What changes the rate of evaporation — answer key

1. EQ19 · Practice

Model answer. Keep the same: the 3.00 g of water on towel B, the 70 °C starting temperature, the whole towel open to the air, and 5 minutes for each run. Temperature and surface area both change how fast water evaporates, so if either changed too, she could not tell what caused a difference. The fan is the one thing she changes on purpose. The reading on balance B at the end is what she measures: she uses it to work out the water lost.

A full answer includes

• I chose the mass of water on the towel at the start, the starting temperature, how much of the towel is open to the air, and the time of each run.
• I explained that only the fan should change, so that any difference in the water lost is caused by the moving air; the final reading on balance B is what she measures.

Also accept

• a, b, c and d: the water on the towel, the starting temperature, the open area and the time; only the fan changes; the final reading is the result.

Watch for

• “She should keep the reading on balance B at the end the same.” — That is the result she is measuring. If it came out the same, the fan would have made no difference.
• “The starting temperature does not matter, because only the fan is being tested.” — Warmer water evaporates faster. If the starting temperature changed too, she could not tell whether the fan or the temperature caused the difference.

2. EQ20 · Practice

Model answer. (a) 81.23 g − 80.31 g = 0.92 g. (b) 0.92 g − 0.64 g = 0.28 g more. The water on the towel, its starting temperature and the time were the same in both runs; only the fan changed. The moving air carries the escaped water particles away from the towel, so fewer of them return to it, and more water evaporates in the same time.

Checked automatically

• (a) Water evaporated with the fan on: 0.92 g (within ±0.005 g)
  • Common wrong answer 80.31 : That is the reading after 5 minutes. The mass lost is the start minus the end: 81.23 g − 80.31 g.
  • Common wrong answer 0.28 : That is the difference from still air, which (b) asks for. (a) is the water lost with the fan on: 81.23 g − 80.31 g.
  • Common wrong answer -0.92 : The balance went down, so the mass lost is a positive number: the start minus the end.
• (b) How much more than in still air: 0.28 g (within ±0.005 g)
  • Common wrong answer 1.56 : You added. “How much more” is the difference: 0.92 g − 0.64 g.
  • Common wrong answer 0.92 : That is the total with the fan on. Subtract the 0.64 g lost in still air.

A full answer includes

• (a) 81.23 g − 80.31 g = 0.92 g.
• (b) 0.92 g − 0.64 g = 0.28 g more.
• The moving air carries the escaped particles away, so fewer return to the towel.

Also accept

• (a) 0.92 g; (b) 0.28 g; the moving air carries the escaped particles away, so fewer return.

Watch for

• “The fan blows cold air, so the water evaporates faster.” — The fan’s air is at room temperature. It speeds up evaporation by carrying the escaped particles away, so fewer return.
• “Tube B fell further with the fan on, so that fall shows how much more evaporated.” — The fan cooled tube A too. Compare the water lost on the balance, not the fall in temperature.

3. EQ21 · Practice

Model answer. (1) warmer: at a higher temperature the particles move faster on average, so more of them have enough energy to escape. (2) bigger: particles can escape only from the surface, so a bigger surface has more particles that can escape. (3) away: moving air sweeps the escaped particles away from the surface. (4) return: so fewer of them wander back into the liquid, and more escape than come back.

A full answer includes

• (1) warmer (or hotter)
• (2) bigger (or larger)
• (3) away
• (4) return (or go back)

Also accept

• warmer; bigger; away; return.

Watch for

• “(1) colder” — In a colder liquid the particles move more slowly on average, so fewer have enough energy to escape: evaporation is slower.
• “(2) smaller” — Particles escape only from the surface. A smaller surface has fewer particles there, so evaporation is slower.

4. EQ22 · Practice

Options as numbered on the pupil sheet: 1 — Faster: the moving air carries the escaped particles away, so fewer return; 2 — Slower: fewer particles are at an open surface where they can escape; 3 — Slower: fewer particles have enough energy to escape; 4 — Faster: the moving air makes the particles more energetic; 5 — No change: the same water is on the towel.

Model answer. Fan on: faster. The moving air carries the escaped particles away from the surface, so fewer return to the liquid: 0.92 g evaporated, against 0.64 g in still air. Half covered: slower. Particles escape only from the surface, and the film keeps only half of it open, so fewer can escape: 0.39 g. From 50 °C: slower. The particles move more slowly on average, so fewer have enough energy to escape: 0.35 g. The fan does not make the particles more energetic: its air is at room temperature.

A full answer includes

• Fan: faster, because the moving air carries the escaped particles away, so fewer return (0.92 g against 0.64 g).
• Cling film: slower, because fewer particles are at an open surface where they can escape (0.39 g).
• From 50 °C: slower, because fewer particles have enough energy to escape (0.35 g).

Also accept

• fan – faster, the escaped particles are carried away; cling film – slower, a smaller open surface; 50 °C – slower, fewer particles with enough energy.

Watch for

• “The fan makes the particles more energetic.” — The fan’s air is at room temperature: it does not speed up the particles in the towel. It carries the escaped ones away, so fewer return.
• “Covering half the towel makes no difference, because the same water is on it.” — Only particles at an open surface can escape. With half of it covered, fewer are at the open surface, and only 0.39 g evaporated.

5. EQ23 · Practice

Model answer. Boiling happens only at the boiling point; evaporation happens at any temperature. At any temperature a few particles near the surface are fast enough to escape, which is why a puddle dries on a cool day. Boiling needs the boiling point, 100 °C for water at standard atmospheric pressure, because only there can bubbles of vapour form inside the liquid. Both turn a liquid into a gas. Evaporation happens only from the surface and boiling throughout the liquid, and it is boiling, not evaporation, that needs energy supplied to the liquid all the time to keep going.

A full answer includes

• I chose “Boiling happens only at the boiling point; evaporation happens at any temperature”.
• I explained that at any temperature a few particles near the surface are fast enough to escape, but bubbles of vapour can form inside the liquid only at the boiling point.

Also accept

• Boiling only at the boiling point; evaporation at any temperature.

Watch for

• “Evaporation happens throughout the liquid; boiling only at the surface.” — It is the other way round. Evaporation is the escape of particles from the surface; in boiling, bubbles of vapour form throughout the liquid.
• “Evaporation turns a gas into a liquid.” — That is condensation. Evaporation and boiling both turn a liquid into a gas.

6. EQ24 · Practice

Options as numbered on the pupil sheet: 1 — boiling; 2 — evaporation; 3 — both; 4 — neither.

Model answer. Evaporation happens at any temperature and only from the surface, because only particles near the surface, moving outwards fast enough, can escape. Boiling happens only at the boiling point, with bubbles of vapour forming throughout the liquid, and energy must be supplied to the liquid all the time to keep it boiling, from a hob, a flame or anything hotter; without that energy it stops. In both, a liquid turns into a gas. (Water also evaporates from its surface while it boils.)

A full answer includes

• Boiling: bubbles of vapour form throughout the liquid; only at the boiling point; needs energy supplied to the liquid all the time to keep going.
• Evaporation: at any temperature; only from the surface.
• Both: a liquid turns into a gas.

Also accept

• any temperature – evaporation; only from the surface – evaporation; bubbles throughout – boiling; only at the boiling point – boiling; a liquid turns into a gas – both; needs energy supplied all the time – boiling.

Watch for

• “Only boiling turns a liquid into a gas.” — Evaporation does too, more slowly and only from the surface, at any temperature. Both are a liquid turning into a gas.
• “Evaporation needs energy supplied all the time to keep going.” — A puddle evaporates with nothing heating it. The energy the escaping particles carry comes from the liquid itself, which is why it cools.

7. EQ25 · Practice

Model answer. (a) 81.58 g − 81.19 g = 0.39 g. The cling film stayed on the balance all the time, so it does not change the difference. (b) 81.23 g − 80.88 g = 0.35 g. Half covered: particles can escape only from the open surface, and half of it was covered, so fewer escaped each second. It lost a little more than half of 0.64 g, because with less evaporation the towel stayed warmer. From 50 °C: the water on the towel was cooler, so its particles moved more slowly on average, and fewer had enough energy to escape.

Checked automatically

• (a) Water evaporated, half covered: 0.39 g (within ±0.005 g)
  • Common wrong answer 81.19 : That is the reading after 5 minutes. The mass lost is the start minus the end: 81.58 g − 81.19 g.
  • Common wrong answer 0.04 : You started from 81.23 g, but the cling film added 0.35 g: this run started at 81.58 g.
  • Common wrong answer 0.32 : That is half of 0.64 g, a guess, not a measurement. Use the balance: 81.58 g − 81.19 g.
• (b) Water evaporated, from 50 °C: 0.35 g (within ±0.005 g)
  • Common wrong answer 80.88 : That is the reading after 5 minutes. Subtract it from the start: 81.23 g − 80.88 g.
  • Common wrong answer 0.29 : That is how much less than in still air (0.64 g − 0.35 g). The question asks for the water lost in this run.

A full answer includes

• (a) 81.58 g − 81.19 g = 0.39 g.
• (b) 81.23 g − 80.88 g = 0.35 g.
• Half covered: fewer particles at an open surface, so fewer escape. From 50 °C: fewer particles have enough energy to escape.

Also accept

• (a) 0.39 g; (b) 0.35 g; a smaller open surface, so fewer particles can escape; cooler, so fewer particles have enough energy.

Watch for

• “(a) 81.23 g − 81.19 g = 0.04 g.” — The cling film added 0.35 g, so the balance started at 81.58 g. Use the start of this run.
• “Covering half the towel must halve the water lost exactly.” — It lost 0.39 g, more than half of 0.64 g. With less water evaporating, the towel stayed warmer, and warmer water evaporates faster.

8. EQ26 · Practice

Model answer. The fan cools a warm tube even without any evaporation: tube A, in its dry towel, fell by 19.0 °C with the fan on instead of 11.0 °C. So part of tube B’s extra fall was caused in that way too, and the fall in temperature mixes the two effects. The balance measures evaporation itself: the water lost from towel B. With the fan on, 0.92 g evaporated, against 0.64 g in still air, in the same 5 minutes with the same 3.00 g of water on the towel. So compare the water lost on balance B.

A full answer includes

• The fan also cooled tube A, which had no water to evaporate, by 19.0 °C instead of 11.0 °C: moving air takes energy away from a warm tube in other ways too.
• So part of tube B’s bigger fall was not caused by evaporation.
• Compare the water lost on balance B: 0.92 g with the fan against 0.64 g in still air.

Also accept

• The fan cooled tube A too (19.0 °C against 11.0 °C), so the fall is not all due to evaporation; compare the water lost on balance B (0.92 g against 0.64 g).

Watch for

• “Tube A has no water, so the fan cannot affect it.” — Tube A fell by 19.0 °C with the fan on, against 11.0 °C in still air. Moving air takes energy away from a warm tube even without evaporation.
• “A bigger fall in temperature always means more evaporation.” — Other things change the fall too, such as air moving over a warm tube. Only the balance measures the water that evaporated.

9. EQ27 · Practice

Model answer. Spread out on a line outdoors, on a warm, windy day. It is the only towel with all three factors working for it. Warm: the water particles move faster on average, so more have enough energy to escape. Spread out: a bigger surface area, so more particles are at the surface. Windy: the moving air carries the escaped particles away, so fewer return. The folded towel in the wind has a much smaller surface area, and the towel spread out on a cold, still day has a lower temperature and no moving air.

A full answer includes

• I chose “Spread out on a line outdoors, on a warm, windy day”.
• I explained all three factors: warm, so more particles have enough energy to escape; spread out, so a bigger surface area; windy, so the escaped particles are carried away and fewer return.

Also accept

• The towel spread out on a warm, windy day: all three factors make evaporation faster.

Watch for

• “The folded towel in the wind dries fastest, because the wind is what matters.” — Wind helps, but folded, the towel has a much smaller surface area: fewer particles are at the surface to escape.
• “Any towel dries equally fast on a warm day.” — On the same warm, windy day, the folded towel has a smaller surface area, so it dries more slowly than the one spread out.

10. EQ28 · Practice

Model answer. The water is at 60 °C, below its boiling point of 100 °C, and no bubbles of vapour form inside it. Its more energetic particles are escaping from the surface only, at a temperature below the boiling point: that is evaporation. The mist forms where that water vapour meets the cooler air above the pan and condenses into tiny drops. One more difference: boiling needs energy supplied all the time and keeps the liquid at its boiling point, while an evaporating liquid cools unless energy reaches it; here the hob supplies some energy, so the water stays warm.

A full answer includes

• At 60 °C the water is below its boiling point (100 °C), and no bubbles of vapour form throughout it: particles are escaping only from its surface, which is evaporation.
• One more difference, for example: boiling needs energy supplied all the time and keeps the liquid at its boiling point, while an evaporating liquid cools unless energy reaches it.

Also accept

• Below 100 °C and no bubbles, so particles escape only from the surface: evaporation. Another difference: boiling needs energy supplied all the time and stays at the boiling point; an evaporating liquid cools.

Watch for

• “Any water with mist above it is boiling.” — Hot water gives off mist well below 100 °C: it evaporates quickly from its surface, and the vapour condenses in the cooler air. Boiling needs bubbles of vapour throughout, at the boiling point.
• “Evaporation and boiling are the same thing at different speeds.” — They differ in where and when: evaporation is from the surface, at any temperature; boiling is throughout the liquid, with bubbles, only at the boiling point.

11. ET3 · Apply

Model answer. (a) The soup is evaporating. It is at 80 °C, below its boiling point, while boiling happens only at the boiling point; particles escape only from its surface, while in boiling bubbles of vapour form throughout the liquid; and no energy is supplied to keep it going: the soup cools as it evaporates, while a boiling liquid stays at its boiling point only as long as energy is supplied. (b) Blowing moves the air over the surface, carrying the escaped particles away, so fewer of them return to the soup. More particles escape each second, and since they are the more energetic ones, the soup that stays in the bowl cools faster. (c) On a wide plate the soup has a much bigger surface area, so many more particles are at the surface, where they can escape. Evaporation is faster, so the soup cools faster. (d) Under the lid, the escaped particles cannot be carried away: the air above the soup fills with water vapour, and many particles return to the soup. Less water evaporates, so less energy is carried away, and the soup stays hot for longer. (e) As the soup cools, its particles move more slowly on average, so fewer of those near the surface have enough energy to escape each second. Evaporation is slower, so less water vapour rises into the air above it to form mist, and the evaporation cools the soup more slowly too.

A full answer includes

• (a) Two of: it happens below the boiling point (evaporation at any temperature, boiling only at the boiling point); only from the surface, not throughout with bubbles; no energy is supplied to keep it going, and the soup cools instead of staying at one temperature.
• (b) The moving air carries the escaped particles away, so fewer return; evaporation is faster, and since the escaping particles are the more energetic ones, the soup cools faster.
• (c) A bigger surface area: more particles are at the surface, so more escape each second, and the soup cools faster.
• (d) The escaped particles are trapped under the lid, so many return to the soup; less evaporates, so there is less cooling.
• (e) At a lower temperature the particles move more slowly on average, so fewer of them have enough energy to escape from the surface each second: evaporation is slower.

Also accept

• (a) below the boiling point, only from the surface, no energy supplied to keep it going; (b) moving air carries the escaped particles away, fewer return; (c) a bigger surface area; (d) the escaped particles are trapped and return, so less evaporates; (e) cooler: fewer particles have enough energy to escape.

Watch for

• “(b) Blowing cools the soup because your breath is cold.” — Breath is not cold: it is warmer than the room. It cools the soup by moving the air, which carries the escaped particles away so fewer return.
• “(d) The lid keeps the soup hot by keeping the cold out.” — There is no “cold” to keep out. The lid traps the escaped particles, so many return and less water evaporates; less evaporation means less cooling.
• “(e) It evaporates more slowly because there is less soup in the bowl.” — In a few minutes the amount of soup hardly changes. It evaporates more slowly because it is cooler: its particles move more slowly on average, so fewer have enough energy to escape.