A First Course in Physics · Ch. V, §§ 121–132 · Chapter 5: Molecular Motions
14. Evaporation and saturated vapor
Goal: Your student can explain evaporation and saturation with moving molecules, and explain why warm water evaporates faster and air only slows evaporation down.
You'll need
- The book, open to these pages
- Two small identical saucers and water
- A clear jar with a lid
- A paper towel
- A notebook and pencil
What's in the book
Evaporation as fast molecules escaping; diffusion in liquids; liquids expanding when heated; evaporation faster when warm; saturated vapor and why it stops increasing; vapor pressures of ether, alcohol and water measured in mercury tubes; the table of saturated water vapor; and why air slows evaporation without stopping it. Five problems follow.
The lesson, step by step
Hook question
SayA puddle disappears in the sun. Where did the water go? And why doesn't the water in a closed jar ever dry up?
Read together
Read §§ 121–132. The book's picture of evaporation:
This molecule may then, because of the unusual speed of its motion, break away from the attraction of its neighbors and fly off into the space above.
Try it: open and closed
Put a spoonful of water in an open saucer and the same in a closed jar. Check them tomorrow and the next day. The saucer dries; the jar keeps its water and its walls fog up. The air in the jar is saturated (§ 125).
Explain saturation
Molecules leave the water all the time. In a closed space, some fly back in. When the space holds enough vapor that as many come back as leave, the vapor is saturated (§ 126). Warmer water sends out more molecules, so warm air can hold more water vapor (§ 130, and the table on page 95).
Watch only: the vapor tubes
Watch only: don't try this at home. The experiments in §§ 128–131 put drops of ether into tubes of mercury, and both are hazardous. Describe what happened: the ether vapor pushed the mercury down much farther than alcohol did, and alcohol farther than water. The liquid that evaporates most easily has the highest vapor pressure.
Say it back
Your student explains why clothes dry faster on a windy day (question 3).
Problems
Work problems 1–5 on book page 97, using the table of saturated water vapor on page 95 (pressure P in millimeters of mercury, density D in grams per cc).
Afterwards
Hang two damp washcloths, one in the sun and one in shade. Which dries first?
Answers to the book's problems
1. No, it will read too low, because water vapor above the mercury pushes it down. 2. At 15° C the vapor pressure is 12.7 mm, so the reading is 12.7 mm (1.27 cm) too low. 3. The wind carries away the moist air next to the clothes so more water can evaporate. 4. 76 cm + 1.53 cm = 77.53 cm (vapor pressure at 18° C is 15.3 mm). 5. About 24,800 g (1,440,000,000 cc × 0.0000172 g per cc).
Worked out for this site from the scan. If a number in your copy differs, trust the book.