Melting and boiling — answer key

1. EQ1 · Practice

Model answer. The energy separates the particles, breaking them out of their fixed positions. The heater supplies energy the whole time. While ice is melting, that energy breaks the particles of ice out of their fixed positions, so that they can move past each other. It does not make the particles move faster on average, so the temperature stays at 0 °C: an energy input without a change in temperature. Only when all the ice has melted does the energy make the particles move faster, and the temperature rises.

A full answer includes

• I chose “The energy separates the particles, breaking them out of their fixed positions, so they do not move faster on average”.
• I explained that energy goes in all the time, but while the ice melts it separates the particles instead of raising their average kinetic energy, so the temperature stays at 0 °C until all the ice has melted.

Also accept

• The energy separates the particles (breaks them out of their fixed positions); they do not move faster on average, so the temperature stays at 0 °C until all the ice has melted.

Watch for

• “No energy goes into the beaker until all the ice has melted.” — The heater is on the whole time, so energy goes in the whole time. Along the flat part, the energy separates the particles instead of making them move faster.
• “The ice cancels out the energy from the heater.” — Ice does not destroy energy. The energy goes into the ice and breaks its particles out of their fixed positions: that is melting.

2. EQ2 · Practice

Model answer. (1) 0: pure water melts at 0 °C at standard atmospheric pressure. (2) 100: it boils at 100 °C. (3) same: boiling happens without a change in temperature. (4) apart: the energy separates the particles completely, against the forces that hold them close, so they become a gas. (5) faster: more energy each second turns the water into steam more quickly, but its temperature stays at 100 °C.

A full answer includes

• (1) 0
• (2) 100
• (3) same
• (4) apart
• (5) faster (or more quickly)

Also accept

• 0; 100; same; apart; faster.

Watch for

• “(5) hotter: turning the hob up makes boiling water hotter” — Boiling water stays at its boiling point, 100 °C, however high the hob is turned up. More energy each second makes it boil away faster.
• “(2) 90: pure water boils at 90 °C” — Water boils at about 90 °C high on a mountain, where the air pressure is lower. At standard atmospheric pressure, pure water boils at 100 °C.

3. EQ3 · Practice

Options as numbered on the pupil sheet: 1 — 0 °C; 2 — 100 °C; 3 — −10 °C; 4 — 37 °C; 5 — 50 °C.

Model answer. Ice melts at 0 °C, and water freezes at the same temperature, 0 °C: this is the melting point of water. Water boils at 100 °C at standard atmospheric pressure, and steam cooled from 120 °C condenses at the same temperature, 100 °C: this is the boiling point. −10 °C is a temperature at which water is already solid, 37 °C is body temperature, and 50 °C is just warm water.

A full answer includes

• Ice melts: 0 °C. Water freezes: 0 °C.
• Water boils: 100 °C. Steam, cooled from 120 °C, condenses: 100 °C.

Also accept

• Ice melts – 0 °C; water boils – 100 °C; steam condenses – 100 °C; water freezes – 0 °C.

Watch for

• “Water freezes below 0 °C, for example at −10 °C.” — Pure water freezes at its melting point, 0 °C. While it freezes, the temperature stays at 0 °C; only when it is all ice can it cool further, to −10 °C.
• “Water boils at a higher temperature if you heat it harder.” — The boiling point does not depend on the heater. At standard atmospheric pressure pure water boils at 100 °C; heating it harder makes it boil away faster.

4. EQ4 · Practice

Options as numbered on the pupil sheet: 1 — At 0 °C the water freezes: the particles settle into fixed positions and can only vibrate; 2 — The steam cools to 100 °C: its particles slow down; 3 — The ice cools below 0 °C: its particles vibrate more slowly; 4 — At 100 °C the steam condenses: the particles come close together, held by the forces between them; 5 — The water cools from 100 °C to 0 °C: its particles move past each other more slowly.

Model answer. First the steam cools from 110 °C to 100 °C, as its particles slow down. At 100 °C it condenses: the gas particles come close together, and the forces between them hold them close, so it becomes a liquid. Energy is given out, and the temperature stays at 100 °C until all of it is water. Then the water cools to 0 °C. At 0 °C it freezes: the particles settle into fixed positions in a regular pattern, giving out energy, and the temperature stays at 0 °C until all of it is ice. Last, the ice cools to −10 °C, and its particles vibrate more slowly.

A full answer includes

• Step 1: the steam cools to 100 °C. Step 2: it condenses at 100 °C.
• Step 3: the water cools to 0 °C. Step 4: it freezes at 0 °C. Step 5: the ice cools below 0 °C.

Also accept

• cools to 100 °C; condenses at 100 °C; cools to 0 °C; freezes at 0 °C; the ice cools to −10 °C.

Watch for

• “The temperature falls steadily the whole time, from 110 °C to −10 °C.” — It stops falling twice: at 100 °C while the steam condenses, and at 0 °C while the water freezes. Energy given out as the particles come together keeps the temperature steady until each change is complete.
• “In ice the particles stop moving.” — They still vibrate about their fixed positions. As the ice cools further, they vibrate more slowly.

5. EQ5 · Practice

Model answer. The particles come close together, and the forces between them hold them close. Condensation turns a gas into a liquid. In the gas the particles are far apart and move fast; at the cold glass they come close together, so they form drops of liquid water. They are not in fixed positions: that would be a solid. Energy is given out as they come together, and it passes into the cold glass.

A full answer includes

• I chose “The particles come close together, and the forces between them hold them close”.
• I explained that in a liquid the particles are close together but not in fixed positions, and that energy is given out, to the cold glass, as they come together.

Also accept

• The particles come close together, held by the forces between them (a liquid, not fixed positions); energy is given out.

Watch for

• “The particles get smaller when a gas condenses.” — Each particle stays the same size. What changes is how far apart they are: in a liquid they are close together.
• “The particles settle into fixed positions.” — That is solidification, a liquid turning into a solid. In a liquid the particles are close together but can still move past each other.

6. EQ6 · Practice

Model answer. While water boils, the energy from the flame pulls the water particles right apart, against the forces that hold them close, so they become steam. The particles do not move faster on average, so the temperature stays at the boiling point, 100 °C: boiling is an energy input without a change in temperature. Turning the gas up supplies more energy each second, so more water turns into steam each second: the water boils away faster and bubbles harder, but it does not get hotter.

A full answer includes

• The energy separates the particles, pulling them right apart into a gas (steam), instead of making them move faster on average.
• So the temperature stays at the boiling point, 100 °C; more energy each second makes the water boil away faster (more steam each second), not hotter.

Also accept

• The energy separates the particles into a gas instead of raising their average kinetic energy, so the temperature stays at 100 °C; more energy each second makes it boil away faster.

Watch for

• “The extra energy does nothing.” — It turns more water into steam each second. On high, the pan boils dry sooner.
• “The thermometer must be wrong: the water is hotter on high.” — Boiling water stays at its boiling point, whatever the heater does. The thermometer is right.

7. EQ7 · Practice

Model answer. (a) The first level part is at 50 °C, from 3 to 8 minutes: Y is melting, so its melting point is 50 °C. (b) The second level part is at 180 °C, from 18 to 28 minutes: Y is boiling, so its boiling point is 180 °C. From 3 to 8 minutes the heater is still supplying energy, but the energy breaks the particles of Y out of their fixed positions, so the solid becomes a liquid. The particles do not move faster on average, so the temperature stays at 50 °C until all of Y has melted. Y is not water: water would melt at 0 °C and boil at 100 °C.

Checked automatically

• (a) Melting point of Y: 50 °C (within ±2 °C)
  • Common wrong answer 20 : 20 °C is where Y started. The melting point is the first level part, where the solid melts.
  • Common wrong answer 0 : 0 °C is the melting point of water. Read Y’s own graph: its first level part.
  • Common wrong answer 180 : That is the second level part, where Y boils. Melting comes first.
• (b) Boiling point of Y: 180 °C (within ±2 °C)
  • Common wrong answer 100 : 100 °C is the boiling point of water. Read Y’s own graph: its second level part.
  • Common wrong answer 195 : That is the last point, after Y has boiled away and the gas warms. The boiling point is the second level part.
  • Common wrong answer 50 : That is the first level part, where Y melts. Boiling comes second.

A full answer includes

• (a) Melting point: 50 °C, the first level part.
• (b) Boiling point: 180 °C, the second level part.
• From 3 to 8 minutes Y is melting: energy still goes in, but it separates the particles (breaks them out of their fixed positions) instead of making them move faster, so the temperature stays at 50 °C.

Also accept

• (a) 50 °C; (b) 180 °C; from 3 to 8 minutes Y melts: the energy separates the particles, so the temperature stays the same.

Watch for

• “From 3 to 8 minutes the heater must have been switched off.” — Y is heated steadily the whole time. Along the level part the energy separates the particles, so the temperature does not change.
• “The boiling point is 100 °C, as for water.” — Each pure substance has its own melting and boiling points. 0 °C and 100 °C are water’s; Y’s come from its own graph.

8. EQ8 · Practice

Model answer. (a) The readings stay at 69.0 °C from 180 s to 300 s, so the freezing point of stearic acid is 69 °C (the same as its melting point). (b) It was freezing from about 180 s to 300 s; it may have started just before 180 s and finished just after 300 s. (c) While it freezes, the particles of stearic acid settle into fixed positions, held by strong forces. They do not slow down on average while they do this: they slowed down before 180 s, while the liquid cooled. Energy is given out as they settle. That energy makes up for the energy the tube loses to the cooler air, so the temperature stays at 69 °C until all of the stearic acid is solid. Then the temperature falls again. (d) Stir gently with the thermometer while the liquid is runny, so that the whole tube is at one temperature; or read the temperature every 15 s near the level part, so that its start and end show clearly; or repeat the run and check that the level part comes at the same temperature.

A full answer includes

• (a) 69 °C, the level part of the curve.
• (b) From about 180 s to 300 s.
• (c) The particles settle into fixed positions (the liquid solidifies), with the same average speed, because the temperature does not change. Energy is given out as they do; it makes up for the energy lost to the air, so the temperature stays at 69 °C until all of it is solid.
• (d) For example: stir until it becomes too stiff; read the temperature more often (every 15 s) near the level part; repeat the run.

Also accept

• (a) 69 °C; (b) about 180 s to 300 s; (c) the particles settle into fixed positions and give out energy, which makes up for the energy lost, so the temperature is steady; (d) stir, read more often near the level part, or repeat.

Watch for

• “(a) The freezing point is 59 °C, the last reading.” — The last reading is just where the pupil stopped. The freezing point is the temperature of the level part, where the liquid turns to solid.
• “(c) The temperature stays the same because no energy leaves the tube.” — The tube is warmer than the air, so energy leaves it the whole time. The temperature stays steady because the particles give out energy as they settle into fixed positions.
• “(c) The particles slow down while the stearic acid freezes.” — They slow down while the liquid cools to 69 °C. While it freezes the temperature stays at 69 °C, so their average speed stays the same: they settle into fixed positions, and the energy given out as they do passes to the air.

9. EQ9 · Practice

Model answer. Correct: ice melts at 0 °C; while ice melts, energy goes in but the temperature stays at 0 °C; water boils at 100 °C. Boiling water does not get hotter if you heat it harder: it stays at 100 °C and boils away faster. Energy is supplied all the time while water boils: it separates the particles into a gas. Salt water is not pure water: the dissolved salt changes its freezing point, which is below 0 °C.

A full answer includes

• I chose: ice melts at 0 °C; while ice melts, energy goes in but the temperature stays at 0 °C; water boils at 100 °C.
• I did not choose the others: boiling water stays at 100 °C however hard it is heated; energy is supplied the whole time it boils; salt water is not pure, and freezes below 0 °C.

Also accept

• a, b and d: ice melts at 0 °C; an energy input without a change in temperature while it melts; water boils at 100 °C.

Watch for

• “While water boils, no energy is being supplied.” — The heater is on the whole time. The energy separates the particles into a gas, so the temperature stays the same.
• “Any water freezes at 0 °C, even salt water.” — 0 °C and 100 °C are for pure water at standard atmospheric pressure. Dissolved salt changes both: salt water freezes below 0 °C.

10. EQ10 · Practice

Model answer. On the can: water vapour in the warm air touches the cold metal. Its particles come close together, and the forces between them hold them close, so the vapour becomes liquid water: this is condensation. In the freezer: the water cools to 0 °C, and its particles slow down as it cools. At 0 °C they settle into fixed positions in a regular pattern, held by strong forces, where they can only vibrate: this is solidification, or freezing. While the water freezes, its temperature stays at 0 °C, so its particles do not slow down on average. In both changes energy is given out as the particles come together: to the cold can, and to the cold air of the freezer.

A full answer includes

• Drops on the can: condensation. Water vapour particles in the air come close together at the cold can, held by the forces between them, forming a liquid.
• In the freezer: solidification (freezing). The water particles slow down as the water cools to 0 °C; at 0 °C they settle into fixed positions in a regular pattern, where they can only vibrate, and their average speed stays the same while the water freezes.
• In both changes energy is given out as the particles come together.

Also accept

• Can: condensation, gas particles come close together into a liquid; freezer: solidification, particles settle into fixed positions; energy is given out in both.

Watch for

• “The drops on the can leaked through the metal.” — The can is sealed. The water comes from water vapour in the air, which condenses on the cold surface.
• “When water freezes, its particles stop moving.” — In ice the particles still vibrate about their fixed positions. Freezing fixes their positions; it does not stop them.
• “While the water freezes, its particles slow down.” — They slow down while the water cools to 0 °C. While it freezes, the temperature stays at 0 °C, so their average speed stays the same: they settle into fixed positions, and the energy given out passes to the freezer air.

11. ET1 · Apply

Model answer. (a) 100 °C, for pure water at standard atmospheric pressure. While water boils, the energy from the flame pulls the particles right apart into steam; it does not make them move faster on average, so the temperature stays at the boiling point. (b) The lid is cooler than the steam. At the lid, the steam particles come close together, and the forces between them hold them close: the steam condenses into drops of liquid water. Energy is given out to the lid as they do. (c) Steam that lands on skin first condenses, giving out the energy that was needed to separate its particles, and then the water cools as hot water would. Condensing gives out a lot of energy, so steam passes much more energy to the skin. (d) More slowly. Up the mountain, the water boils at 90 °C and stays there however high the flame, so the pasta is cooked at 90 °C instead of 100 °C. (e) 0 °C, for as long as there is still ice in the glass. Energy passes all the time from the warm kitchen into the colder glass of water. It melts the ice: it breaks the particles of the ice out of their fixed positions, so that they can move past each other. The particles do not move faster on average, so the temperature stays at 0 °C, the melting point of ice, until all the ice has melted. Then the water warms up. (f) While the reading falls from 20 °C to 0 °C, the water cools: its particles slow down, and energy passes from the water to the colder air in the freezer. While the reading stays at 0 °C, the water solidifies: its particles settle into fixed positions in a regular pattern, held by strong forces, where they can only vibrate. The temperature does not change, so they keep the same average speed. Energy is given out as they settle, and it passes to the freezer air. When all the water is ice, the ice cools below 0 °C.

A full answer includes

• (a) 100 °C: boiling water stays at its boiling point; the energy separates the particles into steam instead of raising their average kinetic energy.
• (b) Condensation: at the cooler lid the steam particles come close together, held by the forces between them, forming liquid drops; energy is given out to the lid.
• (c) When steam condenses on skin, it gives out the energy that separated its particles, on top of the energy the hot water gives out as it cools.
• (d) More slowly: the water boils at only 90 °C and cannot get hotter, so the pasta is cooked at a lower temperature.
• (e) 0 °C while there is still ice: energy passes into the glass from the warm kitchen, and it breaks the particles of the ice out of their fixed positions (melting) instead of raising the temperature.
• (f) While the reading falls, the particles slow down. While it stays at 0 °C the water solidifies: the particles settle into fixed positions in a regular pattern, held by strong forces, with the same average speed; energy is given out and passes to the colder air in the freezer.

Also accept

• (a) 100 °C, the energy separates the particles; (b) condensation on the cooler lid, the particles come close together; (c) condensing steam gives out extra energy; (d) more slowly, it boils at 90 °C so it is cooler; (e) 0 °C, energy from the kitchen melts the ice by separating the particles; (f) falling: the particles slow down; at 0 °C: they settle into fixed positions with the same average speed, giving out energy to the freezer.

Watch for

• “(a) The water is above 100 °C because it has been heated for so long.” — Heating for longer only boils more of it away. The temperature stays at the boiling point until all the water has gone.
• “(d) Faster, because water boils sooner up a mountain.” — It does come to the boil sooner, but it boils at only 90 °C and cannot get hotter. Food cooks more slowly at a lower temperature.
• “(f) The particles slow down while the reading stays at 0 °C.” — The reading stays at 0 °C, so the average speed of the particles stays the same. They slow down while the water cools to 0 °C. At 0 °C they settle into fixed positions, and the energy given out passes to the freezer air.