--- title: "How do you separate a mixture?" subtitle: "Year 7 Chemistry · Pure substances and mixtures · Lesson 7" --- # How do you separate a mixture? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A recycling plant lifts steel cans out of the rubbish with a giant magnet and leaves everything else behind. Every way of separating a mixture works like that: it uses a difference between the parts. :::teacher-note **In one sentence:** Students explain that separation techniques use differences in physical properties, and match property differences to techniques (AC9S7U06, node 2, a hinge node). **Students should be able to say:** I can say that separation techniques use differences in physical properties. I can match at least three property differences to a separation technique. I can explain why a chosen technique works for a given mixture. **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 **A magnet lifts steel cans but leaves aluminium cans behind. Why?** ☐ Steel is attracted to magnets and aluminium isn't ☐ Steel cans are bigger ☐ Aluminium cans are too light to be picked up ☐ The magnet only lifts heavier things My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Magnetism: steel is attracted to magnets and aluminium isn't. Every separation uses a difference in a property like this. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: why each technique works :::keep-together ### Property to technique **Which technique uses each property difference?** Groups: Magnet, Filtration, Evaporation, Sieving, Letting it settle, then pouring off - Iron is magnetic; sand isn't.: {{field-15}} - Sand grains are too big to pass through filter paper; water and dissolved salt pass through.: {{field-15}} - Water boils at 100 °C; salt doesn't boil until 1413 °C.: {{field-15}} - Pebbles are much bigger than grains of rice.: {{field-15}} - Oil is less dense than water, so it floats on top.: {{field-15}} ::: :::teacher-note **Answer:** Iron is magnetic; sand isn't. → Magnet; Sand grains are too big to pass through filter paper; water and dissolved salt pass through. → Filtration; Water boils at 100 °C; salt doesn't boil until 1413 °C. → Evaporation; Pebbles are much bigger than grains of rice. → Sieving; Oil is less dense than water, so it floats on top. → Letting it settle, then pouring off Each technique uses one property difference. ::: :::keep-together ### Filter paper **Why does filter paper catch the sand but not the salt?** ☐ Salt is lighter than sand ☐ The salt has disappeared ☐ Sand grains are too big for the tiny holes; dissolved salt is spread out as single particles small enough to pass through ☐ Sand sticks to the paper ::: :::teacher-note **Answer:** Sand grains are too big for the tiny holes; dissolved salt is spread out as single particles small enough to pass through Filter paper sorts by particle size. ::: :::keep-together ### The order **Why did the magnet have to come before the filter paper?** ☐ It's the rule for separating ☐ Magnets don't work underwater ☐ Iron and sand are both too big for the filter, so filtering first would trap them together ::: :::teacher-note **Answer:** Iron and sand are both too big for the filter, so filtering first would trap them together The magnet picks out just the iron first; then the filter only catches sand. ::: :::keep-together ### What they share **What do all separation techniques have in common?** ☐ Each one turns the mixture into new substances ☐ Each one separates the heavier part from the lighter part ☐ Each one needs heat ☐ Each one uses a difference in a physical property between the parts of the mixture ::: :::teacher-note **Answer:** Each one uses a difference in a physical property between the parts of the mixture Find a property where the parts differ, then choose the technique that uses it. ::: ## Mix it up :::keep-together **Looking back at density: air is much less dense than water. Why?** ☐ Air has no mass ☐ Each air particle is lighter than a water particle ☐ Air particles are smaller ☐ Air particles are far apart, so there are far fewer in each cm³ ::: :::teacher-note **Answer:** Air particles are far apart, so there are far fewer in each cm³ Fewer particles in each cm³ means less mass in each cm³: lower density. Density is one of the property differences a separation can use. ::: :::typical ## Elaborate: choose, and rule out :::keep-together **You need to get chalk dust back out of chalky water. Chalk doesn't dissolve. Judge each technique.** Groups: Best choice, Works, but not the best, Won't work - Filtration: {{field-15}} - Evaporation: {{field-15}} - A magnet: {{field-15}} - A kitchen sieve: {{field-15}} ::: :::teacher-note **Answer:** Filtration → Best choice; Evaporation → Works, but not the best; A magnet → Won't work; A kitchen sieve → Won't work Name the property difference, then rule out techniques that can't use it. ::: ::: :::wider ## Elaborate: size, density and boiling point :::keep-together **Which property difference does each one use?** Groups: Particle size, Density, Boiling point, Magnetism - Panning for gold: swirling water washes away gravel and leaves gold flakes in the pan.: {{field-15}} - Making fresh water from seawater by boiling it and cooling the steam.: {{field-15}} - Sifting flour to remove lumps.: {{field-15}} - Leaving fresh milk to stand so the cream rises to the top.: {{field-15}} ::: :::teacher-note **Answer:** Panning for gold: swirling water washes away gravel and leaves gold flakes in the pan. → Density; Making fresh water from seawater by boiling it and cooling the steam. → Boiling point; Sifting flour to remove lumps. → Particle size; Leaving fresh milk to stand so the cream rises to the top. → Density ::: ::: :::wider ## Elaborate: home, recycling and clean water :::keep-together **Which property difference does each one use?** Groups: Particle size, Density, Boiling point, Magnetism - Home: a tea bag keeps the leaves out of your cup.: {{field-15}} - Recycling: overhead magnets pull steel cans off the conveyor belt.: {{field-15}} - Recycling: air jets blow light paper and plastic film off heavier glass and metal.: {{field-15}} - Water treatment: water sits in big tanks so mud sinks to the bottom.: {{field-15}} - Water treatment: water trickles through deep beds of sand.: {{field-15}} ::: :::teacher-note **Answer:** Home: a tea bag keeps the leaves out of your cup. → Particle size; Recycling: overhead magnets pull steel cans off the conveyor belt. → Magnetism; Recycling: air jets blow light paper and plastic film off heavier glass and metal. → Density; Water treatment: water sits in big tanks so mud sinks to the bottom. → Density; Water treatment: water trickles through deep beds of sand. → Particle size ::: ::: :::wider ## Elaborate: muddy creek water :::keep-together **On a camping trip, the only water is from a muddy creek, and it may contain germs. Order these plans from most effective to least effective at making it safe to drink.** A. `Drink it straight from the creek` B. `Boil only` C. `Filter through a cloth only` D. `Filter through a clean cloth, then boil` Correct order (letters): {{field-24}} ::: :::teacher-note **Answer:** see the program below Filtering removes the mud (particle size) and boiling kills germs, so combining them is best. Boiling alone makes it safer but leaves mud. A cloth alone removes some mud but not germs. ```science Filter through a clean cloth, then boil Boil only Filter through a cloth only Drink it straight from the creek ``` ::: ::: :::typical ## Elaborate: a boil water alert After a flood, or a fault at a treatment plant, tests sometimes find germs in a town's water. The health department and the water company then issue a **boil water alert**. The tests are science; the alert is **science communication**. It turns the results into something every household can act on, and it is lifted only when new tests show the water is safe again. :::keep-together ### The jug filter **An alert says: "Boil water, then let it cool, before drinking. A jug filter is not enough." Why include the jug filter line?** ☐ Jug filters make the water less safe ☐ Filtering removes germs better than boiling, so the alert has it wrong ☐ Many homes own jug filters and might trust them, but most aren't designed to remove germs ::: :::teacher-note **Answer:** Many homes own jug filters and might trust them, but most aren't designed to remove germs Good science communication answers the question its audience will actually have, and heads off the mistake they are likely to make. ::: :::keep-together ### Reaching everyone **Does each choice help the alert reach and protect everyone?** Groups: Helps, Gets in the way - Short, plain instructions: boil, cool, then drink: {{field-15}} - Naming the suburbs affected and when it started: {{field-15}} - Versions in the main languages spoken in the town: {{field-15}} - Only a long table of lab results, with no instructions: {{field-15}} - Posting it only in a technical report on the water company's website: {{field-15}} ::: :::teacher-note **Answer:** Short, plain instructions: boil, cool, then drink → Helps; Naming the suburbs affected and when it started → Helps; Versions in the main languages spoken in the town → Helps; Only a long table of lab results, with no instructions → Gets in the way; Posting it only in a technical report on the water company's website → Gets in the way ::: ::: ## Evaluate: look back Why did you think the magnet worked? Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Every separation technique uses a difference in a physical property between the parts of a mixture. - Magnetism → magnet. Particle size → sieving or filtration. Boiling point → evaporation. Density → letting it settle and pouring off. - Separating doesn't change the particles: you get back the same substances you mixed. - Order matters: use the technique that picks out one part before one that traps several together. ## Exit ticket :::keep-together **Why does a magnet separate iron filings from sand?** {{lines-3}} ::: :::teacher-note **Answer:** Iron is magnetic and sand isn't: the magnet uses that difference in property. :::