A First Course in Physics · Ch. X, §§ 281–290 · Chapter 10: Change of State
32. Freezing mixtures, distillation, and refrigeration
Goal: Your student can explain why salt melts ice and makes a freezing mixture, how distillation purifies water, what a critical temperature is, and how an ice plant or refrigerator moves heat.
You'll need
- The book, open to these pages
- Ice, salt, two zip-top bags (one small, one large), milk or juice, sugar and vanilla (for ice cream in a bag)
- A thermometer
- Gloves or a towel (the bag gets very cold)
- A notebook and pencil
What's in the book
Cooling by dissolving salt; the freezing point of salt water; freezing mixtures of ice and salt; intense cold from evaporating carbon dioxide; distillation and fractional distillation; critical temperatures and a table of them; the liquid-air machine; ice plants and cold storage using ammonia. Ten questions follow.
The lesson, step by step
Hook question
SayWhy do people put salt on icy sidewalks, and how does that make ice cream freeze in an old-fashioned churn?
Read together
Read §§ 281–290. Why salt and ice get so cold:
The efficiency of a mixture of salt and ice in producing cold is therefore due simply to the fact that the freezing point of a salt solution is lower than that of pure water.
Try it: ice cream in a bag
Put a cup of milk, a tablespoon of sugar and a little vanilla in the small bag and seal it. Fill the large bag halfway with ice, add about a third as much salt (the book's three parts ice to one of salt, § 283), and put the small bag inside. Shake for 5–10 minutes wearing gloves. Put a thermometer in the ice-salt mix: it falls well below 0° C.
Explain the idea
Salt water freezes at a lower temperature than pure water. When ice sits in salt water it keeps melting, and each gram of melting ice takes 80 calories from its surroundings, so the mixture gets colder and colder (§ 283).
Distillation (§ 285): boil water, and the steam leaves the salt behind; cool the steam and you get pure water.
Refrigerators
Trace Fig. 203: a pump squeezes a gas until it turns to liquid (giving off heat outside), then lets it evaporate in the cold coils (taking heat in). Your kitchen refrigerator does exactly this; feel the warm coils at its back or bottom.
What we know now: home refrigerators no longer use ammonia; they use other refrigerant gases chosen to be safer and kinder to the ozone layer.
Watch only: dry ice and liquid gases
Watch only: don't try this at home. The carbon dioxide snow, ether, frozen mercury and liquid air of §§ 284–288 are dangerous: extreme cold burns skin, and gases can build up pressure. Look up a video of a teacher demonstrating liquid nitrogen instead.
Say it back
Your student explains why the ocean freezes less easily than a lake (question 10).
Questions
Work questions 1–10 on book page 215.
Afterwards
Leave a saucer of salt water in a sunny window and see the salt left behind as the water evaporates.
Answers to the book's problems
1. By distilling it. 2. The salt dissolves into the ice, lowering its melting point so it melts. 3. It makes the remaining solution more concentrated. 4. No; as ice freezes out, the rest becomes saltier and its freezing point falls, until it is saturated (about −22° C). 5. Air was above its critical temperature (−140° C), so no pressure could liquefy it. 6. It is boiling at its boiling point; to warm it, put it under pressure; to cool it, pump away the vapor above it. 7. Greater, since water's great specific heat and heats of melting and boiling even out the temperature. 8. The pressure in the boiler is higher than one atmosphere. 9. Both have vapor pressure at any temperature, so both evaporate together. 10. Salt water freezes at a lower temperature, and the sea is deep and kept stirred by currents.
Worked out for this site from the scan. If a number in your copy differs, trust the book.