Chapter 23 Changes of Phase

1. Chapter 23 Changes of Phase Conceptual Physics Chapter 23

2. Kinetic Theory • Matter exists in three common states or phases – solid, liquid and gas. • A fourth state – plasma – makes up over 90% of our universe. • All matter is made up of atoms or molecules that are in constant motion. Conceptual Physics Chapter 23

3. Kinetic Theory Conceptual Physics Chapter 23

4. Phase Changes • In many cases when energy is added to or taken from a substance, the matter will experience a temperature change. • If enough energy is added to or taken from a substance, it can undergo a change of state or phase change. Conceptual Physics Chapter 23

5. Evaporation Evaporation is a change of state from liquid to gas. Evaporation Conceptual Physics Chapter 23

6. Evaporation • Molecules in the liquid phase are in continuous motion and undergo collisions leading to energy transfers. • Some molecules at the surface of the liquid may gain enough energy from molecules that bump them from below such that the surface molecules are able to break free of the liquid. Conceptual Physics Chapter 23

7. Evaporation • These molecules undergo a phase change forming a gas. • The energy that is taken from the remaining liquid by those molecules undergoing a phase change causes the temperature of the remaining liquid to decrease. • Evaporation is a cooling process. Conceptual Physics Chapter 23

8. Evaporation • We can promote more rapid evaporation by: • Warming the liquid • Reducing atmospheric pressure • Providing dryer air above the liquid • Increasing the surface area of the liquid Conceptual Physics Chapter 23

9. Boiling • While evaporation only occurs at the surface of a liquid, boiling can take place throughout a liquid. • Typically, boiling will be prompted by an outside heat source. Conceptual Physics Chapter 23

10. Boiling • The vapor pressure inside the bubble must be great enough to withstand the water pressure and atmospheric pressure above it or the bubble will collapse. • At lower atmospheric pressures, less vapor pressure is required inside the bubbles and boiling occurs at lower temperatures. • If enough energy is transferred from the heat source to the molecules in the liquid, tiny bubbles will form beneath the surface. • At standard atmospheric pressure, water will boil when the temperature reaches 100°C. Conceptual Physics Chapter 23

11. Boiling • At this point, the water will be cooled by boiling at the same rate that it is being heated by energy from the heat source. • The water is in thermal equilibrium! Conceptual Physics Chapter 23

12. Boiling • The lid on a pressure cooker can prevent the escape of water vapor and causes the pressure on the surface of the heated liquid to increase, raising the boiling temperature. • Pressure cookers are used at higher altitudes where the atmospheric pressure is so low that boiling occurs before water can get hot enough to cook food items. Conceptual Physics Chapter 23

13. Condensation Condensation is a change of state from gas to liquid. Condensation Conceptual Physics Chapter 23

14. Condensation • Gaseous water vapor molecules in the air can collide with slow moving molecules of a cooler surface. • Some of the water vapor molecules may give up enough energy during the collision to undergo a phase change. • The gas molecules condense into a liquid (e.g. dew forms on the morning grass). Conceptual Physics Chapter 23

15. Condensation • The energy that is transferred to the cool surface by the molecules undergoing the phase change will cause the temperature of the cool surface to increase. • Condensation is a warming process. Conceptual Physics Chapter 23

16. Evaporation and Condensation • Evaporation and condensation are opposite processes, but generally occur simultaneously. • The effects of rapid evaporation of warm water droplets from your skin after a hot shower can be neutralized by drying off in the shower to take advantage of water vapor condensing on your skin. Conceptual Physics Chapter 23

17. Freezing Freezing is a change of state from liquid to solid. Freezing Conceptual Physics Chapter 23

18. Freezing • When enough energy is taken away from liquid molecules, the molecules will slow to the point where the attractive forces between them will cause the molecules to fuse. • The liquid molecules freeze into a solid. Conceptual Physics Chapter 23

19. Freezing • At normal atmospheric pressure, water at 0°C will turn into a solid when energy is removed from it. • The energy released by the liquid molecules that undergo the freezing process is absorbed by the surrounding environment. • Freezing is a warming process. Conceptual Physics Chapter 23

20. Freezing • The freezing process can be hindered if salt, sand or some other impurity is added to the liquid – this causes the freezing temperature to be lowered. Conceptual Physics Chapter 23

21. Regelation • Applying pressure to solid ice can cause the open-celled hexagonal structure to collapse, reducing the freezing temperature. • When the pressure is removed, refreezing occurs. • This phenomenon is called regelation. Conceptual Physics Chapter 23

22. Regelation • The wire will cut through the ice, but its track will refill with ice. • The wire and weights fall to the floor, leaving the ice in a single solid piece! Conceptual Physics Chapter 23

23. Changes of Phase • The change in the internal energy of a substance causes the change of phase. Conceptual Physics Chapter 23

24. Changes of Phase Suppose we have a 1-gram piece of ice at a temperature of -50°C in a closed container, and it is put on a stove to heat. A thermometer in the container reveals a slow increase in temperature up to 0°C. Conceptual Physics Chapter 23

25. Changes of Phase Once it reaches 0°C, the temperature of the ice remains at 0°C even though heat input continues. Rather than getting warmer, the ice melts. Not until all the ice melts does the temperature again begin to rise. The heat of fusion for water is 80 cal/g - for each gram of ice at 0°C that melts, 80 cal of thermal energy must be absorbed by the ice. Conceptual Physics Chapter 23

26. Changes of Phase Each additional calorie absorbed by the gram of water increases its temperature by 1°C until it reaches its boiling temperature, 100°C. Conceptual Physics Chapter 23

27. Changes of Phase The temperature remains constant while water is boiled away. Instead of the heat transfer causing a temperature increase, the water undergoes a phase change. The heat of vaporization of water is 540 cal/g. Conceptual Physics Chapter 23

28. Changes of Phase When all the water has become steam at 100°C, the temperature begins to rise once more. Conceptual Physics Chapter 23