Evaporation

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Evaporation

  1. 1. 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.
  2. 2. 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.
  3. 3. 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: _________
  4. 4. Introduction <ul><li>Liquid can change to vapour at temperatures below the boiling point. </li></ul><ul><li>A liquid changing into a vapour is said to be evaporating. The process is called evaporation . </li></ul>
  5. 5. <ul><li>Differences between evaporation and boiling </li></ul><ul><li>Evaporation and boiling require latent heat of vaporization . </li></ul>
  6. 6. 1 Cooling effect of evaporation <ul><li>Apply perfume/alcohol to your body... </li></ul>What do you feel? COOL ! Perfume/alcohol evaporates easily and takes the energy from your body.
  7. 7. 1 Cooling effect of evaporation Sweat comes out of the pores, evaporates thus taking energy away from the skin. So we feel cool!
  8. 8. 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. S weat tends to stay on the skin.
  9. 9. <ul><li>You can easily catch a cold if you are wet. </li></ul>1 Cooling effect of evaporation If it is windy, you cool down even more . As you dry out, latent heat is taken from your body.
  10. 10. 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.! Different particles have different velocity.
  11. 11. Simulation
  12. 12. 2 Evaporation and particle motion  average KE of the remaining molecules decreases  temperature of liquid   cooling effect slow molecules left behind E fast molecules (high KE) near the surface may escape
  13. 13. 3 Factors affecting evaporation 1 Fast-moving particles in liquid escape and become vapour ; Points to consider:
  14. 14. 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.
  15. 15. <ul><li>The rate of evaporation increases with the temperature of the liquid </li></ul>Temperature of water Molecules move faster on average. Molecules have more KE More molecules can escape.
  16. 16. Surface area of water larger the surface area , more molecules can escape at the same time. G reater is the rate of evaporation. When surface area is increased…
  17. 17. <ul><li>If the air is humid, it is full of water vapour. </li></ul>Humidity of air <ul><li>Water particles in vapour have greater chance to return to the liquid. </li></ul><ul><li>more humid the air , </li></ul>the lower the rate of evaporation. Some return evaporation
  18. 18. 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.
  19. 19. Condensation  the opposite of evaporation <ul><li>Warm air can hold more water . </li></ul><ul><li>If warm humid air suddenly cools, </li></ul>some of the vapour has to condense.
  20. 20. <ul><li>This is how clouds and mist are formed from millions of tiny water droplets. </li></ul>Condensation  the opposite of evaporation <ul><li>Latent heat of vaporization is released when water vapour condenses. </li></ul>
  21. 21. A E = ml v 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?
  22. 22. Soup covered with oil… … because the oil layer __________ (slows/enhances) the evaporation of water ( soup) longer slows Soup covered with oil takes ________ (longer/shorter) to cool down...
  23. 23. True or false: When... True or false: When water vapour condenses, the surrounding air is warmed. (T/F)
  24. 24. ____________________________________________________________________________________________________________________________________________ Increasing water temperature, Increasing surface area of water, Decreasing humidity, Increasing air movement Name any TWO factors that increase the rate of evaporation.
  25. 25. Example 6 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?
  26. 26. <ul><li>Energy required to evaporate 1 kg of water </li></ul>Example 6 = ml v = 2.26  10 6 = 2.26 MJ = 1  2.26  10 6
  27. 27. <ul><li>(b) If this amount of energy were not removed from the body by sweating ... </li></ul>Example 6 … by how much would the body temperature of the person  ? The average specific heat capacity of the human body is 3500 J kg -1 o C -1
  28. 28. Example 6 Apply E = mc  T ,  T = E / mc = 2.26  10 6 / (60  3500) If the energy were not removed from the body by sweating,... ... the body temperature would increase by 10.8 °C.  T = 10.8 °C
  29. 29. Example 7 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
  30. 30. <ul><li>Switch on the heater only when it is immersed in water; otherwise it may be damaged. </li></ul>Example 7 (a) State a precaution in this experiment.
  31. 31. <ul><li>A lid should not be added although it can reduce heat loss. </li></ul>Example 7 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 . Comment on his suggestion.
  32. 32. <ul><li>The error may be caused by </li></ul>Example 7 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.
  33. 33. 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?

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