Evaporation

Warm-up

Wet floor is left to dry up.

Are these examples of evaporation?

Feeling cool under air-conditioner.

Ironing wet clothes to dry them.

Warm-up

Put it inside a room with all windows closed.

Do you think the leaves of a plant get dry quickly in these cases?

It is muggy and is going to rain.

Strong sunlight shines on it.

The plant is under a sea breeze.

Warm-up

A bit warmer than it would be.

How does the formation of snow or rain affect the temperature of the air?

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 evaporation.

Introduction

Evaporation BoilingOccurs at anytemperature

Occurs at a definitetemperature — the

boiling pointOccurs at surface Occurs within liquid

No bubbles formed Bubbles appear

• Differences between evaporation and boiling

• 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.

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

1 Cooling effect of evaporation

No cooling effect occurs.

So, we feel hot.

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.

1 Cooling effect of evaporation

You can easily catch a cold if you are wet.

If it is windy, you cool down even more.

As you dry out, latent heat is taken from your body.

1 Cooling effect of evaporation

Evaporation is the escape of fast-moving particles from the surface of a liquid.

Thus, some have higher K.E.!

2 Evaporation and particle motion

Different particles have different velocity.

Simulation

average KE of the remaining molecules decreases temperature of liquid

cooling effect

slow molecules left behind

fast molecules (high KE) near the surface may escape

2 Evaporation and particle motionEE

1 Fast-moving particles in liquid escape and become vapour;

Points to consider:

3 Factors affecting evaporation

3 The rate of evaporation (drying up) is a balance between the rate of escape and the rate of return.

2 While slow-moving particles in vapour stick back to the liquid.

3 Factors affecting evaporation

The rate of evaporation increases with the temperature of the liquid

Molecules move faster on average.

More molecules can escape.

Molecules have more KE

Temperature of water

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

• 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

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

Condensation the opposite of evaporation

• Warm air can hold more water.• If warm humid air suddenly

cools,some of the vapour has to condense.

• This is how clouds and mist are formed from millions of tiny water droplets.

• Latent heat of vaporization is released when water vapour condenses.

Condensation the opposite of 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.

Which is the difference between boiling and evaporation?

…because the oil layer __________ (slows/enhances) the evaporation of water (soup)

longer

slows

Soup covered with oil takes ________ (longer/shorter) to cool down...

Soup covered with oil…

True or false: When water vapour condenses, the surrounding air is warmed.

(T/F)

True or false: When...

____________________________________________________________________________________________________________________________________________

Increasing water temperature,

Increasing surface area of water,Decreasing humidity,

Increasing air movement

Name any TWO factors that increase the rate of evaporation.

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

= 2.26 106 = 2.26 MJ

= 1 2.26 106

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 = mcT,

T = E /mc = 2.26 106/ (60 3500)

If the energy were not removed from the body by sweating,...

... the body temperature would increase by 10.8 °C.

Example 6

T = 10.8 °C

The experimental set-up is used to find the specific latent heat of vaporization of water.

the electronic balance measures the mass of water boiled away

Example 7

Switch on the heater only when it is immersed in water; otherwise it may be damaged.

(a) State a precaution in this experiment.Example 7

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.

(b) A student suggests that a lid should be added to the polystyrene cup to reduce heat loss.

Example 7

Comment on his suggestion.

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.

(c) The result obtained in the experiment is larger than the standard value. Explain.

Example 7

Some water in the polystyrene cup evaporates...

…and thus the balance reading drops.

(d) After the heater has been switched off for a period of time, the reading shown on the balance drops slightly. Why?

Example 7