30 terms

PHYS- Chapter 17

1. What are the four common phases of matter?
Solid, liquid, gas, and plasma.
2. Do the molecules in a liquid all have about the same speed, or do they have a wide variety of speeds?
Molecules in liquids have a wide variety of speeds.
3. What is evaporation, and why is it a cooling process? Exactly what is it that cools?
Evaporation is a change of phase from liquid to gas that occurs at the surface of a liquid. The average kinetic energy of the molecules remaining in the liquid is lowered, which results in cooling. The loss of energy is the only factor which cools liquid evaporation.
4. Why does warmer water evaporate more readily than cold water?
Warmer water evaporates more readily than cold water because there are a greater proportion of molecules with sufficient kinetic energy to escape the warmer water.
5. What is sublimation?
The form of evaporation in which molecules are covered directly from a solid to a gaseous phase.
6. Distinguish between condensation and evaporation.
In condensation the state changes from gas to liquid while evaporation the state changes from liquid to gas. Condensation is a warming process while evaporation is a cooling process.
7. Why is a steam burn more damaging than a burn from boiling water of the same temperature?
A scaling from steam is much more damaging than a scalding from boiling water of the same temperature because the steam releases considerable energy when it condenses to a liquid and wets the skin.
8. Why do you feel uncomfortably warm on a hot day and humid day?
On a hot day and humid day we feel uncomfortably warm because of the presence of moisture, the rate of evaporation is less than the rate of condensation, and we feel the warming effect as vapor in the air condenses on our skin.
9. Distinguish between humidity and relative humidity.
Humidity is a measure of actual amount of water vapor presenting in the air irrespective of air temperature. Relative humidity is the ratio of the amount of water vapor currently presenting in the air at a given temperature, compared to the largest amount of water vapor that the air can exist at that temperature.
10. Why does water vapor in the air condense when the air is chilled?
Saturation takes place when the air is cooled and the water vapor molecules in the air begin condensing. Water molecules tend to stick together.
11. Why does warm, moist air form clouds when it rises?
Warm air rises and expands which results in the cooling of the air and the condensing of water vapor to form clouds.
12. What is the basic difference between a cloud and fog?
The basic difference between fog and a cloud is altitude.
13. Distinguish between evaporation and boiling.
The basic difference between evaporation and boiling is that evaporation is a surface phenomenon whereas boiling results in a change of phase throughout a liquid.
14. Does increased atmospheric pressure increase or decrease the boiling point of water? Why is this so?
If atmospheric pressure is increase the molecules in the vapor must move faster to exert enough pressure to keep bubbles from collapsing resulting in an increase in the boiling point of water.
15. Is it the boiling of water or the higher temperature of water that cooks food faster in a pressure cooker?
The higher temperature of water under pressure cooks the food faster in a pressure cooker, not the boiling process of water.
16. Why will water a the bottom of a geyser not boil when it is at 100C?
When water is at the bottom of a geyser it will not boil because the relatively deep vertical column of water exerts pressure on the deeper water, thereby increasing the boiling point of this water
17. What happens to the water pressure at the bottom of a geyser when some of the water above gushes out?
When some of the water gushes out, water pressure at the bottom of the geyser reduces and this water boils rapidly.
18. The temperature of boiling water doesn't increase with continued energy input. Why is this evidence that boiling is a cooling process?
The temperature of the boiling water does not increase with continued energy input because cooling takes place at the same rate as boiling. Thus, the experiment is evidence that boiling is a cooling process.
19. When will water boil at a temperature of less than 100C?
When the pressure reduce, the boiling point of water decrease and water boils at a temperature less than 100 C.
20. What evidence can you cite for the claim that water can boil at a temperature of 0 C?
When water is placed in a vacuum jar and the pressure in the jar is slowly reduced, the water will start to boil. As the pressure is further reduced, more and more of the slower moving molecules boil, and this continues until the temperature is lowered to 0 C.
21. Why does increasing the temperature of a solid make it melt?
As the temperature of a solid increases, the molecules vibrate more and more violently. The attractive forces between the molecules will no longer be able to hold themselves together, which results in the melting of a sold.
22. Why does decreasing the temperature of a liquid make it freeze?
As the temperature of a liquid decreases, the attractive force between molecules is able to cause cohesion and the molecules vibrate about fixed positions and from a solid.
23. Why does freezing of water not occur at 0C when foreign ions are present?
When foreign ions are present in water, it will not freeze at 0 C because the ions take electrons from the hydrogen atoms in water and impede crystal formation.
24. What happens to the hexagonal open structure of ice when sufficient pressure is applied to it?
When sufficient pressure is applied to a hexagonal open structure of ice, the crystals are crushed into liquid phase.
25. Why does a wire not simply cut a block of ice in two when it passes through the ice?
When a wire passes through the ice, it will slowly cut its way through the ice. When the pressure from the wire is removed molecules crystalize and refreezing occurs, leaving the ice intact.
26. Does a liquid release energy ir absorb energy when it changes into a gas?
Liquid absorbs energy when it turns into gas.
27. Does liquid release energy or absorb energy when it changes into a solid?
Liquid gives off energy when it turns into a solid.
28. Is heat discharged at the back of a refrigerator and by heat pup given off by vaporization of the refregerating fluid or by condensation?
Heat discharged at the back of a refrigerator, or given off by a heat pump is from the cycle phase change of vaporization of refrigerating fluid and condensation.
29. How may calories are needed to change temperature of 1 g of water by 1 C? To melt 1 g of ice at 0C? to vaporize 1 g of boiling water at 100C?
To change temperature by 1 C 1 calorie is needed. To melt 1 g of ice at 0C 80 calories. To vaporize 1 g of boiling water at 100C 540 calories.
30. Cite two reasons why fire walkers don't burn their wetted feet when walking barefoot on red-hot coals.
Because of the low conductivity of wooden coal, and also the heat energy of the red-hot coals converts the moisture to a vapor, which then provides an insulating layer between the feet and the red-hot coals.