LECTURE 8 PHASES, EVAPORATION & LATENT HEAT Lecture Instructor: Kazumi Tolich Lecture 8

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¨ Reading chapter 17.4 to 17.5 ¤ Phase equilibrium ¤ Evaporation ¤ Latent heats n Latent heat of fusion n Latent heat of vaporization n Latent heat of sublimation Phase equilibrium & vapor-pressure curve

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¨ If a substance has two or more phases that coexist in a steady and stable fashion, the substance is in phase equilibrium.

¨ The pressure of the gas when equilibrium is reached is called equilibrium vapor pressure.

¨ A plot of the equilibrium vapor pressure versus temperature is called vapor-pressure curve.

¨ A liquid boils at the temperature at which its vapor pressure equals the external pressure. Phase diagram

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¨ A fusion curve indicates where the solid and liquid phases are in equilibrium.

¨ A sublimation curve indicates where the solid and gas phases are in equilibrium.

¨ A plot showing a vapor-pressure curve, a fusion curve, and a sublimation curve is called a phase diagram.

¨ The vapor-pressure curve comes to an end at the critical point. Beyond the critical point, there is no distinction between liquid and gas.

¨ At triple point, all three phases, solid, liquid, and gas, are in equilibrium.

¤ In water, the triple point occur at T = 273.16 K and P = 611.2 Pa. Quiz: 1

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¨ In most substance, as the pressure increases, the melting temperature of the substance also increases because a solid is denser than the corresponding liquid. But in water, ice is less dense than liquid water. Does the fusion curve of water have a positive or negative slope? A. Positive

B. Negative Quiz: 8-1 answer

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¨ Negative

¨ In most substances, the fusion line has a positive slope. As the pressure increases, the melting temperature of the substance also increases because a solid is denser than the corresponding liquid.

¨ In water, the slope is negative because ice is less dense.

Typical substance Water Example: 1

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¨ The phase diagram for water is shown.

a) What is the temperature T1?

b) What is the temperature T2?

c) What happens to the melting/freezing temperature if atmospheric pressure is decreased?

d) What happens to the boiling/condensation temperature if atmospheric pressure is increased? Evaporation/ Demo: 1

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¨ Evaporation is a cooling process.

¨ Freezing water by evaporation ¤ When the pressure is lowered, water boils at a lower temperature. ¤ Evaporation of water lowers the temperature of water itself. Demo: 2

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¨ CO2 sublimation

¤ Demonstration of sublimation of CO2 Latent heats

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¨ The energy required to convert a substance of mass � from one phase to another is given by

� = ��

where � is the coefficient of latent heat.

¤ latent heat of fusion, �, solid to liquid

¤ latent heat of vaporization, �, liquid to gas

¤ latent heat of sublimation, �, solid to gas Example: 2

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¨ A heat transfer of Q = 9.5 × 105 J is required to convert a block of ice at

Ti = -12 °C to Tf = 12 °C. What was the mass of the block of ice? Quiz: 2

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¨ Which will cause more severe burns to your skin: 100°C water or 100°C steam of a same mass? A. Water B. Steam C. Both the same Quiz: 8-2 answer

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¨ Steam

¨ The hot steam first has to undergo a phase change into water at 100 °C before cooling down. This releases additional energy causing more severe burns. Example: 3

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¨ Flesh becomes badly damaged when its temperature reaches 50.0 °C.

a) Calculate the heat released as

m = 12.5 g of liquid water at Ti = 100 °C is cooled to Tf = 50.0 °C. b) Calculate the heat released as

m = 12.5 g of steam at Ti = 100 °C is condensed and cooled to Tf = 50.0 °C. c) Calculate the mass of flesh that can be

heated from T’i = 37.0 °C (normal body temperature) to T’f = 50.0 °C. The average specific heat of flesh is

cflesh = 3500 J/kg·K. Quiz: 3

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¨ You step out of a swimming pool on a hot day, where the air temperature is 90°F. Where will you feel cooler, in Phoenix (dry air) or in Philadelphia (humid air)? A. equally cool in both places B. Philadelphia C. Phoenix Quiz: 8-3 answer

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¨ Phoenix

¨ In Phoenix, where the air is dry, more of the water will evaporate from your skin because there is not much water vapor in the air.

¨ Evaporation is a phase change, where the water must absorb the heat of vaporization from your skin.

¨ That is why you feel cool as the water evaporates.

¨ You may also feel colder when you are taking a shower with a bathroom door open compared to when the door is closed. Quiz: 4

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¨ The graph shows the temperature of a 1.0-g sample of material as heat is added to it. The material is initially a solid at 10ºC. The pressure remains constant, and there is no chemical change. What is the coefficient of latent heat of fusion for the material in cal/g? Quiz: 8-4 answer

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¨ 50 cal/g

¨ The coefficient of latent heat of fusion is the energy required to convert a substance of unit mass from one phase to another. Q Q 50 cal L = = m 1.0 g Quiz: 5

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¨ The graph shows the temperature of a 1.0-g sample of material as heat is added to it. The material is initially a solid at 10ºC. The pressure remains constant, and there is no chemical change. What is the specific heat of the solid phase in ? ° Quiz: 8-5 answer

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¨ The specific heat is the energy required to raise the temperature of a substance of unit mass by 1C°. Q ¨ � = = = 0.25 . ° ° ΔT Water protects buds from cold weather

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¨ When temperature falls a few degrees below 0 °C, fruit crop is in danger of being ruined.

¨ To protect the buds, farmers spray the trees with water.

¨ Water has large latent heat of fusion.

¨ Before the buds can freeze, the water must be cooled to 0 °C and then freeze.

¨ In this process, the water gives up heat and keeps the temperature of the buds from going below 0 °C.

¨ The layer of ice over the buds acts like insulation because ice is not a good conductor of heat.