Where does the heat go? — answer key

1. SQ5 · Practice

Model answer. 44 °C. The first flat section, B to C, is where solid P melts: the temperature stays constant while it changes state.

A full answer includes

• I chose 44 °C.
• I used the first flat section, B to C, as evidence.

Also accept

• 44 °C, because the graph is first flat from B to C, where P melts.

Watch for

• “The melting point is where heating starts, at 20 °C.” — At A the solid is simply being heated. The melting point is the temperature of the first flat section, where the temperature stops rising while the solid melts.

2. SQ6 · Practice

Model answer. Statements 1 and 2. Evaporation happens only at the surface, at temperatures below the boiling point: some fast-moving particles near the surface escape. Bubbles throughout the liquid and a fixed temperature describe boiling.

A full answer includes

• I chose statements 1 and 2.
• I explained that some fast-moving particles at the surface escape, even below the boiling point.

Also accept

• 1 and 2: evaporation happens at the surface below the boiling point, because some surface particles have enough energy to escape.

Watch for

• “Evaporation is just slow boiling.” — Boiling happens throughout the liquid at the boiling point. Evaporation happens only at the surface, at temperatures below it.

3. SQ7 · Practice

Model answer. Boiling: liquid P is turning into a gas. The energy supplied is used to overcome the forces of attraction between the particles and separate them, not to make them move faster. So the temperature stays at 150 °C until all of P has boiled.

A full answer includes

• I named boiling: liquid P turning into a gas.
• I said the energy is used to overcome the forces of attraction between the particles.
• I said the particles don't move faster on average, so the temperature stays constant.

Also accept

• Boiling. The energy overcomes the forces of attraction between particles instead of increasing their kinetic energy, so the temperature is constant.

Watch for

• “No energy is going in while the graph is flat.” — The heater still supplies energy. It goes into separating the particles, not into raising their average speed.

4. SQ8 · Practice

Options as numbered on the pupil sheet: 1 — Liquid water warms up; 2 — Ice warms up; 3 — Water boils; 4 — Ice melts.

Model answer. Ice warms to 0 °C; ice melts at 0 °C; liquid water warms to 100 °C; water boils at 100 °C. The temperature stays constant during melting and boiling, because the energy overcomes forces of attraction between molecules instead of making them move faster.

A full answer includes

• Order: ice warms, ice melts, water warms, water boils.
• I said the temperature does not rise during melting and boiling, because the energy changes the state instead.

Also accept

• Ice warms, ice melts, water warms, water boils; no temperature rise while melting or boiling, because the energy changes the state.

Watch for

• “Water boils as soon as all the ice has melted.” — The liquid first has to warm from 0 °C to 100 °C. Only then does it boil.

5. SQ13 · Practice

Model answer. It is evaporating. At 70 °C the water is below its boiling point, so molecules leave only from the surface: some move fast enough to escape. Hot water evaporates quickly, which is why the level fell. If it started to boil, at 100 °C, bubbles of water vapour would form throughout the liquid.

A full answer includes

• I said the water is evaporating, not boiling.
• I explained that some fast-moving molecules escape from the surface, below the boiling point of 100 °C.
• I said boiling would show bubbles of vapour forming throughout the liquid, at 100 °C.

Also accept

• Evaporating: 70 °C is below the boiling point and molecules leave only from the surface; boiling would make bubbles throughout at 100 °C.

Watch for

• “The water is hot and losing volume, so it is boiling.” — Hot is not the same as boiling. Below 100 °C water only evaporates from its surface; boiling makes bubbles form inside the liquid.

6. ST2 · Apply

Model answer. (a) The temperature rises to −114 °C, stays flat while the ethanol melts, rises to 78 °C, stays flat while it boils, then rises to 100 °C as a gas. (b) Evaporation. At 60 °C the ethanol is below its boiling point, so particles escape only from the surface; the warm bath makes this faster. At 78 °C it would boil: bubbles of ethanol vapour would form throughout the liquid, and its temperature would stop rising.

A full answer includes

• The curve is flat at −114 °C while solid and liquid ethanol are both present (melting).
• The curve rises between the flat parts and is flat again at 78 °C while liquid and gas are both present (boiling); then it rises.
• (b) Evaporation: below its boiling point, some fast-moving particles escape from the surface; at 78 °C bubbles of vapour would form throughout the liquid as it boils.

Also accept

• Flat at −114 °C (melting) and 78 °C (boiling), rising between; at 60 °C it evaporates from the surface, and at 78 °C it would boil with bubbles throughout.

Watch for

• “The ethanol in the warm bath must be boiling, because it is losing volume.” — 60 °C is below its boiling point and no bubbles form inside. Particles escape from the surface: that is evaporation.