Evaporation

2009-03-15 7:54 pm

1.A 1-kW heater,immersed im water(0.5kg,20C),is switched on for 10min.Calculate the maximum amount of water boiled away.
Take c(water)=4200Jkg^-1C-1
If(ice)=3.34X10^5Jkg^-1
Iv(water)=2.26X10^6Jkg^-1

A.0.191kg
B.0.27kg
C.0.5kg
D.3kg

2.Why does evaporation cause a liquid to cool?
A.Since change of state takes place,PE of liquid particle decreases.
B.The total KE of the liquid particles decreases,as the amount of water decreases.
C.When faster particles escape,those slower particles are left behind.So the average KE of liquid particles decreases.
D.Condensation of the liquid occurs on the container and it cools down the liquid.

3.Vaporization of water always occurs at 100C.
Why does this statement isnt correct?

Pls explain as well..

回答 (1)

天同

2009-03-15 10:02 pm

✔ 最佳答案

1. Heat supplied by heater = 1000 x (10 x60) J = 600 000 J
Heat absorbed by water when its temperature rises from 20 C to 100 C
= 0.5 x 4200 x (100-20) J = 168 000 J
The amount of heat for evaporation of water = (600 000 - 168 000) J
= 432 000 J
Mass of water evaporated = 432 000/2260 000 kg = 0.191 kg
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2. Option C
The answer is self explanatory.
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3. It is wrong because evaporaztion of water can occur in any temperature. Evaporation is due to escape of higher energy water molecules at the surface of water. This can take place at any temperature.
At boiling point (100 C), almost all water molecules in the water can escape into free air.

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