Change of Phase

1. Change of Phase

2. Phases of Matter Matter exists in four common phases: • Solid phase (ice) • Liquid phase (ice melts to water) • Gaseous phase (water turns to vapor; addition of more energy vaporizes water to vapor) • Plasma phase (vapor disintegrates to ions and electrons)

3. Phases of Matter The phase depends upon: temperature and pressure. Change from Solid  Liquid  Gas  Plasma requires energy to be added to the material. Energy causes the molecules to move more rapidly.

4. Evaporation Evaporation • Change of phase from liquid to gas

5. Evaporation • EVAPORATION IS A COOLING PROCESS! • Molecules are randomly moving • Some gain KE, others lose KE • Some can break free of the liquid—become gas • Average KE drops, the liquid becomes cooler

6. OMG Like Totally Ewww…Sweat. Bodies overheat, produce perspiration (sweat) The water evaporates and cools us Keeps body temperature stable

7. Animals don’t have many sweat glands

8. Evaporation Sublimation • Form of phase change directly from solid to gas Example: dry ice (solid carbon dioxide molecules), frozen water

9. When evaporation occurs in a dish of water, the molecules left behind in the water • are less energetic. • have decreased average speeds. • result in lowered temperature. • All of these.

10. When evaporation occurs in a dish of water, the molecules left behind in the water • are less energetic. • have decreased average speeds. • result in lowered temperature. • All of these.

11. When matter changes phase directly from solid to gas, we call the process • evaporation. • condensation. • sublimation. • regelation.

12. When matter changes phase directly from solid to gas, we call the process • evaporation. • condensation. • sublimation. • regelation.

13. Condensation Condensation process • Opposite of evaporation • Warming process from a gas to a liquid • Gas molecules near a liquid surface are attracted to the liquid • They strike the surface with increased KE, becoming part of the liquid • Overall KE of liquid increases, so it gets warmer!

14. Examples of Condensation • Steam releases much energy when it condenses to a liquid and moistens the skin—hence, it produces a more damaging burn than from same-temperature 100C boiling water. • A cold soda can is wet in warm air because slow-moving molecules make contact with the cold surface and condense. • The Shower Dilemma: You’re cold when you get out of the shower because the water is evaporating, but if you stay in the shower stall, it feels warmer. Why? • The water vapor in the stall is condensing with the water molecules causing it to be warmer!

15. Condensation in the Atmosphere Humidity- amount of water vapor in the air Relative Humidity- ratio of the amount of water vapor in the air at a given temp. to the largest amount of water vapor the air can contain at that temp. (Weather reports)

16. Condensation in the Atmosphere • Temperature drops: water vapor condenses • Slow moving molecules combine: create droplets of water

17. Clouds and Fog • Warm air rises, it expands. • As it expands, it cools. • As it cools, water vapor molecules begin sticking together • They combine with other particles in the air to form clouds! • Fog is a cloud closer to the ground based on temp. difference between water and the ground

18. The dew that forms on a cold soda can when taken from a refrigerator is due to • evaporation. • condensation. • sublimation. • regelation.

19. The dew that forms on a cold soda can when taken from a refrigerator is due to • evaporation. • condensation. • sublimation. • regelation.

20. When you step out after a hot shower you feel cold, but you can feel warm again if you step back into the shower area. Which process is responsible for this? Evaporation Condensation Both of these. None of the above. Condensation CHECK YOUR NEIGHBOR

21. When you step out after a hot shower you feel cold, but you can feel warm again if you step back into the shower area. Which process is responsible for this? Evaporation Condensation Both of these. None of the above. Condensation CHECK YOUR ANSWER Explanation: When you step back into the shower area, the steam that is present condenses on your body, causing it to warm up.

22. Evaporation and Condensation

23. Boiling Evaporation beneath the surface causes bubbles (turns to vapor in the liquid!) Bubbles rise to the surface Only occurs when the pressure of the vapor can resist the pressure of the liquid and atmosphere above it.

24. Boiling For water, that point is 100 degrees C at regular atmospheric pressure Water at 100 degrees C is in thermal equilibrium, it is being cooled as much as it is being warmed This makes boiling a cooling process Add more heat to boiling water…what happens to temperature?

25. Boiling Depends on Pressure! Higher altitude means less air pressure The lower the pressure, the lower the boiling point of water Higher the pressure, boiling point increases Denver: boiling point is 95 degrees C

26. Boiling as a Cooling Process

27. Boiling CHECK YOUR NEIGHBOR The process of boiling A. cools the water being boiled. • depends on atmospheric pressure. • is a change of phase below the water surface. • All of the above.

28. Boiling CHECK YOUR ANSWER The process of boiling A. cools the water being boiled. • depends on atmospheric pressure. • is a change of phase below the water surface. • All of the above.

29. Melting Change from solid to liquid Heat a solid: molecules move faster and more violently Forces can’t hold the molecules together, solid melts!

30. Freezing Opposite of melting Freeze at the same temp. as they melt Liquid gives off energy, motion slows down, molecules link together Become a solid! A pure substance has a definite melting or freezing point at a certain pressure.

31. The process of boiling water tends to • warm the water. • cool the water. • both warm and cool the water at the same time. • have no effect on water’s temperature.

32. The process of boiling water tends to • warm the water. • cool the water. • both warm and cool the water at the same time. • have no effect on water’s temperature.

33. Boiling and freezing can occur at the same time when water is subjected to • decreased temperatures. • decreased atmospheric pressure. • increased temperatures. • increased atmospheric pressure.

34. Boiling and freezing can occur at the same time when water is subjected to • decreased temperatures. • decreased atmospheric pressure. • increased temperatures. • increased atmospheric pressure. Comment: This is shown in the chapter-opener photo of Ron Hipschman with the Water Freezer exhibit at the Exploratorium in San Francisco.

35. The melting point of a substance such as water is normally • higher than the freezing point. • lower than the freezing point. • the same as the freezing point. • unrelated to the freezing point.

36. The melting point of a substance such as water is normally • higher than the freezing point. • lower than the freezing point. • the same as the freezing point. • unrelated to the freezing point. Comment: Water freezes at 0°C and ice melts at 0°C. Likewise for the melting and freezing points of other substances.

37. Energy and Changes of Phase

38. Energy of Water Add energy to a piece of -50°C ice Temperature rises After it hits 0°C, the temperature stays here even as we add heat! This heat is being used to Melt the ice This is a change of phase! When all the ice melts, the temperature of the water can begin to increase

39. When it hits 100°C, again the temperature stays the same as heat is applied. This heat is vaporizing the water to steam After all the water is steam, the temperature will rise

40. Change of Phase of Water

41. Latent Heat of Fusion It takes 80 calories to melt one gram of ice to water This process is reversible, it takes 80 calories to freeze one gram of water This energy (80 cal per gram) is called the Latent Heat Of Fusion for water. If you have a 25 gram sample of ice, how much energy is needed to melt it? Q = mass x Latent Heat = 25 x 80 = 2000 cal

42. Latent Heat of Vaporization It takes 540 calories to vaporize a gram of water Again, reversible 540 calories to condense steam to water This energy (540 calories per gram) is called the Latent Heat of Vaporization for water If you have a 25 gram sample of 100°C water, how much heat is needed to vaporize it? Q = mass x Latent Heat = 25 x 540 = 13500 cal

43. Heat Required to Vaporize Ice Phase Change: Q(gained or lost) = m x Latent Heat (fusion or vaporiztion) Temperature Change: Q (gained or lost) = m c ΔT

44. Problem How much energy is required to heat 25 g of liquid water from 25C to 100C and change it to steam?

45. Step 1: Calculate the energy needed to heat the water from 25C to 100C Q = m  c T Q = 25g  1.0 cal g-1C –1  75 C= 1875 cal

46. Step 2: Vaporization: Use the Latent Heat to calculate the energy required to vaporize 25g of water at 100C Q = 25.0 g  540 cal/g = 13500 cal .25g  1mol H2O / 18g mol-1 H2O = 1.4 mol H2O vap H (H2O) = 1.4 mol H2O  40.6kJ/mol = 57 kJ

47. Total energy change is: Heat from temperature change + Heat of vaporization: 1875 cal + 13500 cal = 15375 cal

48. Latent Heats for Water Latent heat of fusion: 80 cal per gram Latent heat of vaporization: 540 cal per gram

49. Practice One more step: How much energy is needed to vaporize a 50 gram sample of ice completely to steam? Hint: You must go through three different heats exchanges!

50. Vaporize 50 grams of ice Heat of fusion: Q = m x 80 = 50 x 80 = 4000 cal Heat for temperature change 0 – 100°C: Q = m c ΔT = 50 x 1 x (100-0) = 5000 cal Heat of vaporization: Q = m x 540 = 50 x 540 = 27000 cal Total Heat needed: 4000 + 5000 + 27000 = 36000 cal!