By Moizul Hasan Assistant Professor Basics Of Evaporation By Moizul Hasan Assistant Professor

Warm-up  Are these examples of evaporation? Wet floor is left to dry up. Feeling cool under air-conditioner. Ironing wet clothes to dry them.

Warm-up Do you think the leaves of a plant get dry quickly in these cases? Put it inside a room with all windows closed. It is muggy and is going to rain.  Strong sunlight shines on it.  The plant is under a sea breeze.

Warm-up 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: _________

Introduction 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.

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? COOL ! 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 skin. No cooling effect occurs. 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 behind  average KE of the remaining molecules decreases  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 increases with the temperature of the liquid More molecules can escape. Molecules have more KE Molecules move faster on average.

When surface area is increased… Surface area of water When surface area is increased… 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 evaporation.

Movement 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.

Condensation  the opposite of evaporation Warm air can hold more water. If warm humid air suddenly cools, some of the vapour has to condense.

Condensation  the opposite of evaporation This is how clouds and mist are formed from millions of tiny water droplets. Latent heat of vaporization is released when water vapour condenses.

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

True or false: When... True or false: When water vapour condenses, the surrounding air is warmed. (T/F)

Name the factors that increase the rate of evaporation. ____________________________________________________________________________________________________________________________________________ Increasing water temperature, Decreasing humidity, Increasing surface area of water, Increasing air movement

Example 7 (a) State a precaution in this experiment. Switch on the heater only when it is immersed in water; otherwise it may be damaged.