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

Differences between evaporation and boiling

Evaporation

Boiling

Occurs at any temperature

Occurs at a definite temperature the boiling point

Occurs at surface

Occurs within liquid

No bubbles formed

Bubbles appear

1Cooling effect of evaporationApply 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!1Cooling 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.1Cooling 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.1Cooling effect of evaporation

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

average KE of the remaining molecules decreases temperature of liquid cooling effectslow molecules left behind2Evaporation and particle motionE

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

3The rate of evaporation (drying up) is a balance between the rate of escape and the rate of return.2While slow-moving particles in vapour stick back to the liquid.3Factors affecting evaporation

The rate of evaporation increases with the temperature of the liquid Molecules move faster on average. Molecules have more KETemperature of water

larger the surface area, more molecules can escape at the same time. Greater is the rate of evaporation.When surface area is increasedSurface 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.Some returnHumidity 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

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

____________________________________________________________________________________________________________________________________________ Increasing water temperature,Increasing surface area of water,Decreasing humidity,Increasing air movementName any TWO factors that increase the rate of evaporation.

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