Book 1 Section 3.2 Evaporation Speedy cooling trick Cooling effect of evaporation Evaporation and particle motion Factors affecting evaporation Check-point 4 Condensation—the opposite of evaporation Check-point 5 1 2 3 4 Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation Speedy cooling trick How can we cool canned drinks fast without a refrigerator? Wrap each can in a cool wet towel and blow them with an electric fan for 10 min, the cans become chilled. Do you know how this method works? energy is taken away when water evaporates Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation Liquid can change to vapour at temp below the boiling point.  evaporation A substance changes its state from liquid to gas in both evaporation and boiling.  Latent heat of vaporization is required. Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation Differences between evaporation and boiling: Evaporation Boiling Occurs at any temp Occurs at boiling point Occurs on the surface Occurs throughout the liquid No bubbles formed Bubbles appear Takes energy from iteself or surroundings Takes energy from heat source Book 1 Section 3.2 Evaporation

1 Cooling effect of evaporation We feel cold when we apply perfume to our skin because… …perfume evaporates even at low temperatures. takes energy from our skin. Energy taken = latent heat of vaporization Book 1 Section 3.2 Evaporation

1 Cooling effect of evaporation We feel cold in a breeze when we are wet because… …the rate of evaporation is higher if it is windy. On a humid day, the rate of evaporation is low. There is hardly any cooling effect. Cooling effect of sweating Example 8 Book 1 Section 3.2 Evaporation

2 Evaporation and particle motion Evaporation occurs on a liquid’s surface. Different particles have different speeds. Neighbouring particles keep bumping into each other.  Some particles gain KE while others lose KE. Book 1 Section 3.2 Evaporation

2 Evaporation and particle motion Particles near the surface may gain enough KE. Liquid surface  Escape and become particles of vapour The remaining particles have a lower average KE.  The liquid has a lower temp. Simulation 3.6 Evaporation and particle motion Book 1 Section 3.2 Evaporation

3 Factors affecting evaporation Fast-moving particles in a liquid escape and become vapour. Slow-moving particles in vapour return to the liquid. Rate of evaporation = Rate of escape – Rate of return Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation 3 Factors affecting evaporation Factors affecting the rate of evaporation (for water): Temperature of water (temp , rate  ) Surface area of water (area , rate  ) Density of vapour (density  , rate  ) Movement of vapour (rate  with a breeze) Transpiration Example 9 Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation Check-point 4 – Q1 Soup covered with oil takes a ( longer / shorter ) time to cool down, because the oil layer ( slows / enhances ) the evaporation of the soup. Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation Check-point 4 – Q2 Which of the following is the difference between boiling and evaporation? A Q = mlv applies to boiling but not to evaporation. B Boiling always occurs at a definite temp but evaporation does not. C Boiling takes place on the surface, but evaporation occurs throughout the liquid. D Bubbles are formed violently in boiling, but slowly in evaporation. Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation Check-point 4 – Q3 Which of the following must be correct? A A liquid evaporates only when its temperature is higher than 0 C. B During evaporation, the particles at the liquid surface gain enough PE to escape into the space above. C Evaporation helps bring away the internal energy of the liquid. D A liquid does not evaporate on a windless day. Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation Check-point 4 – Q4 Name any TWO factors that increase the rate of evaporation. (Any TWO of the following)  water temperature  humidity  surface area of water  air movement Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation 4 Condensation—the opposite of evaporation Vapour condenses more readily at lower temperatures. If warm humid air suddenly cools, some of the vapour has to condense.  Latent heat of vaporization is released Dehumidifier Example 10 Book 1 Section 3.2 Evaporation

4 Condensation—the opposite of evaporation Errors due to evaporation and condensation Example 11 Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation Check-point 5 – Q1 True or False: When water vapour condenses, the surrounding air is warmed. ( T / F ) Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation Check-point 5 – Q2 In a tropical cyclone, water vapour condenses into water droplets. Latent heat is released. Heat released in 1 day = 5.2  1019 J. Mass of water that condenses in 1 day = ? Mass of water that condenses Q lv = = 5.2  1019 2.26  106 = 2.30  1013 kg Book 1 Section 3.2 Evaporation

Book 1 Section 3.2 Evaporation The End Book 1 Section 3.2 Evaporation