--- title: "Where does the heat go?" subtitle: "Year 7 Chemistry · Particle theory · Lesson 4" --- # Where does the heat go? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A pot of water on the stove climbs to 100 °C and then stops, even with the gas still on full. Where is all that heat going? :::teacher-note **In one sentence:** Students explain changes of state with particle theory, identify energy input or output for each, and explain the flat parts of a heating or cooling curve (AC9S7U05, node 4). **Students should be able to say:** I can say whether energy goes in or out when a substance melts, boils, freezes or condenses. I can explain a change of state using particles. I can explain why the temperature stays the same while a substance changes state. **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 pot of water is boiling on a stove. The stove stays on full. What does a thermometer in the water show over the next five minutes?** ☐ It drops, because the water is turning to steam ☐ It keeps rising ☐ It goes up and down ☐ It stays at about 100 °C My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** It stays at about 100 °C. The energy from the stove is going into separating the water particles so they escape as steam, not into making them move faster, so the temperature stays the same until the water has all boiled away. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: energy in, energy out :::keep-together ### In or out **Does the substance take energy in, or give energy out?** Groups: Energy in, Energy out - Melting (solid to liquid): {{field-15}} - Boiling or evaporating (liquid to gas): {{field-15}} - Freezing (liquid to solid): {{field-15}} - Condensing (gas to liquid): {{field-15}} ::: :::teacher-note **Answer:** Melting (solid to liquid) → Energy in; Boiling or evaporating (liquid to gas) → Energy in; Freezing (liquid to solid) → Energy out; Condensing (gas to liquid) → Energy out ::: :::keep-together ### The flat part **On the heating curve, the line is flat from 1 to 9 minutes. What is happening?** ☐ Nothing: the stove was turned off ☐ The water is boiling ☐ The ice is melting: energy goes into freeing the particles from their fixed places, so the temperature doesn't rise ::: :::teacher-note **Answer:** The ice is melting: energy goes into freeing the particles from their fixed places, so the temperature doesn't rise During a change of state, the energy goes into overcoming the attraction between particles, not into making them move faster, so the temperature stays constant. ::: :::keep-together ### The second flat part **At what temperature (°C) is the second flat part of water's heating curve?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 100 100 °C: water's boiling point (at sea level). The first flat part, at 0 °C, is its melting point. ::: :::keep-together ### Why boiling takes longer **The curve is drawn to scale: the stove gives the same energy every minute. Why is the flat part at 100 °C about seven times longer than the one at 0 °C?** ☐ The stove gives less energy once the water is hot ☐ Boiling pulls the particles completely apart, which takes far more energy than loosening them from their fixed places ☐ Steam is lighter than ice, so it takes longer to make ::: :::teacher-note **Answer:** Boiling pulls the particles completely apart, which takes far more energy than loosening them from their fixed places Melting only frees the particles to slide past each other. Boiling has to pull them far apart against their attraction, so it needs about seven times as much energy (2260 kJ for each kilogram, against 334 kJ to melt it). ::: :::keep-together ### Before melting **What does adding energy do to the particles in a solid, before it melts?** ☐ They start to leave the solid straight away ☐ They get bigger ☐ They vibrate faster, so the temperature rises ::: :::teacher-note **Answer:** They vibrate faster, so the temperature rises Temperature is a measure of how fast the particles move on average. Energy in, faster particles, higher temperature, until a change of state begins. ::: :::keep-together ### What the particles do **What happens to the particles in each change of state?** A. Gas particles slow down and are pulled close together into a liquid, giving out energy B. Particles slow down and settle into fixed places, giving out energy C. Fast particles at the surface pull free of the others and escape as a gas D. Particles vibrate hard enough to break free of their fixed places and start sliding past each other {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Melting` | | | `Freezing` | | | `Evaporating` | | | `Condensing` | | ::: :::teacher-note **Answer:** Melting → D. Particles vibrate hard enough to break free of their fixed places and start sliding past each other; Freezing → B. Particles slow down and settle into fixed places, giving out energy; Evaporating → C. Fast particles at the surface pull free of the others and escape as a gas; Condensing → A. Gas particles slow down and are pulled close together into a liquid, giving out energy Energy in frees the particles from each other's attraction; energy out lets the attraction pull them back together. ::: ## Mix it up :::keep-together **Looking back at states of matter: a small jar of gas is opened in the middle of a large, sealed room. A few minutes later, where are its particles?** ☐ Mostly on the floor ☐ Spread through the whole room ☐ Mostly at the top of the room ☐ Still in a clump where the jar was ::: :::teacher-note **Answer:** Spread through the whole room Gas particles move fast in every direction and barely attract each other, so they spread to fill the room. ::: :::typical ## Elaborate: a cooling curve :::keep-together ### Why it stays flat **Why does the stearic acid's temperature stay at 69 °C for 8 minutes while it freezes?** ☐ As particles settle into fixed places they give out energy, which keeps the temperature steady ☐ The thermometer stopped working ☐ The room warms the wax ::: :::teacher-note **Answer:** As particles settle into fixed places they give out energy, which keeps the temperature steady Freezing gives energy out. Until every particle has settled, that energy balances what is lost to the room. ::: ::: :::wider ## Elaborate: sweating :::keep-together **Why does sweating cool you down, even on a hot day?** ☐ Sweat is colder than your body ☐ Evaporating sweat takes energy in, and it takes it from your skin ☐ The wind blows the heat away ::: :::teacher-note **Answer:** Evaporating sweat takes energy in, and it takes it from your skin Changing liquid to gas needs energy. The fastest particles escape from the sweat, carrying energy away from your skin. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Melting, boiling and evaporating take **energy in**; freezing and condensing give **energy out**. - Adding energy to a substance makes its particles move faster (the temperature rises), until it reaches a change of state. - During a change of state the temperature stays the same: the energy goes into separating the particles (or comes out as they settle). - A **heating curve** shows this as flat parts at the melting and boiling points. ## Exit ticket :::keep-together **Water vapour condenses on a cold window. Is energy taken in or given out, and what are the particles doing?** {{lines-3}} ::: :::teacher-note **Answer:** Given out. The water particles slow down and come close together, forming a liquid on the glass. :::