physics

1. 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.

2. 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.

3. 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: _

4. • 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.
Introduction

5. • 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.

6. 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.

7. Sweat comes out of the pores, evaporates
thus taking energy away from the skin.
So we feel cool!
1 Cooling effect of evaporation

8. On a humid day, the air is full of water
vapor
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

9. 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.
1 Cooling effect of evaporation

10. 2 Evaporation and particle motion
Evaporation is the
escape of fast-
moving particles from
the surface of a
liquid.
Different particles have
different velocity.
Thus, some have higher K.E.!

11. 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
2 Evaporation and particle motion
E

12. Points to consider:
1 Fast-moving particles in liquid
escape and become vapour;
3 Factors affecting evaporation

13. 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.
3 Factors affecting evaporation

14. More molecules
can escape.
The rate of evaporation
increases with the
temperature of the liquid
Molecules have more
KE
Molecules move faster
on average.
Temperature of water

15. larger the surface area, more molecules
can escape at the same time.
Greater is the rate of evaporation.
When surface
area is
increased…
Surface area of water

16. • If the air is humid, it is full of water vapour.
• Water particles in
vapour have greater
chance to return to
the liquid.
• more humid the air,
the lower the rate of
evaporation.
evaporation
Some return
Humidity of air

17. 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.
Movement of air

18. Condensation  the opposite of
evaporation
• Warm air can hold more water.
• If warm humid air suddenly cools,
some of the vapour has to condense.

19. 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.

20. Which is the difference between boiling and
evaporation?
A. E = mlv applies to boiling but not to
evaporation.
B. Boiling always occurs at a definite
temperature but evaporation does not.
C. Boiling takes place at the surface, but
evaporation occurs within the liquid.
D. Bubbles are formed violently in boiling,
but slowly in evaporation.

21. Soup covered with oil takes
(longer/shorter) to cool down...
…because the oil
layer (slows/enhances) the
evaporation of water (soup)
Soup covered with oil…

22. True or false: When water vapour
condenses, the surrounding air is
warmed.
(T/F)
True or false: When...

23. _
Name any TWO factors that increase
the rate of evaporation.