Warm-up Wet floor is left to dry up. Feeling cool under air-conditioner. Ironing wet clothes to dry them. Are these examples of evaporation?
Warm-up Put it inside a room with allwindows closed. It is muggyand is going torain. Strong sunlight shines on it. The plant is under a sea breeze. Do you think the leaves of a plant getdry quickly in these cases?
Warm-up A bit warmer than it would be. A bit cooler than it would be. No change. Others: _________ How does the formation of snow or rain affect the temperature of the air?
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 humidday, 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.
Different particles have different velocity. 2 Evaporation and particle motion Evaporation is the escape of fast-moving particles from the surface of a liquid. Thus, some have higher K.E.!
fast molecules (high KE) near the surface may escape E 2 Evaporation and particle motion 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-movingparticles in vapour stick back to the liquid. 3 The rate of evaporation (dryingup) is a balance between the rate of escape and the rate of return.
More molecules can escape. Temperature of water The rate of evaporation increases with thetemperature of the liquid Molecules have more KE Molecules move faster on average.
Surface area of water When surface area is increased… larger the surface area, more molecules can escape at the same time. Greater is the rateof evaporation.
evaporation Humidity of air • If the air is humid, it is full of water vapour. • Waterparticles in vapour have greater chance to return to the liquid. • 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 thesurface of water get blown away. fewer particles in the vapourreturn 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 andmist are formed frommillions of tiny waterdroplets. • Latent heat of vaporizationis released when watervapour condenses.
Which is the difference between boiling and evaporation? A E = mlvapplies to boiling but not toevaporation. B Boiling always occurs at a definitetemperature butevaporation does not. C Boiling takes place at the surface, but evaporationoccurs within the liquid. D Bubbles are formed violently in boiling, but slowly inevaporation.
Soup covered with oil… longer 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 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 exerciseis covered with sweat. (a) If 1 litre (1 kg) of sweat isevaporated in 1 hour... …how muchenergy is required to evaporatethis amount of water?
Example 6 = mlv Energy required to evaporate 1 kg of water = 1 2.26 106 = 2.26 106 = 2.26 MJ
Example 6 (b) If this amount of energy were notremoved from the body bysweating... …by how much wouldthe body temperature of theperson ? The averagespecific heat capacity of thehuman body is 3500 J kg-1 oC-1
Example 6 Apply E = mcT, 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 temperaturewould 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 heatof 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 bedamaged.
Example 7 (b) A student suggests that a lid should be added to the polystyrene cup to reduce heat loss. A lid should notbe added although it can reduce heat loss. Comment on his suggestion. Steam will condense on the lid and drip back into the cup. 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 heaterdripping 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, thereading shown on the balance drops slightly. Why? Some water in the polystyrene cup evaporates... …and thus the balance reading drops.