# Homework Help: Evaporation problem

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1. Jul 27, 2015

### Anshuman Bajpai

1. Water evaporates under atmospheric pressure. Without changing the temperature , the same water is placed under partial vacuum . The rate of evaporation will
a] increase
b] decrease
c] drop to zero
d] remain unaffected

2. No eqns reqd

3. The evaporation is a slow process that can occur at any temp. Other factors affecting evaporation are - Windspeed, Humidity, Surface Area of liquid .

Last edited by a moderator: Jul 27, 2015
2. Jul 27, 2015

### Orodruin

Staff Emeritus
Your attempt does not really try to answer the question. There is a single correct answer among the listed ones.

Last edited by a moderator: Jul 27, 2015
3. Jul 27, 2015

### William White

what happens to the temperature at which water boils as you move up a mountain and the air pressure decreases?

4. Jul 27, 2015

### Noctisdark

You can think of liquid water as bunch of H2O molecules stacked all together and aren't allowed to freely move around, when you heat them they gain some energy and dance around, this is when you see water boil and vaporate, but even at a room temperature some of the molecules make it through and flee thus turning into gaz(evaporate), pressure is one of the most important reason that these molecules were stacked in the first place because it presses them, in partial vaccum pressure goes down, so what might happen then ?

5. Jul 27, 2015

### haruspex

Not my area of expertise (if there is such), but I don't think it's as simple as that. Isn't it the partial pressure of the water vapour that's critical? If I understand correctly, the answer could be different if the air is completely dry. To put it another way, don't confuse evaporation with boiling.

6. Jul 27, 2015

### Staff: Mentor

Yes. I agree also. The equilibrium vapor pressure of water does not change significantly with air pressure in the system. The air in the gas phase above the water provides resistance to water diffusion away from the interface. If you lower the air partial pressure by lowering the total pressure, water in the gas phase can diffuse more easily away from the interface. This results in increased evaporation rate.

Chet

7. Jul 27, 2015

### Bystander

... or, remain unchanged if the volume of the system remains unchanged.
I'd call the problem statement "incomplete."

8. Jul 28, 2015

### Noctisdark

9. Jul 28, 2015

### haruspex

Again, that text does not discriminate clearly between the pressure of the gas and the partial pressure of the vapour. As Chet described, the mechanism is quite subtle.