1 Chapter 2Energy and Matter 2.6 Changes of State Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

2 Melting and Freezing A substance is melting while it changes from a solid to a liquid. is freezing while it changes from a liquid to a solid. such as water has a freezing (melting) point of 0°C. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

3 Calculations Using Heat of Fusion The heat of fusion is the amount of heat released when 1 gram of liquid freezes (at its freezing point). is the amount of heat needed to melt 1 gram of a solid (at its melting point). for water (at 0°C) is 80. cal 1 g water

4 The heat needed to freeze (or melt) a specific mass of water (or ice) is calculated using the heat of fusion. Heat = g water x 80. cal 1 g water Example: How much heat in cal is needed to melt 15.0 g of water? 15.0 g water x 80. cal = 1200 cal 1 g water Calculation Using Heat of Fusion

5 A. How many calories are needed to melt 5.00 g of ice at 0°C? 1) 80. cal2) 400 cal 3) 0 cal B. How many calories are released when 25.0 g of water at 0°C freezes? 1) 80. cal2) 0 cal 3) 2000 cal Learning Check

6 A. How many calories are needed to melt 5.00 g of ice at 0°C? 2) 400 cal 5.00 g x 80. cal 1 g B. How many calories are released when 25.0 g of water at 0°C freezes? 3) 2000 cal 25 g x 80. cal 1 g Solution

7 Sublimation occurs when particles change directly from solid to a gas. is typical of dry ice, which sublimes at -78  C. takes place in frost-free refrigerators. is used to prepare freeze- dried foods for long-term storage.

8 Evaporation and Condensation Water evaporates when molecules on the surface gain sufficient energy to form a gas. condenses when gas molecules lose energy and form a liquid. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

9 Boiling At boiling, all the water molecules acquire enough energy to form a gas. bubbles appear throughout the liquid. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

10 Heat of Vaporization The heat of vaporization is the amount of heat absorbed to vaporize 1 g of a liquid to gas at the boiling point. released when 1 g of a gas condenses to liquid at the boiling point. Boiling Point of Water = 100°C Heat of Vaporization (water) = 540 cal 1 g water

11 Learning Check How many kilocalories (kcal) are released when 50.0 g of steam from a volcano condenses at 100°C? 1) 27 kcal 2) 540 kcal 3) 2700 kcal

12 Solution How many kilocalories (kcal) are released when 50.0 g of steam in a volcano condenses at 100°C? 1) 27 kcal 50.0 g steam x 540 cal x 1 kcal = 27 kcal 1 g steam 1000 cal

13 Summary of Changes of State

14 Heating Curve A heating curve illustrates the changes of state as a solid is heated. uses sloped lines to show an increase in temperature. uses plateaus (flat lines) to indicate a change of state. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

15 A. A flat line on a heating curve represents 1) a temperature change. 2) a constant temperature. 3) a change of state. B. A sloped line on a heating curve represents 1) a temperature change. 2) a constant temperature. 3) a change of state. Learning Check

16 A. A flat line on a heating curve represents 2) a constant temperature. 3) a change of state. B. A sloped line on a heating curve represents 1) a temperature change. Solution

17 Cooling Curve A cooling curve illustrates the changes of state as a gas is cooled. uses sloped lines to indicate a decrease in temperature. uses plateaus (flat lines) to indicate a change of state. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

18 Use the cooling curve for water to answer each. A. Water condenses at a temperature of 1) 0°C.2) 50°C.3) 100°C. B. At a temperature of 0°C, liquid water 1) freezes.2) melts.3) changes to a gas. C. At 40°C, water is a 1) solid. 2) liquid.3) gas. D. When water freezes, heat is 1) removed.2) added. Learning Check

19 Use the cooling curve for water to answer each. A. Water condenses at a temperature of 3) 100°C. B. At a temperature of 0°C, liquid water 1) freezes. C. At 40 °C, water is a 2) liquid. D. When water freezes, heat is 1) removed. Solution

20 To reduce a fever, an infant is packed in 250 g of ice. If the ice (at 0°C) melts and warms to body temperature (37.0°C), how many calories are removed from the body? A. 3.0 x 10 5 cal B. 2.0 x 10 4 cal C. 1.0 x 10 3 cal Heat Calculations

21 To reduce a fever, an infant is packed in 250 g of ice. If the ice (at 0°C) melts and warms to body temperature (37.0°C), how many calories are removed from the body? Step 1: Diagram the changes 37°C  T = 37.0°C - 0°C = 37.0°C temperature increase 0°C solid liquid melting Heat Calculations

22 Step 2: How is the ice changing Ice is melting from 0°C to 37.0°C, use the heat of fusion. Heat = g of ice x 80.0 cal 1 g ice Step 3: Plug value into equation Heat = 250 g of ice x 80.0 cal = 2.0x10 4 cal (B) or 2.0x10 1 kcal 1 g ice Heat Calculations