Answers · Melting, boiling and evaporation

Melting, boiling and evaporation — final check key

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

1. EK1 · Final check

Model answer. (a) The level part, from Q to R, is at 54 °C, so Z freezes at 54 °C. (b) From 4 minutes to 11 minutes: 11 − 4 = 7 minutes. (c) Between Q and R, Z is solidifying. Its particles settle into fixed positions in a regular pattern, held by strong forces, where they can only vibrate. The temperature stays at 54 °C, so their average speed stays the same: they slow down only from P to Q and from R to S, while the temperature falls. (d) As the particles settle into fixed positions, energy is given out. That energy makes up for the energy the tube loses to the cooler room, so the temperature stays at 54 °C until all of Z has become solid. Then, from R to S, the temperature falls again. (e) The temperature rises from 20 °C to 54 °C, stays level at 54 °C while Z melts, then rises again once it is all liquid. Along the level part energy goes in all the time, but it breaks the particles out of their fixed positions instead of making them move faster, so the temperature does not change. (f) Pure water at standard atmospheric pressure freezes and melts at 0 °C. Z freezes at 54 °C, so it is not pure water. (g) 100 °C, the boiling point of pure water at standard atmospheric pressure. The burner supplies energy all the time, but while the water boils, that energy pulls the particles right apart, against the forces between them, so that they become steam. It does not make them move faster on average, so the temperature stays at 100 °C until all the water has boiled away. (h) The steam condenses. Its particles, far apart and moving fast, come close together, and the forces between them hold them close, so they form liquid water, which drips out. Energy is given out as they come together. It passes through the glass into the colder water flowing round the tube, which warms up.

A full answer includes

(a) 54 °C.
(b) 7 minutes (from 4 to 11 minutes).
(c) Z solidifies: the particles settle into fixed positions in a regular pattern, held by strong forces, where they can only vibrate; their average speed stays the same, because the temperature does not change.
(d) Energy is given out as the particles settle; it makes up for the energy lost to the room, so the temperature stays at 54 °C until all of Z is solid.
(e) A rise to 54 °C, level at 54 °C while it melts, then a rise again; the energy breaks the particles out of their fixed positions without raising the temperature.
(f) Pure water freezes and melts at 0 °C, not at 54 °C.
(g) 100 °C: while the water boils, the energy pulls the particles right apart into a gas (steam) instead of making them move faster, so the temperature stays at the boiling point.
(h) Condensation: the steam particles come close together, held by the forces between them, forming a liquid; energy is given out and passes through the glass into the cold water.

Also accept

(a) 54 °C; (b) 7 minutes; (c) the particles settle into fixed positions, with the same average speed; (d) the energy given out makes up for the energy lost; (e) rise, level at 54 °C, rise; the energy separates the particles; (f) water freezes at 0 °C; (g) 100 °C, the energy separates the particles into steam; (h) condensation: the particles come close together into a liquid, and energy is given out to the cold water.

Watch for

“(b) 11 minutes.” — 11 minutes is the time at R, when Z had finished freezing. It started at Q, at 4 minutes: 11 − 4 = 7 minutes.
“(d) Nothing is happening between Q and R, so the temperature does not change.” — Z is solidifying, and energy is given out as its particles settle. That is what keeps the temperature at 54 °C.
“(c) The particles slow down between Q and R.” — The temperature stays at 54 °C between Q and R, so the average speed of the particles stays the same. They slow down from P to Q and from R to S, while the temperature falls. Between Q and R they settle into fixed positions.

2. EK2 · Final check

Model answer. (a) The water particles move at a range of speeds. The more energetic ones near the surface, moving outwards fast enough, break away from their neighbours and escape into the air as water vapour: the water evaporates. (b) The particles that escape are the most energetic, so the particles that stay have a lower average kinetic energy, and the water cools. It cools until the energy passing into it from the warmer room makes up for the energy the escaping particles carry away, a few degrees below the room. (c) Every dish started with the same 20.0 g of water at 20 °C. Air movement: compare P and Q, both wide and out of the sun; only the draught differs. P, in the draught, lost 6.2 g; Q, in still air, 3.1 g. Moving air carries the escaped particles away, so fewer return. Surface area: compare Q and R, both in still air on the shelf, out of the sun; only the width differs. Wide Q lost 3.1 g, narrow R 1.0 g. A bigger surface has more particles at the surface, where they can escape. Temperature: compare S and Q, both wide and in still air; only the sun differs. The sun warmed S’s water to 30 °C, while Q’s cooled to 17 °C; S lost 5.0 g and Q 3.1 g. In warmer water more particles have enough energy to escape. (d) The dishes evaporate at about room temperature, far below 100 °C, while a kettle boils only at its boiling point; particles escape from the dishes only from the surface of the water, while in the kettle bubbles of vapour form throughout the water; the kettle needs energy supplied all the time and stays at 100 °C, while nothing keeps the dishes at one temperature: they cool as they evaporate. (e) The water in the hat evaporates: its most energetic particles escape, so the water that stays in the hat is cooler. Her head is warmer than the wet hat in contact with it, so energy passes from her head into the hat, and her head cools. This goes on while the hat is wet, faster in a breeze.

A full answer includes

(a) The more energetic particles near the surface escape into the air (evaporation).
(b) The most energetic particles escape, so the water that stays has a lower average kinetic energy: it cools below the room, until the energy passing in from the room makes up for the energy carried away.
(c) In each pair only one thing differs; every dish started with water at 20 °C. Air movement: P 6.2 g against Q 3.1 g; moving air carries the escaped particles away, so fewer return. Surface area: Q 3.1 g against R 1.0 g; a bigger surface has more particles at the surface. Temperature: S 5.0 g against Q 3.1 g; in warmer water more particles have enough energy to escape.
(d) Two of: the dishes evaporate at any temperature, far below the boiling point, while the kettle boils only at 100 °C; only from the surface, while the kettle boils throughout, with bubbles; the kettle needs energy supplied all the time, while the dishes cool as they evaporate.
(e) The water in the hat evaporates, so the water that stays in the hat is cooler; energy passes from her warmer head into the cooler wet hat.

Also accept

(a) the most energetic particles escape from the surface; (b) the water that stays has a lower average kinetic energy; (c) P and Q for air movement, Q and R for surface area, S and Q for temperature; (d) any temperature against 100 °C, surface against throughout; (e) the hat’s water evaporates and cools, and energy passes from her head into it.

Watch for

“(c) Compare P and R for air movement.” — P and R differ in two things, the draught and the width, so you cannot tell which one caused the difference. Compare P with Q: only the air movement differs.
“(b) Q was cooler because the shelf is cold.” — The shelf is in the same 20 °C room. Q is cooler because evaporation takes away its most energetic particles.