Warm-up. Wet floor is left to dry up. Feeling cool under air-conditioner. Ironing wet clothes to dry them. Are these examples of evaporation?. . Warm-up. Put it inside a room with all windows closed. It is muggy and is going to rain. Strong sunlight shines on it.
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Wet floor is left to dry up.
Feeling cool under air-conditioner.
Ironing wet clothes to dry them.
Are these examples of evaporation?
Put it inside a room with allwindows closed.
It is muggyand is going torain.
Strong sunlight shines on it.
The plant is under a sea breeze.
Do you think the leaves of a plant getdry quickly in these cases?
A bit warmer than it would be.
A bit cooler than it would be.
How does the formation of snow or rain affect the temperature of the air?
Apply perfume/alcohol to your body...
What do you feel?
Perfume/alcohol evaporates easily and takes the energy from your body.
Sweat comes out of the pores, evaporates thus taking energy away from the skin. So we feel cool!
On a humidday, the air is full of water vapour.
This slows down the evaporation of sweat. Sweat tends to stay on the skin.
No cooling effect occurs.
So, we feel hot.
You can easily catch a cold if you are wet.
As you dry out, latent heat is taken from your body.
If it is windy, you cool down even more.
E2 Evaporation and particle motion
slow molecules left behind
average KE of the remaining molecules decreases
temperature of liquid
Points to consider:
1 Fast-moving particles in liquid escape and become vapour;
2 While slow-movingparticles in vapour stick back to the liquid.
3 The rate of evaporation (dryingup) is a balance between the rate of escape and the rate of return.
When surface area is increased…
larger the surface area, more molecules can escape at the same time.
Greater is the rateof evaporation.
If there is a breeze,
the particles that escape from thesurface of water get blown away.
fewer particles in the vapourreturn to the liquid.
a breeze increases the rate of evaporation.
some of the vapour has to condense.
A E = mlvapplies to boiling but not toevaporation.
B Boiling always occurs at a definitetemperature butevaporation does not.
C Boiling takes place at the surface, but evaporationoccurs within the liquid.
D Bubbles are formed violently in boiling, but slowly inevaporation.
Soup covered with oil takes ________ (longer/shorter) to cool down...
…because the oil layer __________(slows/enhances) the evaporation of water (soup)
True or false: When water vapour condenses, the surrounding air is warmed.
Increasing water temperature,
Increasing surface area of water,
Increasing air movement
A person (mass 60 kg) doing exerciseis covered with sweat.
(a) If 1 litre (1 kg) of sweat isevaporated in 1 hour...
…how muchenergy is required to evaporatethis amount of water?
Energy required to evaporate 1 kg of water
= 1 2.26 106
= 2.26 106 = 2.26 MJ
(b) If this amount of energy were notremoved from the body bysweating...
…by how much wouldthe body temperature of theperson ?
The averagespecific heat capacity of thehuman body is 3500 J kg-1 oC-1
Apply E = mcT,
T = E /mc = 2.26 106/ (60 3500)
T = 10.8 °C
If the energy were not removed from the body by sweating,...
... the body temperaturewould increase by 10.8 °C.
the electronic balance measures the mass of water boiled away
The experimental set-up is used to find the specific latent heatof vaporization of water.
(a) State a precaution in this experiment.
Switch on the heater only when it is immersed in water; otherwise it may bedamaged.
(b) A student suggests that a lid should be added to the polystyrene cup to reduce heat loss.
A lid should notbe added although it can reduce heat loss.
Comment on his suggestion.
Steam will condense on the lid and drip back into the cup.
error in the mass of water boiled away.
(c) The result obtained in the experiment is larger than the standard value. Explain.
The error may be caused by
1 Steam condensing on the top part of the heaterdripping back into the cup.
2 Energy loss to the surroundings.
(d) After the heater has been switched off for a period of time, thereading shown on the balance drops slightly. Why?
Some water in the polystyrene cup evaporates...
…and thus the balance reading drops.