitrogen with every breath, yet only the oxygen keeps you alive. In air, oxygen still behaves exactly like oxygen. :::teacher-note **In one sentence:** Students define a mixture, explain that its parts keep their properties, vary in amount and can be separated physically, and distinguish mixtures from compounds (AC9S8U06, node 4). **Students should be able to say:** I can explain what a mixture is. I can tell a mixture from a compound. I can sort substances into elements, compounds and mixtures. **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 **Air is about one fifth oxygen. Is the oxygen in air still oxygen, with oxygen's properties?** ☐ Only some of it ☐ No: mixing with nitrogen turned it into a new substance ☐ No: air is one compound called 'air' ☐ Yes: in a mixture each substance keeps its own particles and properties My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Yes. Air is a mixture: the oxygen particles are mingled with nitrogen and others, but not joined to them. That is why your lungs can take oxygen out of air. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: mixtures and compounds :::keep-together ### What a mixture is **What is a mixture?** ☐ Two or more substances mingled together but not chemically joined ☐ Two or more types of atom joined in a fixed ratio ☐ Any substance with more than one atom in each particle ::: :::teacher-note **Answer:** Two or more substances mingled together but not chemically joined In a mixture each substance keeps its own particles. ::: :::keep-together ### Mixture or compound **Mixture or compound?** Groups: Mixture, Compound - Each part keeps its own properties: {{field-15}} - New properties, unlike its elements: {{field-15}} - The amounts can be anything: {{field-15}} - A fixed ratio of atoms: {{field-15}} - Separated by filtering, evaporating or distilling: {{field-15}} - Separated only by a chemical reaction: {{field-15}} ::: :::teacher-note **Answer:** Each part keeps its own properties → Mixture; New properties, unlike its elements → Compound; The amounts can be anything → Mixture; A fixed ratio of atoms → Compound; Separated by filtering, evaporating or distilling → Mixture; Separated only by a chemical reaction → Compound ::: :::keep-together ### Three kinds **Element, compound or mixture?** Groups: Element, Compound, Mixture - Oxygen: {{field-15}} - Water: {{field-15}} - Air: {{field-15}} - Salt water: {{field-15}} - Carbon dioxide: {{field-15}} - Copper: {{field-15}} ::: :::teacher-note **Answer:** Oxygen → Element; Water → Compound; Air → Mixture; Salt water → Mixture; Carbon dioxide → Compound; Copper → Element ::: :::keep-together ### Getting it back **How could you get the salt back from salt water?** ☐ You can't: salt and water have joined ☐ Evaporate the water: the salt is left ☐ Filter it ::: :::teacher-note **Answer:** Evaporate the water: the salt is left Mixtures can be separated by physical means (Year 7: filtering, evaporating, distilling, chromatography). ::: :::keep-together ### Solutions, suspensions and colloids **Three kinds of mixture. Match each to an example and what it does.** A. Milk: tiny drops spread through; it looks cloudy but nothing settles B. Salt water: the salt dissolves, the liquid stays clear and nothing settles C. Muddy water: bits of solid spread through, then settle if left to stand {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Solution` | | | `Suspension` | | | `Colloid` | | ::: :::teacher-note **Answer:** Solution → B. Salt water: the salt dissolves, the liquid stays clear and nothing settles; Suspension → C. Muddy water: bits of solid spread through, then settle if left to stand; Colloid → A. Milk: tiny drops spread through; it looks cloudy but nothing settles All three are mixtures. In a solution the particles are dissolved and pass through filter paper; a suspension's bits are big enough to settle and be filtered out; a colloid's are in between, so it looks cloudy but doesn't settle. Fog and whipped cream are colloids too. ::: :::typical ## Elaborate: any amount :::keep-together ### Both mixtures? **Both boxes hold hydrogen and oxygen in very different amounts. Are they both mixtures of hydrogen and oxygen?** ☐ No: only one of them has the right amounts ☐ No: they are two different compounds ☐ Yes: a mixture can have its parts in any amounts ::: :::teacher-note **Answer:** Yes: a mixture can have its parts in any amounts A mixture's make-up can vary. Water, the compound, is always 2 hydrogen to 1 oxygen. ::: ::: :::wider ## Elaborate: humid air :::keep-together ### Compound or mixture **Humid air has water vapour in it. Water is a compound. Is humid air a compound?** ☐ It is an element ☐ Yes: it contains a compound ☐ No: it is a mixture of elements and a compound, none joined to each other ::: :::teacher-note **Answer:** No: it is a mixture of elements and a compound, none joined to each other A mixture can hold elements, compounds or both, as long as the different substances aren't joined. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - A **mixture** is two or more substances mingled together but not chemically joined. Each keeps its own particles and properties. - A mixture's parts can be in any amounts; a **compound** always has a fixed ratio (H₂O). - Mixtures can be separated by physical means (filtering, evaporating, distilling); compounds only by chemical reactions. - Air and salt water are mixtures; water and carbon dioxide are compounds. ## Exit ticket :::keep-together **Iron and sulfur are mixed, then heated to make iron sulfide. Which is the mixture and which the compound, and how could you tell with a magnet?** {{lines-3}} ::: :::teacher-note **Answer:** Before heating: a mixture (a magnet pulls out the iron). After: iron sulfide, a compound (the iron is joined to sulfur, so a magnet can't pull it out). :::