--- title: "Distillation and chromatography" subtitle: "Year 7 Chemistry · Pure substances and mixtures · Lesson 9" --- # Distillation and chromatography **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Ships at sea make fresh water by boiling seawater and catching the steam. Forensic scientists find out which pen wrote a note by letting water creep up a strip of paper. :::teacher-note **In one sentence:** Students describe the principles of distillation and chromatography, identify a mixture for each, and explain when distillation is chosen over evaporation (AC9S7U06, node 4). **Students should be able to say:** I can describe the principle of distillation and of chromatography. I can name a mixture each technique suits. I can explain when distillation is used instead of evaporation. **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 **Seawater is heated in a flask. A tube carries the steam through a cold jacket into a second, empty flask. What collects in the second flask?** ☐ Nothing ☐ Salty water ☐ Fresh water ☐ Salt My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Fresh water. Only water particles boil off; the salt stays behind in the first flask. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: distillation is not just evaporation :::keep-together ### Compare **Both techniques heat a liquid until it turns to gas. Sort each statement.** Groups: Evaporation only, Distillation only, Both - Uses the difference in boiling point: {{field-15}} - Needs heat: {{field-15}} - The vapour escapes into the air: {{field-15}} - The vapour is cooled and collected as a liquid: {{field-15}} - You end up with only the solid: {{field-15}} - You end up with the liquid and the solid: {{field-15}} - Needs a condenser and a collecting flask: {{field-15}} ::: :::teacher-note **Answer:** Uses the difference in boiling point → Both; Needs heat → Both; The vapour escapes into the air → Evaporation only; The vapour is cooled and collected as a liquid → Distillation only; You end up with only the solid → Evaporation only; You end up with the liquid and the solid → Distillation only; Needs a condenser and a collecting flask → Distillation only Same property, different outcome: distillation keeps the vapour by cooling it. ::: :::keep-together ### The dyes **Why do the dyes in the ink end up at different heights?** ☐ The dyes evaporate at different speeds ☐ Heavier dyes sink lower ☐ The water makes new colours as it rises ☐ Each dye dissolves in the water and sticks to the paper differently, so the water carries each one a different distance ::: :::teacher-note **Answer:** Each dye dissolves in the water and sticks to the paper differently, so the water carries each one a different distance Dyes that dissolve well and stick weakly travel furthest. ::: :::keep-together ### Which technique suits? **Match each job to the technique that suits it.** A. Evaporation B. Distillation C. Chromatography D. Filtration {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Get drinking water from seawater` | | | `Get salt from seawater, letting the water go` | | | `Find out which dyes are in a black ink` | | | `Get sand out of sandy water` | | ::: :::teacher-note **Answer:** Get drinking water from seawater → B. Distillation; Get salt from seawater, letting the water go → A. Evaporation; Find out which dyes are in a black ink → C. Chromatography; Get sand out of sandy water → D. Filtration Need the liquid back? Distil. Need only the dissolved solid? Evaporate. Several dissolved substances, such as dyes? Chromatography. ::: ## Mix it up :::keep-together **Looking back at heating curves: water in a flask is boiling and the heat stays on full. What does a thermometer in the water show over the next few minutes?** ☐ It goes up and down ☐ It drops, because the water is turning to steam ☐ It stays at about 100 °C ☐ It keeps rising ::: :::teacher-note **Answer:** It stays at about 100 °C During a change of state the temperature stays the same: the energy goes into pulling the particles apart into steam. ::: :::typical ## Elaborate: distillation or evaporation? :::keep-together **Both use the same property. Which would you choose?** Groups: Evaporation, Distillation - A salt farm wants salt from seawater.: {{field-15}} - A ship needs drinking water from seawater.: {{field-15}} - A lab wants very pure water with nothing dissolved in it.: {{field-15}} - You want sugar crystals from sugar syrup.: {{field-15}} - You want to keep both the water and the salt from a salt solution.: {{field-15}} ::: :::teacher-note **Answer:** A salt farm wants salt from seawater. → Evaporation; A ship needs drinking water from seawater. → Distillation; A lab wants very pure water with nothing dissolved in it. → Distillation; You want sugar crystals from sugar syrup. → Evaporation; You want to keep both the water and the salt from a salt solution. → Distillation Ask: do I need the liquid? If yes, distil. ::: ::: :::wider ## Elaborate: three techniques, three properties :::keep-together **Which property does each technique use?** Groups: Boiling point, How well each substance dissolves and sticks to the paper, How much solid can stay dissolved - Crystallisation: a hot, concentrated solution is left to cool, and crystals form.: {{field-15}} - Chromatography: food colourings separate on paper.: {{field-15}} - Distillation: alcohol is separated from water.: {{field-15}} ::: :::teacher-note **Answer:** Crystallisation: a hot, concentrated solution is left to cool, and crystals form. → How much solid can stay dissolved; Chromatography: food colourings separate on paper. → How well each substance dissolves and sticks to the paper; Distillation: alcohol is separated from water. → Boiling point ::: ::: :::wider ## Elaborate: First Nations knowledge and plant oils The Bundjalung people of northern New South Wales have long used the leaves of the tea tree, crushing them or soaking them in water to use as medicine. Today tea tree oil is extracted by steam distillation. :::keep-together **Which property or process does each step use?** Groups: Breaking up the leaves so substances can get out, Some substances dissolve in water, The oil is carried away with steam, then condensed, The oil floats on the water - Crushing tea tree leaves: {{field-15}} - Soaking crushed leaves in water: {{field-15}} - Modern steam distillation: steam passes through the leaves: {{field-15}} - Modern steam distillation: separating the collected liquid: {{field-15}} ::: :::teacher-note **Answer:** Crushing tea tree leaves → Breaking up the leaves so substances can get out; Soaking crushed leaves in water → Some substances dissolve in water; Modern steam distillation: steam passes through the leaves → The oil is carried away with steam, then condensed; Modern steam distillation: separating the collected liquid → The oil floats on the water ::: ::: :::wider ## Elaborate: reading a chromatogram Chemists compare spots with the **Rf value**: the distance the spot moved divided by the distance the water moved, both measured from the pencil line. In the Pen A lane, the water moved **10.0 cm**. :::keep-together ### Blue **The blue spot moved 8.0 cm. What is its Rf value?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 0.8 8.0 ÷ 10.0 = 0.80 ::: :::keep-together ### Red **The red spot moved 5.0 cm. What is its Rf value?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 0.5 5.0 ÷ 10.0 = 0.50 ::: :::keep-together ### Unknown dye **An unknown dye has Rf = 0.50 in the same setup. Which dye could it be?** ☐ The red dye ☐ The blue dye ☐ None of them ::: :::teacher-note **Answer:** The red dye Same Rf value, same conditions: probably the same substance. ::: :::keep-together ### One spot **A green ink gives just one spot. What can you conclude?** ☐ It is definitely a pure substance ☐ The test failed ☐ It is probably one dye, but two dyes with the same Rf value would also give one spot ::: :::teacher-note **Answer:** It is probably one dye, but two dyes with the same Rf value would also give one spot Testing again with a different solvent would check. ::: ::: ## Evaluate: look back What did you think would collect in the second flask? Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Distillation separates a liquid from what is dissolved in it by boiling the liquid and cooling the vapour back into a liquid somewhere else. - Evaporation and distillation use the same property (boiling point), but distillation collects the vapour and evaporation lets it escape. - Chromatography separates a mixture of dissolved substances, such as dyes, because each is carried a different distance. - Distillation suits seawater (to get water). Chromatography suits inks, food colourings and plant pigments. ## Exit ticket :::keep-together **Why would a ship use distillation, not evaporation, to get drinking water from seawater?** {{lines-3}} ::: :::teacher-note **Answer:** Distillation collects the water vapour as liquid; evaporation lets it escape and keeps only the salt. :::