Evaporation

Learning objectives

At the end of this presentation the students will be able to:

• Explain the phenomenon of evaporation

• State the factors influencing evaporation

• Explain why we should wear cotton clothes in summers

• 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.

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!

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.

Cooling effect of evaporation

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.

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

Thus, some have higher K.E.

Evaporation and particle motion

Different particles have different velocity.

slow molecules left behind

Fast moving molecules (high KE) near the surface may escape whereas

Evaporation and particle motion

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

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 increases with the temperature of the liquid

Molecules move faster on average.

More molecules can escape.

Molecules have more KE.

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

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.

• More humid the air, the lower the rate of evaporation.

evaporation

Some return

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

Why should we wear cotton clothes in summer?

During summer we perspire more Because of the mechanism of our body which keeps us cool. We know that during evaporation , the particles at the surface of the liquid gain energy from the surrounding or body surface and change into vapour. the heat energy from the body is absorbed leaving the body cool. Cotton being a good absorber of water helps in absorbing sweat exposing to the atmosphere for easy evaporation.