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

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

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

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 SOLID LIQUID GAS Cooling Concept maps

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 Melting point becomes . Easier to but more difficult to . More impurities A thread acts like a knife to cut an ice cube. Concept maps

Factors affecting values of boiling point 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 Boiling point becomes . More difficult to but easier to . More impurities Do you know why water takes longer time to boil on mountains? Concept maps

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

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

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

Microscopic interpretation of latent heat 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 . gains k.e. (and p.e.) gains k.e. (and p.e.) gains k.e. (no p.e.) gains p.e. gains p.e. liquid gas solid Gain total internal energy (k.e.+p.e.) Concept maps

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

Specific latent heat of fusion of water Experiments Specific latent heat of fusion of water Control set - up Experimental set - up Ice Joulemeter 12 V a.c. power supply 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

Precautions of experiment Reasons Concept maps

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

Specific latent heat of vaporization of water Experiments Specific latent heat of vaporization of water rotating disc kilowatt-hour meter Electronic balance 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

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

Possible sources of errors of experiment Effects on obtained value Concept maps

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

Example Energy is transferred at an equal rate to equal masses of substance X and Y. Their temperature-time graphs are shown below. Temperature / oC Time / min Substance X Substance Y 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

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

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

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. Boiling Evaporation Concept maps

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

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

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

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

Microscopic interpretation of evaporation 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

Factors affecting rate of evaporation How can wet clothes dry quickly? Extension Evaporation is faster for because Concept maps