Answers · Melting, boiling and evaporation

Melting, boiling and evaporation — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. ER1 · Return

Model answer. (a) Melting point 0 °C; boiling point 100 °C. (b) The temperature stays at 0 °C while the ice melts, although energy goes in all the time. The energy breaks the particles out of their fixed positions so that they can move past each other; they do not move faster on average, so the temperature does not rise until all the ice has melted. (c) In condensation, gas particles, far apart and moving fast, come close together, and the forces between them hold them close: they form a liquid. Energy is given out. (d) In solidification, liquid particles settle into fixed positions in a regular pattern, held by strong forces, where they can only vibrate: they form a solid. Energy is given out. (e) The temperature rises from −20 °C to 0 °C, stays level at 0 °C while the ice melts, because the energy breaks the particles out of their fixed positions, rises from 0 °C to 100 °C, then stays level at 100 °C while the water boils, because the energy pulls the particles right apart into a gas. The level part at 100 °C lasts much longer, because separating the particles into a gas takes more energy.

A full answer includes

(a) 0 °C and 100 °C.
(b) The temperature stays at 0 °C until all the ice has melted: an energy input without a change in temperature. The energy separates the particles, breaking them out of their fixed positions, instead of making them move faster.
(c) Gas particles come close together, held by the forces between them, forming a liquid; energy is given out.
(d) Liquid particles settle into fixed positions in a regular pattern, where they can only vibrate; energy is given out.
(e) A rise to 0 °C; level at 0 °C (melting: the energy separates the particles); a rise to 100 °C; level at 100 °C (boiling: the energy pulls the particles apart into a gas), the longer level part.

Also accept

(a) 0 °C, 100 °C; (b) stays at 0 °C, the energy separates the particles; (c) the particles come close together into a liquid, energy out; (d) the particles settle into fixed positions, energy out; (e) rise, level at 0 °C, rise, longer level at 100 °C.

Watch for

“(b) The temperature of melting ice rises slowly as it melts.” — It stays at 0 °C until the last of the ice has melted. The energy goes into separating the particles, not into making them move faster.
“(d) In a solid the particles do not move.” — They vibrate about fixed positions. Solidification fixes their positions; it does not stop them.
“(d) While a liquid solidifies, its particles slow down.” — While it solidifies, its temperature stays the same, so the average speed of its particles stays the same. They settle into fixed positions, and the energy given out passes to the surroundings. They slow down only while the liquid, or the solid, cools.

2. ER2 · Return

Model answer. (a) The particles of a liquid move at a range of speeds. The more energetic particles near the surface, moving outwards fast enough, escape from the liquid into the air. (b) It cools. The particles that escape are the most energetic ones, so the particles that stay have a lower average kinetic energy, and the temperature of the water that stays on her skin is lower. (c) The water on the skin is cooler than the skin and in contact with it, so energy passes from the warmer skin into the cooler water. The skin cools, and this goes on for as long as she is wet. (d) The wind carries the escaped particles away from her skin, so fewer of them return. The water evaporates faster, so it is cooled faster and takes energy from her skin faster. In a sheltered spot, the escaped particles stay near her skin, more return, and evaporation is slower. (e) Evaporation happens at any temperature, boiling only at the boiling point; evaporation happens only from the surface, while boiling happens throughout the liquid, with bubbles; boiling needs energy supplied all the time, while an evaporating liquid cools unless energy reaches it. (f) Spread out flat on the hot sand in the sun. There the water in the towel is warmer, so its particles move faster on average and more of them have enough energy to escape. Spread flat, the towel also has a much bigger surface area than rolled up, so many more particles are at the surface, where they can escape. Both make evaporation faster. Rolled up in the bag, the towel is cooler, most of its surface is hidden, and the escaped particles are trapped in the bag, so many return.

A full answer includes

(a) The more energetic particles escape from the surface of the liquid.
(b) It cools: the particles that stay have a lower average kinetic energy.
(c) Energy passes from the warmer skin into the cooler water in contact with it, so the skin cools.
(d) Moving air carries the escaped particles away, so fewer return: the water evaporates faster, so it cools her skin faster.
(e) Two of: evaporation at any temperature, boiling only at the boiling point; evaporation only from the surface, boiling throughout, with bubbles; boiling needs energy supplied all the time, while an evaporating liquid cools.
(f) Spread out on the hot sand: the water in the towel is warmer, so more particles have enough energy to escape; and its surface area is much bigger, so more particles are at the surface, where they can escape. Both make evaporation faster.

Also accept

(a) the most energetic particles escape from the surface; (b) it cools: a lower average kinetic energy; (c) energy passes from the warmer skin to the cooler water; (d) the wind carries escaped particles away, so evaporation is faster; (e) any temperature against the boiling point; surface against throughout; (f) spread out in the sun: the water is warmer and the surface bigger, so more particles escape each second.

Watch for

“(d) The wind feels cold because it is cold air.” — The air on the beach is warm. The wind cools her because it makes the water on her skin evaporate faster.
“(b) The water that stays on her skin keeps the temperature of the sea.” — As the most energetic particles escape, the water that stays has a lower average kinetic energy: it cools.