evaporation (PowerPoint)

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Are these examples of evaporation?

   Wet floor is left to dry up.
      Feeling cool under air-conditioner.
      Ironing wet clothes to dry them.
     Do you think the leaves of a plant get
     dry quickly in these cases?
      Put it inside a room with all windows
      It is muggy and is going to rain.

     Strong sunlight shines on it.
     The plant is under a sea breeze.
   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: _________

• 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
• Differences between evaporation and
    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
No cooling effect
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 fast-
moving particles from
the surface of a liquid.

       Different particles have
       different velocity.
Thus, some have higher K.E.!
    2     Evaporation and particle motion
    fast molecules (high KE) near the surface
    may escape
     slow molecules left
     average KE of the
      remaining molecules
     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
Molecules move faster
on average.
    Surface area of water
                            When surface
                                area is

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,                    Some return
  the lower the rate of
    Movement of air
If there is a breeze,
the particles that
escape from the surface
of water get blown
fewer particles in the
vapour return to the
a breeze increases the
rate of evaporation.
Condensation  the opposite of

• Warm air can hold more water.
• If warm humid air suddenly cools,
  some of the vapour has to condense.
Condensation  the opposite of
•   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
A    E = mlv applies to boiling but not to
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.
Soup covered with oil…
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...
True or false: When water vapour condenses,
the surrounding air is warmed.       (T/F)
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
      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

       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.
      Example 7
(d)   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
       …and thus the balance reading drops.

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