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Conceptual Physics Mod #21 Textbook Answers
Chapter 23, Pages 463 – 464: #2-20 even2. What is evaporation, and why is it also a cooling process?
It is a process by which liquids change from state to vapor.The particles which are remaining in a liquid have lower average kinetic energy than before, so the cooling process of the liquid happens as evaporation happens. This is the reason why sweating cools people down.
4. What is condensation, and why is it also a warming process?
It refers to the process in which water vapor in the atmosphere is changed into liquid water. It is also a warming process since energy that has is being transferred to the cool surface by the molecules which are undergoing phase changes will make the temperature of the cool surface to rise
6. Which usually contains more water vapor-warm air or cool air?
Warm air will hold extra water vapor than cool air this is because as the air warms its molecules will be further apart which gives more room for more molecules. For a cool air the particle are much closer hence cannot accommodate more molecules.
8. How can you tell if the rate of evaporation equals the rate of condensations?
When the amount of liquid evaporating; is equal to the amount of vapor condensing no longer changes in this state. This is called equilibrium state.
. 10. What is the difference between evaporation and boiling?
Evaporation happens on the surface. It may occur when the partial pressure of vapor of the substance is less than the equilibrium vapor pressure while boiling point is the transition phase from liquid to gas and it happens above the boing point of the liquid
12.

Why is a pressure cooker even more useful when cooking food in the mountains than when cooking at sea level?
In the mountains, there is the less pressure hence lower boiling point. Hence when using a pressure cooker, it will be useful since less energy will be used to cook.
14. How can water be made to both boil and freeze at the same time?
The equilibrium vapor pressure is a function of temperature. When the pressure in the container is less than the equilibrium vapor pressure, the water will always boil. At zero degrees the equilibrium vapor pressure is 610Pa. On freezing, point pressure has a weak effect thus if water is cooled below zero degrees and raised the pressure to 610Pa then water can boil as it freezes.
16. a. How many calories are needed to raise the temperature of 1 gram of water by 1degree C?
One calorie will increase 1 gram of water one degree Celsius
b. How many calories are needed to melt 1 gram of ice at 0 degree C?
: It will take 80 calories to melt
c. How many calories are needed to vaporize 1 gram of boiling water at 100 degrees C?
Water boils at 100 degrees Celsius, so 540 calories will be needed to vaporize 1 gram at 100 degrees
18. What is the effect of rapid evaporation on the temperature of the water?
Rapid evaporation reduces the temperature of water
20. Why is it important that a finger be wet before it is touched to a hot clothes iron?
To prevent damage to the skin since some of the heat which would have been transferred to the skin will be absorbed by water

References
Collier, J. G., & Thome, J. R. (1994). Convective boiling and condensation. Clarendon Press.