1.3 Change of State

1. 1.3 Change of State 1.3.1 Change of State 1.3.2 Latent Heat and Specific Latent Heat 1.3.3 Evaporation

2. Prior Knowledge Linkage Heat flow Transfer processes Internal energy Internal kinetic energy Internal potential energy Molecular motion Molecular separation Temperature State of matters

3. 1.3.1 Change of State Change of state Fusion Solidification Vaporization Condensation Boiling Evaporation Melting point Boiling point Any temperature

4. SOLID LIQUID GAS Changes of states Matter around us mainly exists in three states (物態): solid (固體), liquid (液體) and gas (氣體). Heating Cooling Concept maps

5. A thread acts like a knife to cut an ice cube. The melting point and freezing point A substance changes from solid to liquid at a temperature called the ; the same substance changes from liquid to solid at the constant temperature called the . Factors affecting values of melting point Higher pressure Easier to but more difficult to . Melting point becomes . More impurities Concept maps

6. The boiling point A substance may change from liquid to gas in the process of at a temperature called the ; the same substance may also change from gas to liquid at the constant temperature. Factors affecting values of boiling point Higher pressure More difficult to but easier to . Boiling point becomes . More impurities Do you know why water takes longer time to boil on mountains? Concept maps

7. 1.3.2 Latent Heat and Specific Latent Heat Fusion Solidification Vaporization Condensation Latent heat of fusion Latent heat of vaporization Specific latent heat of fusion Specific latent heat of vaporization

8. Hot Water thermometer Liquid octadecan-1-ol Liquid octadecan-1-ol Temperature falls at a rate here because energy is at a rate here (slope is ). Temperature remains constant here. Is energy releasing? Cooling curve Typical cooling curve Temperature / oC Time / min Experiments Cooling curve Concept maps

9. Latent heat Latent heat (潛熱) is the energy absorbed or released by a substance during its changes of state a change of temperature. Unit of L: Latent heat of fusion is the energy absorbed by a substance during its changes from to state a change of temperature. Latent heat of vaporization is the energy absorbed by a substance during its changes from to state (except in ) a change of temperature. Concept maps

10. gains k.e. (and p.e.) gains k.e. (and p.e.) gains k.e. (no p.e.) gains p.e. gains p.e. Extension Microscopic interpretation of latent heat The internal potential energy of an object arises from the forces between molecules. During change of state upon heating, molecules gain more to overcome the attractive force, and have more freedom to move around. The internal potential energy while the internal kinetic energy , so that the temperature also . gas liquid solid Gain total internal energy (k.e.+p.e.) Concept maps

11. Specific latent heat The specific latent heat (比潛熱) of a substance is the amount of energy required to change the state of of the substance. or l : Specific heat capacity E : Energy required m: mass Unit of l : The specific latent heat of fusion of a substance is the amount of energy required to change of the substance form solid to liquid. The specific latent heat of vaporization of a substance is the amount of energy required to change of the substance form liquid to gas. Concept maps

12. Joulemeter Ice Ice 12 V a.c. power supply Experimental set - up Control set - up Specific latent heat of fusion of water Specific latent heat of fusion of water Experiments Mass of water in beaker of control set-up = 0.012 kg Mass of water in beaker of experiment set-up = 0.048 kg Initial joulemeter reading = 24 500 J Final joulemeter reading = 38 800 J Standard value = 3.34  105 J kg-1 Concept maps

13. Precautions of experiment Concept maps

14. Ice cubes taken out from refrigerators are usually sticky to wet hands. Do you know why? Possible sources of errors of experiment Concept maps

15. kilowatt-hour meter rotating disc Electronic balance Specific latent heat of vaporization of water Specific latent heat of vaporization of water Experiments Why is a kW h meter needed in this experiment? kW h meter calibration = 1 2 00 turns / kW h Number of rotations counted = 40 Mass of water boiled away = 0.05 kg Standard value = 2.26  106 J kg-1 Concept maps

16. Precautions of experiment Should a lid be used in the experiment to reduce heat loss to the surroundings? Why? Concept maps

17. Possible sources of errors of experiment Concept maps

18. Example How much energy is required to change 0.2 kg of ice at 0 oC to steam at 100 oC. Concept maps

19. Temperature / oC Substance X Substance Y Time / min Example Energy is transferred at an equal rate to equal masses of substance X and Y. Their temperature-time graphs are shown below. • Which substance has a higher • (i) melting point? (ii) boiling point? • specific latent heat of (i) fusion (ii) vaporization? • specific heat capacity in solid states? Concept maps

20. Example What would happen when 1 kg of ice meets 1 kg of steam? Concept maps

21. 1.3.3 Evaporation Vaporization Evaporation Boiling Temperature liquid Bubbles Latent heat of vaporization

22. Occurrence of evaporation below boiling point Do you notice that wet plants get dry eventually after raining? Evaporation (蒸發) is the process in which a liquid change into gas (vapour) the boiling point. Concept maps

23. Cooling effect of evaporation When you leave water after swimming or bathing, you feel cold. Do you know why? Concept maps

24. Condensing coilsrefrigerator Evaporating coilsrefrigerator Energy removed from foods as freon . Energy given out to surrounging as freon . Electric pump to pump the around the circuit. Cooling by evaporation in refrigerators Refrigerators Concept maps

25. Condensation – the opposite of evaporation Condensation is the process in which substance changes from to state when is . When a container of ice-cream is removed from the freezer, a mist forms around it. Do you know why? The temperature of the ice-cream is much than room temperature. When the ice-cream or ice on it melts, large amount of is absorbed from the surrounding air. The temperature of the air quickly and so water vapour in the air quickly to form a mist. Concept maps

26. Warming effect of condensation Evaporation on the surface of a body can the body down. On the other hand, condensation on the surface of a body can the body up. The use of steam in cooking can make food being cooked more because the steam a large amount of energy to the food during condensation. • Other examples: • Preparing coffee by steam • Steam bath Concept maps

27. Extension Microscopic interpretation of evaporation Molecules in a liquid move at speeds and collide with each other. Molecules at the surface can escape from the liquid if they gain enough energy from these collisions. The escaped molecules become molecules of vapour. Molecules gain enough energy to escape Energy taken away from body As molecules carrying energy have escaped, the average kinetic energy of the liquid . The temperature of the liquid therefore . Concept maps

28. Factors affecting rate of evaporation How can wet clothes dry quickly? Extension Concept maps