SPH3U1 Lesson 10 Energy LATENT HEAT AND CHANGES OF STATE LEARNING GOALS Students will learn: It takes heat to change an object’s state. During a change of state, the temperature of a substance stays constant. HEATING / COOLING CURVE We learned that when you add heat to a substance, the temperature increases. When you remove heat, the temperature decreases. BUT, an interesting thing occurs when an object is undergoing a change of state. During a change of state, the temperature remains constant. The heat being added or removed is going into breaking or creating the bonds between the particles in the different states. If you measured the temperature of a solid substance to the point where it melts to a liquid, then continued heating the liquid until it boiled and turned entirely to a gas, you would get the following graph of temperature versus time: Consider the stages: 1. The substance is a solid. It increases temperature from the heat added. The heat to do this is given by 2. The substance has reached melting point. The temperature stops increasing as the heat is going into breaking the lattice bonds to change the solid to a liquid. The heat to do this is given by Lf is called the specific latent heat of fusion. It is the amount of heat needed to change 1 kg of the solid at its melting point to a liquid at the same temperature. 3. The substance is a liquid. It increases temperature from the heat added. The heat to do this is given by Note that the heat capacity of the liquid state is not always the same as the heat capacity of the solid state. Water is an example of this. SPH3U1 Lesson 10 Energy 4. The substance has reached boiling point. The temperature stops increasing as the heat is going into breaking the bonds between the liquid particles to change the liquid to a gas. The heat to do this is given by Lv is called the specific latent heat of vapourization. It is the amount of heat needed to change 1 kg of the liquid at its boiling point to a gas at the same temperature. 5. The substance is a gas. It increases temperature from the heat added. The heat to do this is given by Note that the heat capacity of the gas state is not always the same as the heat capacity of the solid or the liquid state. Water is an example of this. Latent heats of fusion and vapourization can be found in many textbooks. See table 1 on p 291 in your text book. EXAMPLE A 300 g block of ice at -25 0C is heated until it eventually becomes 300 g of water vapour at 110 0C. How much total heat does this take? Data you have to look up (pages 281 and 291) heat capacity of solid water = _______ J/kg 0C melting point of ice to water: 0 0C heat capacity of liquid water = ________J/kg 0C boiling point of water to steam: 100 0C heat capacity of steam = 200 J/kg 0C latent heat of fusion of water = ________________ J/kg latent heat of vapourization of water = _________________ J/kg Solution: This must be solved in 5 steps. Each step has its own calculation. 1. Heat the ice from -25 0 to 0 0C: = 2. Heat to melt the ice to water at 0 0C = 3. Heat liquid water from 0 0C to 100 0C. 4. Heat to boil the water to steam at 100 0C 4 1. 1.58 x 10 J 4 2. 9.9 x 10 J 5 0 0 3. 1.25 x 10 J 5. Heat water vapour from 100 C to 200 C. 5 4. 6.9 x 10 J 3 5. 6.0 x 10 J TOTAL: 9.4 x 105 J Total energy needed: SPH3U1 Lesson 10 Energy CLASS WORK 1. Read page 291 in your text and explain the difference between latent heat and specific latent heat. 2. In the pioneer days, settlers stored their vegetables in root cellars under their houses. These root cellars were not heated in the winter nor cooled in the summer. Settlers placed a large barrel of liquid water in the root cellar with their vegetables. Use the concept of latent heat to explain how this barrel of water kept the cellar warmer in the winter and cooler in the summer. 3. Do p295 #1-9 (for #9, read the text, page 293-294) .