1. Book 1 Section 3.2 Evaporation
Speedy cooling trick
Cooling effect of evaporation
Evaporation and particle motion
Factors affecting evaporation
Check-point 4
Condensation—the opposite of evaporation
Check-point 5 1 2 3 4
Book 1 Section 3.2 Evaporation

2. Book 1 Section 3.2 Evaporation
Speedy cooling trick
How can we cool canned drinks fast without a refrigerator?
Wrap each can in a cool wet towel and blow them with an electric fan for 10 min, the cans become chilled.
Do you know how this method works?
energy is taken away when water evaporates
Book 1 Section 3.2 Evaporation

3. Book 1 Section 3.2 Evaporation
Liquid can change to vapour at temp below the boiling point.
 evaporation
A substance changes its state from liquid to gas in both evaporation and boiling.
 Latent heat of vaporization is required.
Book 1 Section 3.2 Evaporation

4. Book 1 Section 3.2 Evaporation
Differences between evaporation and boiling:
Evaporation
Boiling
Occurs at any temp
Occurs at boiling point
Occurs on the surface
Occurs throughout the liquid
No bubbles formed
Bubbles appear
Takes energy from iteself or surroundings
Takes energy from heat source
Book 1 Section 3.2 Evaporation

5. 1 Cooling effect of evaporation
We feel cold when we apply perfume to our skin because…
…perfume evaporates even at low temperatures.
takes energy from our skin.
Energy taken = latent heat of vaporization
Book 1 Section 3.2 Evaporation

6. 1 Cooling effect of evaporation
We feel cold in a breeze when we are wet because…
…the rate of evaporation is higher if it is windy.
On a humid day, the rate of evaporation is low.
There is hardly any cooling effect.
Cooling effect of sweating
Example 8
Book 1 Section 3.2 Evaporation

7. 2 Evaporation and particle motion
Evaporation occurs on a liquid’s surface.
Different particles have different speeds.
Neighbouring particles keep bumping into each other.
 Some particles gain KE while others lose KE.
Book 1 Section 3.2 Evaporation

8. 2 Evaporation and particle motion
Particles near the surface may gain enough KE.
Liquid surface
 Escape and become particles of vapour
The remaining particles have a lower average KE.
 The liquid has a lower temp.
Simulation
3.6 Evaporation and particle motion
Book 1 Section 3.2 Evaporation

9. 3 Factors affecting evaporation
Fast-moving particles in a liquid escape and become vapour.
Slow-moving particles in vapour return to the liquid.
Rate of evaporation = Rate of escape – Rate of return
Book 1 Section 3.2 Evaporation

10. Book 1 Section 3.2 Evaporation
3 Factors affecting evaporation
Factors affecting the rate of evaporation (for water):
Temperature of water (temp , rate  )
Surface area of water (area , rate  )
Density of vapour (density  , rate  )
Movement of vapour (rate  with a breeze)
Transpiration
Example 9
Book 1 Section 3.2 Evaporation

11. Book 1 Section 3.2 Evaporation
Check-point 4 – Q1
Soup covered with oil takes a ( longer / shorter ) time to cool down, because the oil layer ( slows / enhances ) the evaporation of the soup.
Book 1 Section 3.2 Evaporation

12. Book 1 Section 3.2 Evaporation
Check-point 4 – Q2
Which of the following is the difference between boiling and evaporation?
A Q = mlv applies to boiling but not to evaporation.
B Boiling always occurs at a definite temp but evaporation does not.
C Boiling takes place on the surface, but evaporation occurs throughout the liquid.
D Bubbles are formed violently in boiling, but slowly in evaporation.
Book 1 Section 3.2 Evaporation

13. Book 1 Section 3.2 Evaporation
Check-point 4 – Q3
Which of the following must be correct?
A A liquid evaporates only when its temperature is higher than 0 C.
B During evaporation, the particles at the liquid surface gain enough PE to escape into the space above.
C Evaporation helps bring away the internal energy of the liquid.
D A liquid does not evaporate on a windless day.
Book 1 Section 3.2 Evaporation

14. Book 1 Section 3.2 Evaporation
Check-point 4 – Q4
Name any TWO factors that increase the rate of evaporation.
(Any TWO of the following)
 water temperature
 humidity
 surface area of water
 air movement
Book 1 Section 3.2 Evaporation

15. Book 1 Section 3.2 Evaporation
4 Condensation—the opposite of evaporation
Vapour condenses more readily at lower temperatures.
If warm humid air suddenly cools, some of the vapour has to condense.
 Latent heat of vaporization is released
Dehumidifier
Example 10
Book 1 Section 3.2 Evaporation

16. 4 Condensation—the opposite of evaporation
Errors due to evaporation and condensation
Example 11
Book 1 Section 3.2 Evaporation

17. Book 1 Section 3.2 Evaporation
Check-point 5 – Q1
True or False:
When water vapour condenses, the surrounding air is warmed.
( T / F )
Book 1 Section 3.2 Evaporation

18. Book 1 Section 3.2 Evaporation
Check-point 5 – Q2
In a tropical cyclone, water vapour condenses into water droplets. Latent heat is released. Heat released in 1 day = 5.2  1019 J.
Mass of water that condenses in 1 day = ?
Mass of water that condenses Q lv = =
5.2  1019
2.26  106
= 2.30  1013 kg
Book 1 Section 3.2 Evaporation

19. Book 1 Section 3.2 Evaporation
The End
Book 1 Section 3.2 Evaporation