A First Course in Physics · Ch. X, §§ 275–280 · Chapter 10: Change of State
31. Boiling and the heat of vaporization
Goal: Your student can use the heat of vaporization of water (about 536 calories per gram), define the boiling point by vapor pressure, and explain why water boils at a lower temperature on a mountain.
You'll need
- The book, open to these pages
- A pot, water, a stove and a thermometer that reads to 100° C (adult supervision)
- A clock
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Heat of vaporization; heat given out when steam condenses; measuring it by passing steam into cold water (about 536 calories per gram); the boiling point defined as the temperature at which vapor pressure equals the outside pressure; boiling points lower at high altitude and higher under pressure; evaporation compared with boiling. Fourteen problems follow.
The lesson, step by step
Hook question
SayA burn from steam is much worse than a burn from boiling water. Why, if both are at 100° C?
Read together
Read §§ 275–280. The exact definition of boiling:
The boiling point of a liquid is therefore defined as that temperature at which the pressure of the saturated vapor of the liquid is equal to the pressure acting upon the surface of the liquid.
Try it: heat a pot (with an adult)
Put a measured amount of water on the stove and record the temperature each minute until it boils, then for five more minutes. It climbs steadily, then stops near 100° C (a little lower if you live high up) no matter how hard it boils. Note how long it took to heat from cold to boiling, and how much longer it would take to boil away; the book says about five times as long (§ 275). Safety: an adult handles the pot and keeps faces away from the steam.
Explain the idea
Turning water to steam pulls the molecules far apart: about 536 calories per gram (in SI units, about 2260 joules per gram). Steam gives all that back when it condenses on your skin, which is why steam burns are so bad (question 6).
A bubble can only form when the vapor pressure inside it equals the air pressure outside. On a mountain the air pressure is lower, so water boils below 100° C (Quito about 90°, Mont Blanc about 84°). In a pressure cooker it boils hotter.
Watch only: boiling with cold water
Watch only: don't try this at home. The experiment in Fig. 199 stoppers a flask of boiling water and turns it over, and a sealed, cooling flask can crack or burst. Describe what happens: pouring cold water on the flask lowers the pressure inside and the water boils again without any flame. Look up a video of a teacher's "boiling water with ice" demonstration.
Say it back
Your student explains the difference between evaporation and boiling (§ 280).
Problems
Work a selection of problems 1–14 on book pages 206–207: try 2, 3, 4, 6, 8, 12, 13 and 14.
Afterwards
Look up the altitude of your town and work out the boiling point there using the book's rule of about 1° C per 960 ft.
Tip
The "about five times as long" in § 275 comes from 536 calories to boil a gram away compared with 100 to heat it from 0° to 100°.
Answers to the book's problems
1. The steam gives up its heat of vaporization as it condenses into water. 2. 10,720 calories (using 536). 3. 12,320 calories; it would warm 1232 g of water from 10° to 20° C. 4. About 15.5° C. 5. They are bubbles of dissolved air driven out as the water warms; boiling bubbles are steam and form throughout at the boiling point. 6. Each gram of steam gives up about 536 extra calories as it condenses on the skin. 7. The pressure on them grows less as they rise, and more water evaporates into them. 8. Each gram of snow needs 80 calories to melt, which the air supplies only slowly. 9. Condensing dew gives out heat. 10. No; the pressure of the vapor keeps rising with the temperature. 11. Steam bubbles form at the hot bottom and collapse in the cooler water above, making the noise. 12. It will be raised. 13. About 94.8° C. 14. About 2400 ft.
Worked out for this site from the scan. If a number in your copy differs, trust the book.