Chapter 23 Changes of Phase. Conceptual Physics Chapter 23 1

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1 Chapter 23 Changes of Phase Conceptual Physics Chapter 23 1

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 2

3 Kinetic Theory Solid Liquid Gas Macroscopic Properties Low KE levels Retains its own shape Highly incompressible Higher KE levels Takes shape of its container Slightly compressible High KE levels Takes shape of its container Highly compressible Microscopic Properties Slow-moving molecules Very strong molecular bonds Small intermolecular spacing Faster moving molecules Weak intermolecular bonds Larger intermolecular spacing Very fast-moving molecules No intermolecular bonds Very large intermolecular spacing Conceptual Physics Chapter 23 3

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 4

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

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 6

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 7

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 8

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 9

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

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

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

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

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 14

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 15

16 Evaporation and Condensation Evaporation The effects and of rapid condensation are opposite evaporation processes, of warm but generally occur simultaneously. 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 16

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

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 18

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 19

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 20

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 21

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 22

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

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

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

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 26

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

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 28