Download 3.2 Evaporation - cloudfront.net

Warm-up
Are these examples of evaporation?
 Wet floor is left to dry up.
Feeling cool under air-conditioner.
Ironing wet clothes to dry them.
Warm-up
Do you think the leaves of a plant get
dry quickly in these cases?
Put it inside a room with all windows
closed.
It is muggy and is going to rain.


Strong sunlight shines on it.
The plant is under a sea breeze.
Warm-up
How does the formation of snow or rain
affect the temperature of the air?
A bit warmer than it would be.
 A bit cooler than it would be.
No change.
Others: _________
Introduction
• Liquid can change to vapour at
temperatures below the boiling point.
• A liquid changing into a vapour is said to
be evaporating. The process is called
evaporation.
• Differences between evaporation and
boiling
Evaporation
Boiling
Occurs at any
Occurs at a definite
temperature
temperature — the
boiling point
Occurs at surface Occurs within liquid
No bubbles formed
Bubbles appear
• Evaporation and boiling require latent
heat of vaporization.
1
Cooling effect of evaporation
Apply perfume/alcohol to
your body...
What do you feel?
Perfume/alcohol evaporates easily and takes the energy
from your body.
1
Cooling effect of evaporation
Sweat comes out of the pores, evaporates
thus taking energy away from the skin.
So we feel cool!
1
Cooling effect of evaporation
On a humid day, 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.
1
Cooling effect of evaporation
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.
2
Evaporation and particle motion
Evaporation is the
escape of fastmoving particles from
the surface of a liquid.
Different particles have
different velocity.
Thus, some have higher K.E.!
Simulation
E
2
Evaporation and particle motion
fast molecules (high KE) near the surface
may escape
slow molecules left
behind
 average KE of the
remaining molecules
decreases
 temperature of liquid 
 cooling effect
3
Factors affecting evaporation
Points to consider:
1
Fast-moving particles in liquid escape
and become vapour;
3
Factors affecting evaporation
2
While slow-moving particles in vapour
stick back to the liquid.
3
The rate of evaporation (drying up)
is a balance between the rate of
escape and the rate of return.
Temperature of water
The rate of evaporation
More molecules
increases with the
can escape.
temperature of the liquid
Molecules have more
KE
Molecules move faster
on average.
Surface area of water
When surface
area is
increased…
larger the surface area, more molecules
can escape at the same time.
Greater is the rate of evaporation.
Humidity of air
• If the air is humid, it is full of water vapour.
• Water particles in
vapour have greater
chance to return to
the liquid.
evaporation
• more humid the air,
the lower the rate of
evaporation.
Some return
Movement of air
If there is a breeze,
the particles that
escape from the surface
of water get blown
away.
fewer particles in the
vapour return to the
liquid.
a breeze increases the
rate of evaporation.
Condensation  the opposite of
evaporation
• Warm air can hold more water.
• If warm humid air suddenly cools,
some of the vapour has to condense.
Condensation  the opposite of
evaporation
•
This is how clouds and mist are formed
from millions of tiny water droplets.
• Latent heat of vaporization is released
when water vapour condenses.
Which is the difference between boiling and
evaporation?
A
B
C
D
E = mlv applies to boiling but not to
evaporation.
Boiling always occurs at a definite
temperature but evaporation does not.
Boiling takes place at the surface, but
evaporation occurs within the liquid.
Bubbles are formed violently in boiling,
but slowly in evaporation.
Soup covered with oil…
longer
Soup covered with oil takes ________
(longer/shorter) to cool down...
…because the oil
slows
layer __________
(slows/enhances) the
evaporation of water (soup)
True or false: When...
True or false: When water vapour condenses,
(T/F)
the surrounding air is warmed.
Name any TWO factors that increase the rate
of evaporation.
Increasing water temperature,
___________________________________
Decreasing humidity,
___________________________________
Increasing surface area of water,
___________________________________
___________________________________
Increasing air movement
Example 6
A person (mass 60 kg) doing exercise is
covered with sweat.
(a)
If 1 litre (1 kg) of sweat
is evaporated in 1 hour...
…how much energy is
required to evaporate
this amount of water?
Example 6
Energy required to evaporate 1 kg of water
= mlv
= 1  2.26  106
= 2.26  106 = 2.26 MJ
Example 6
(b)
If this amount of energy were not removed
from the body by sweating...
…by how much would the body
temperature of the person  ?
The average specific heat capacity of the human body is
3500 J kg-1 oC-1
Example 6
Apply E = mcT,
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 temperature would increase by
10.8 °C.
Example 7
the electronic
balance measures
the mass of water
boiled away
The experimental set-up is used to find the
specific latent heat of vaporization of water.
Example 7
(a)
State a precaution in this experiment.
Switch on the heater only when it is
immersed in water; otherwise it may be
damaged.
Example 7
(b)
A student suggests that a lid should be
added to the polystyrene cup
to reduce heat loss.
Comment on his suggestion.
A lid should not be added although it can
reduce heat loss.
Steam will condense on the lid and drip back into the
cup.

error in the mass of water boiled away.
Example 7
(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
heater dripping back into the cup.
2
Energy loss to the surroundings.
(d)
Example 7
After the heater has been switched off
for a period of time, the reading shown
on the balance drops slightly. Why?
Some water in the polystyrene cup
evaporates...
…and thus the balance reading drops.