Change of Phase

1. Change of Phase • Evaporation • Condensation • Fogs and Clouds • Boiling • Melting and Freezing • Energy and Change of Phase

2. Evaporation • Why do you feel chilly when getting out of a swimming pool?

3. Evaporation • Why do you feel chilly when getting out of a swimming pool? • Fast molecules escape the liquid, taking energy, and leaving the liquid cooler.

4. Evaporation • Cool coffee by blowing on it • Evaporation is a cooling process! • Wrap a wet cloth around a container to cool it off at a picnic.

5. Condensation • When in a shower and you turn off the water, why does is stay warm in the stall?

6. Condensation • When in a shower and you turn off the water, why does is stay warm in the stall? • The number of molecules in condensation is greater than the ones in evaporation. • Condensation is a warming process • Molecules that condense have a higher average kinetic energy

7. Condensation • Why does a cold drink get wet on the outside? • What causes Dew, fog, clouds?

8. Cloud formation • Convection upward and expansion • Cooling due to expansion • Condensation due to cooling • Cloud formation • Condensation is enhanced by particles of dust

9. Boiling • Boiling is a cooling process • Temperature is 100 degrees C no matter how much heat we give the water. • Water boils as fast as it is being warmed by heating. • The steam carries away the heat.

10. Boiling Water • At what temperature does water boil in the mountains?

11. Energy in Condensation/Vaporization • The Heat of Vaporization is 540 calories per gram of condensed water at 100 degrees.

12. Melting and Freezing

13. Energy and Phase Changes

14. Energy

15. Class Problem • What is the minimum amount of 100°C steam required to melt 1 gram of 0°C ice? 1) 0.125 g 2) 0.148 g 3) 6.75 g 4) 8.0 g 5) none of these

16. Class Problem • What is the minimum amount of 100°C steam required to melt 1 gram of 0°C ice? • 0.125 gram of 100°C steam will provide the 80 calories required to melt 1 gram of ice. The H2O in the form of steam will give up 540 calories per gram when it condenses to boiling water, and another 100 calories per gram when the water is cooled from 100°C to 0°C. So the steam will give up a total of 640 calories per gram to the ice. But the ice needs only 80 calories to melt. So only 80/640 gram (0.125 gram) of steam will do the job.

17. Class Problem • Suppose 4 grams of boiling water are spread over a large surface so 1 gram rapidly evaporates. If evaporation takes 540 calories from the remaining 3 grams of water, and no other heat transfer takes place, what will be the temperature of the remaining 3 grams?

18. Class Problem • The remaining 3 grams will turn to 0°C ice under conditions where all 540 calories are taken from the remaining water (like when the surroundings are below freezing and don't contribute energy). 540 calories from 3 grams means each gram gives up 180 calories. 100 calories from a gram of boiling water reduces its temperature to 0°C, and 80 more calories taken away turns it to ice. This is why hot water so quickly turns to ice in a freezing-cold environment.

19. Class Problems • 1. To say that evaporation is a cooling process is to say that the • A) the less energetic particles escape. • B) more energetic particles escape. • C) more energetic particles remain in the water. • 2. When a gas is changed to a liquid state, the gas • A) absorbs energy. • B) releases energy. • C) neither releases nor absorbs energy. • D) both releases and absorbs energy. • 3. A refrigerator • A) causes internal energy to disappear. • B) produces cold. • C) removes internal energy from inside the refrigerator. • D) changes heat into cold. • E) none of these

20. Class Problems • 4. On a humid day, water condenses on the outside of a glass of ice water. This phenomenon occurs mainly because of • A) capillary action. • B) adhesion of water molecules to glass. • C) evaporation. • D) the saturation of cooled air. • E) the porosity of glass. • 5. When a block of ice at zero degrees Celsius melts, the ice • A) absorbs energy from its environment. • B) releases energy and gets warmer. • C) absorbs energy and gets warmer. • D) releases energy to its environment. • E) absorbs energy but does not change its temperature.