Evaporation and cooling — answer key

1. EQ11 · Practice

Model answer. The more energetic particles near the surface escape into the air. Even on a cool day, the particles in the puddle move at a range of speeds. A particle at the surface, moving outwards fast enough, breaks away from the forces of its neighbours and escapes: that is evaporation. A fast particle deep in the liquid cannot escape, because it soon hits its neighbours, and a slow particle at the surface is pulled back. The puddle never reaches 100 °C: evaporation happens at any temperature.

A full answer includes

• I chose “The more energetic particles near the surface escape into the air”.
• I explained that the particles move at a range of speeds, and only those near the surface, moving outwards fast enough, can break away from the forces of their neighbours; a fast particle deep in the liquid soon hits its neighbours.

Also accept

• The more energetic particles near the surface escape (only particles at the surface, moving fast enough, can break away).

Watch for

• “The puddle must reach 100 °C before any water can turn into a gas.” — That is boiling. Evaporation happens at any temperature, because at any temperature a few particles near the surface are fast enough to escape.
• “Any fast particle can escape, wherever it is in the liquid.” — A fast particle deep in the liquid soon hits its neighbours. Only particles near the surface, moving outwards, can break away.

2. EQ12 · Practice

Model answer. (1) range: a few particles are slow, most are middling and a few are very fast. (2) surface: only particles at the surface, moving outwards fast enough, can break away. (3) average: the fastest particles escape, so the average kinetic energy of the particles that stay is lower. (4) cooler: the temperature of a liquid is a measure of the average kinetic energy of its particles, so a lower average means a lower temperature.

A full answer includes

• (1) range
• (2) surface
• (3) average
• (4) cooler (or colder)

Also accept

• range; surface; average; cooler.

Watch for

• “(1) All the particles move at the same speed.” — The particles keep colliding, so they move at a range of speeds. That is why a few are fast enough to escape while most are not.
• “(4) hotter” — The fastest particles have escaped, so the particles that stay have a lower average kinetic energy: the liquid is cooler.

3. EQ13 · Practice

Model answer. Mass lost = 81.23 g − 80.59 g = 0.64 g. The cork closed tube B, so no water could escape from the tube itself, and balance A, with its dry towel, did not change. So the 0.64 g went from the wet towel into the air: it evaporated into the air as water vapour.

Checked automatically

• Mass of water evaporated: 0.64 g (within ±0.005 g)
  • Common wrong answer 80.59 : That is the reading after 5 minutes. The mass lost is the start minus the end: 81.23 g − 80.59 g.
  • Common wrong answer 3.00 : 3.00 g is the water put on the towel at the start. Most of it was still there after 5 minutes: work out what the balance lost.
  • Common wrong answer -0.64 : The balance went down, so the mass lost is the start minus the end, a positive number.
  • Common wrong answer 161.82 : You added the two readings. Subtract them: the start minus the end.

A full answer includes

• 0.64 g: 81.23 g − 80.59 g.
• The cork closed the tube, so water could escape only from the wet towel: it evaporated into the air as water vapour.

Also accept

• 0.64 g (81.23 g − 80.59 g); the tube was corked, so the water came from the towel.

Watch for

• “The balance fell because the water in the tube got lighter as it cooled.” — Cooling does not change mass. The balance fell because 0.64 g of water went from the towel into the air as vapour.
• “All 3.00 g of water on the towel evaporated.” — Only 0.64 g evaporated in 5 minutes; the towel was still wet at the end.

4. EQ14 · Practice

Options as numbered on the pupil sheet: 1 — Energy passes from the warmer tube into the cooler wet towel; 2 — The more energetic water particles escape from the surface of the wet towel; 3 — So the water in tube B loses energy faster, and its temperature falls faster; 4 — The water that stays in the towel has a lower average kinetic energy: it is cooler.

Model answer. 1. The more energetic water particles escape from the surface of the wet towel: the water evaporates. 2. The particles that stay have a lower average kinetic energy, so the towel’s water is cooler. 3. The tube is warmer than the wet towel in contact with it, so energy passes from the tube into the towel. 4. So the water in tube B loses energy faster than the water in tube A, and its temperature falls faster. The chain goes on for as long as the towel is wet.

A full answer includes

• Step 1: the more energetic particles escape from the surface of the towel. Step 2: the water that stays is cooler.
• Step 3: energy passes from the warmer tube into the cooler towel. Step 4: so tube B cools faster.

Also accept

• escape from the surface; the water that stays is cooler; energy passes from the warmer tube into the towel; tube B cools faster.

Watch for

• “The cold passes from the towel into the tube.” — Energy passes from warmer to cooler: from the tube into the towel. There is no “cold” that moves.
• “The water inside tube B evaporates, and that cools it.” — The cork closes tube B, so no water escapes from it. The water that evaporates is on the towel outside; the tube cools because energy passes from it into that cooler towel.

5. EQ15 · Practice

Options as numbered on the pupil sheet: 1 — Water evaporated from the wet towel; the corks kept the water in both tubes; 2 — The room-temperature water on towel B took energy as it warmed up, and the wet towel let energy pass more easily; 3 — Evaporation kept the towel’s water cooler, so energy kept passing from the warmer tube into it; 4 — As the towel cooled, fewer of its water particles had enough energy to escape; 5 — Tube A’s thermometer was faulty; 6 — The water inside tube B was boiling.

Model answer. Balance B fell by 0.64 g while balance A did not change: the corks kept the water in both tubes, so the mass came off the wet towel as it evaporated. In the first 30 s, the 3.00 g of water on towel B started at 20 °C and took energy from the tube just by warming up, and a wet towel lets energy pass more easily than a dry one; this is why the dry towel is only a rough comparison. From 30 s to 300 s, tube B still cooled about twice as fast as tube A: evaporation kept the towel’s water cooler, so energy kept passing from the warmer tube into it. Balance B fell more slowly near the end because the towel had cooled, so fewer of its water particles had enough energy to escape.

A full answer includes

• Balance B fell, balance A did not: water evaporated from the wet towel.
• First 30 s: the cool water on the towel warmed up, and the wet towel let energy pass more easily (it is a rough comparison).
• 30 s to 300 s: evaporation kept the towel’s water cooler, so energy kept passing from the warmer tube into it.
• Slower loss at the end: the towel had cooled, so fewer of its particles had enough energy to escape.

Also accept

• balance – evaporation from the towel; first 30 s – cool water warming up and the wet towel; 30 s to 300 s – evaporation keeps the towel cooler; slower at the end – a cooler towel, fewer particles escape.

Watch for

• “All of tube B’s faster cooling in the first 30 s was caused by evaporation.” — Part of it was the room-temperature water on the towel warming up, and the wet towel letting energy pass more easily. That is why the dry towel is only a rough comparison, and the balance is the evidence for evaporation.
• “The water inside tube B was boiling.” — It was at 70 °C or below, far under 100 °C, and corked. Water escaped only from the towel, by evaporation.

6. EQ16 · Practice

Model answer. (a) The lowest reading is 12.5 °C, at 90 s. (b) 20.0 °C − 12.5 °C = 7.5 °C below room temperature. The ethanol on the bulb evaporated: its more energetic particles escaped from its surface, so the ethanol that stayed had a lower average kinetic energy and was cooler. Energy passed from the warmer bulb into the cooler ethanol, so the bulb cooled, below room temperature. Once the thin film of ethanol had all evaporated, nothing kept cooling the bulb, and energy passed into it from the warmer room air, so the reading rose back to 20 °C.

Checked automatically

• (a) Lowest reading: 12.5 °C (within ±0.3 °C)
  • Common wrong answer 20 : 20.0 °C is where the reading started and finished. Find the bottom of the dip.
  • Common wrong answer 90 °C: 90 s is the time of the lowest reading. Read the temperature axis, not the time axis.
  • Common wrong answer 7.5 : That is the fall, which (b) asks for. (a) is the reading itself, at the bottom of the dip.
• (b) Fall below room temperature: 7.5 °C (within ±0.3 °C)
  • Common wrong answer 12.5 : That is the lowest reading. The fall is 20.0 °C − 12.5 °C.
  • Common wrong answer 32.5 : You added. The fall is the difference: 20.0 °C − 12.5 °C.

A full answer includes

• (a) 12.5 °C. (b) 20.0 °C − 12.5 °C = 7.5 °C.
• The ethanol evaporated: its more energetic particles escaped, so the ethanol that stayed was cooler; energy passed from the warmer bulb into it, so the bulb cooled below room temperature.
• When the ethanol had all evaporated, the cooling stopped, and energy from the warmer room warmed the bulb back to 20 °C.

Also accept

• (a) 12.5 °C; (b) 7.5 °C; the ethanol evaporates, its most energetic particles escape, the ethanol that stays is cooler and energy passes from the bulb into it; once it has gone, the room warms the bulb back.

Watch for

• “The ethanol was colder than the room.” — It started at 20 °C, the same as the room. It became cooler only as it evaporated.
• “The reading rose again because the ethanol warmed the bulb.” — By then the ethanol had evaporated. With no evaporation to cool it, the bulb was warmed by the room air around it.

7. EQ17 · Practice

Model answer. The sweat evaporates: the more energetic water particles near its surface escape into the air. The particles that stay have a lower average kinetic energy, so the sweat that stays on the skin is cooler. The skin is in contact with the sweat and warmer than it, so energy passes from the skin into the cooler sweat. The skin, and the blood flowing just under it, are cooled. On a dry day the sweat evaporates quickly, and the cooling goes on for as long as the skin is wet.

A full answer includes

• The sweat evaporates: its more energetic particles escape from its surface.
• The particles that stay have a lower average kinetic energy, so the sweat that stays is cooler.
• Energy passes from the warmer skin into the cooler sweat in contact with it, so the skin (and the blood under it) cools.

Also accept

• The most energetic particles escape from the sweat, so the sweat that stays is cooler; energy passes from the warmer skin into it, so the skin cools.

Watch for

• “Sweat cools you because it comes out cold.” — Sweat comes out at about skin temperature. It cools you only as it evaporates.
• “The escaping particles take the cold away from the skin.” — There is no “cold” to carry. The escaping particles carry energy away, so the sweat that stays is cooler, and energy then passes into it from the skin.

8. EQ18 · Practice

Model answer. Balance B fell by 0.64 g, while balance A did not change. With both tubes corked, the only way balance B could lose mass was water going from the wet towel into the air as vapour: that is evaporation, measured directly. The fall in temperature shows that tube B cooled faster, but the wet towel also let energy pass more easily, and its water started at room temperature, so the curves alone cannot show that water evaporated. The fast fall in the first 30 s was partly the cool water warming up.

A full answer includes

• I chose “Balance B fell by 0.64 g, while balance A did not change”.
• I explained that the balance measures the mass of water that went into the air, with both tubes corked; the temperatures show that B cooled faster, but the dry towel is only a rough comparison, so on their own they cannot show why.

Also accept

• Balance B fell by 0.64 g and balance A did not: water evaporated from the towel. The temperatures have other causes too.

Watch for

• “Tube B cooled more, so that proves water evaporated.” — A wet towel also lets energy pass more easily, and its water started cool. The temperatures show that B cooled faster; the balance shows that water evaporated.
• “The towel was still damp at the end, so water evaporated.” — A damp towel shows that water is still there, not that any has gone. Only the balance shows how much evaporated.

9. ET2 · Apply

Model answer. (a) The alcohol evaporates. Its particles move at a range of speeds, and the more energetic ones near the surface escape into the air, so a thin film soon disappears. (b) The particles that escape are the most energetic, so the alcohol that stays on the skin has a lower average kinetic energy: it is cooler than the skin. Energy passes from the warmer skin into the cooler alcohol, so the skin cools, and nerves in the skin sense the fall. (c) At room temperature, far fewer water particles escape each second than alcohol particles: the forces between water particles are stronger, so fewer are energetic enough to break away. The water evaporates more slowly, so it is cooled less, energy passes from the skin into it more slowly, and it takes longer to dry. (d) While the swab is wet, the thermometer reads below room temperature, because the evaporating alcohol keeps the bulb cooler. Once the alcohol has all evaporated, energy passes into the bulb from the room air, and the reading rises back to room temperature.

A full answer includes

• (a) It evaporates: the more energetic alcohol particles near the surface escape into the air.
• (b) The alcohol that stays on the skin has a lower average kinetic energy, so it is cooler than the skin; energy passes from the warmer skin into it, so the skin cools.
• (c) Water evaporates much more slowly than alcohol at the same temperature, so it is cooled less and takes energy from the skin more slowly.
• (d) Below room temperature while the swab is wet; back up to room temperature once the alcohol has all evaporated.

Also accept

• (a) evaporation: the most energetic particles escape; (b) the alcohol that stays is cooler, and energy passes from the skin into it; (c) water evaporates more slowly; (d) below room temperature while wet, back to room temperature once dry.

Watch for

• “(b) The alcohol feels cold because it is a cold liquid.” — It was at room temperature. It cools as it evaporates, and then energy passes into it from the warmer skin.
• “(d) The thermometer stays low after the alcohol has gone.” — Once nothing is evaporating, nothing keeps the bulb cool: energy from the warmer room warms it back to room temperature.