Change of Phase

Change of PhasePhases of MatterMatter 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)Phases of MatterThe 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.

EvaporationEvaporationChange of phase from liquid to gas

EvaporationEVAPORATION IS A COOLING PROCESS!

Molecules are randomly moving

Some gain KE, others lose KE

Some can break free of the liquidbecome gas

Average KE drops, the liquid becomes cooler

OMG Like Totally EwwwSweat.Bodies overheat, produce perspiration (sweat)The water evaporates and cools usKeeps body temperature stable

Animals dont have many sweat glands

EvaporationSublimationForm of phase change directly from solid to gasExample: dry ice (solid carbon dioxide molecules), frozen water

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

9Answer: DWhen evaporation occurs in a dish of water, the molecules left behind in the waterare less energetic.have decreased average speeds.result in lowered temperature.All of these.10Answer: DWhen matter changes phase directly from solid to gas, we call the processevaporation.condensation.sublimation.regelation.

11Answer: CWhen matter changes phase directly from solid to gas, we call the processevaporation.condensation.sublimation.regelation.12Answer: CCondensationCondensation processOpposite of evaporationWarming process from a gas to a liquidGas molecules near a liquid surface are attracted to the liquidThey strike the surface with increased KE, becoming part of the liquidOverall KE of liquid increases, so it gets warmer!

Examples of CondensationSteam releases much energy when it condenses to a liquid and moistens the skinhence, it produces a more damaging burn than from same-temperature 100C 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: Youre 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!

Condensation in the AtmosphereHumidity- amount of water vapor in the airRelative 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)Condensation in the AtmosphereTemperature drops: water vapor condensesSlow moving molecules combine: create droplets of water

Clouds and FogWarm air rises, it expands.As it expands, it cools. As it cools, water vapor molecules begin sticking togetherThey 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

The dew that forms on a cold soda can when taken from a refrigerator is due toevaporation.condensation.sublimation.regelation.

18Answer: BThe dew that forms on a cold soda can when taken from a refrigerator is due toevaporation.condensation.sublimation.regelation.19Answer: B

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.CondensationCHECK YOUR NEIGHBOR 20When 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.CondensationCHECK 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.21Evaporation and CondensationBoilingEvaporation beneath the surface causes bubbles (turns to vapor in the liquid!)Bubbles rise to the surfaceOnly occurs when the pressure of the vapor can resist the pressure of the liquid and atmosphere above it.

BoilingFor water, that point is 100 degrees C at regular atmospheric pressureWater at 100 degrees C is in thermal equilibrium, it is being cooled as much as it is being warmedThis makes boiling a cooling processAdd more heat to boiling waterwhat happens to temperature?BoilingDepends on Pressure!Higher altitude means less air pressureThe lower the pressure, the lower the boiling point of waterHigher the pressure, boiling point increasesDenver: boiling point is 95 degrees C

Boiling as a Cooling ProcessThe process of boilingA.cools the water being boiled.depends on atmospheric pressure. is a change of phase below the water surface.All of the above.

BoilingCHECK YOUR NEIGHBOR D. all of these.

The process of boilingA.cools the water being boiled.depends on atmospheric pressure.is a change of phase below the water surface.All of the above.

BoilingCHECK YOUR ANSWERD. all of these.

MeltingChange from solid to liquidHeat a solid: molecules move faster and more violentlyForces cant hold the molecules together, solid melts!FreezingOpposite of meltingFreeze at the same temp. as they meltLiquid gives off energy, motion slows down, molecules link togetherBecome a solid!A pure substance has a definite melting or freezing point at a certain pressure.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 waters temperature.

31Answer: BThe 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 waters temperature.32Answer: BBoiling and freezing can occur at the same time when water is subjected todecreased temperatures.decreased atmospheric pressure.increased temperatures.increased atmospheric pressure.

33Answer: BBoiling and freezing can occur at the same time when water is subjected todecreased 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.34Answer: BThe melting point of a substance such as water is normallyhigher than the freezing point.lower than the freezing point.the same as the freezing point.unrelated to the freezing point.

35Answer: CThe melting point of a substance such as water is normallyhigher than the freezing point.lower than the freezing point.the same as the freezing point.unrelated to the freezing point.Comment: Water freezes at 0C and ice melts at 0C. Likewise for the melting and freezing points of other substances.36Answer: CEnergy and Changes of Phase

Energy of WaterAdd energy to a piece of -50C iceTemperature rises After it hits 0C, the temperature stays here even as we add heat!This heat is being used to Melt the iceThis is a change of phase!When all the ice melts, the temperature of the water can begin to increaseWhen it hits 100C, again the temperature stays the same as heat is applied. This heat is vaporizing the water to steamAfter all the water is steam, the temperature will rise

Change of Phase of Water

Latent Heat of FusionIt takes 80 calories to melt one gram of ice to waterThis process is reversible, it takes 80 calories to freeze one gram of waterThis 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 Latent Heat of VaporizationIt takes 540 calories to vaporize a gram of waterAgain, reversible 540 calories to condense steam to waterThis energy (540 calories per gram) is called the Latent Heat of Vaporization for waterIf you have a 25 gram sample of 100C water, how much heat is needed to vaporize it?Q = mass x Latent Heat = 25 x 540 = 13500 cal

Heat Required to Vaporize IcePhase Change:

Q(gained or lost) = m x Latent Heat (fusion or vaporiztion)Temperature Change:

Q (gained or lost) = m c TProblemHow much energy is required to heat 25 g of liquid water from 25C to 100C and change it to steam?

Step 1: Calculate the energy needed to heat the water from 25C to 100CQ = 25g 1.0 cal g-1 C 1 75 C = 1875 calQ = m c T

Step 2: Vaporization: Use the Latent Heat to calculate the energy required to vaporize 25g of water at 100C.25g 1mol H2O / 18g mol-1 H2O = 1.4 mol H2Ovap H (H2O) = 1.4 mol H2O 40.6kJ/mol = 57 kJ

Q = 25.0 g 540 cal/g = 13500 cal

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

Latent Heats for WaterLatent heat of fusion: 80 cal per gramLatent heat of vaporization: 540 cal per gramPracticeOne 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!Vaporize 50 grams of iceHeat of fusion:Q = m x 80 = 50 x 80 = 4000 calHeat for temperature change 0 100C:Q = m c T = 50 x 1 x (100-0) = 5000 calHeat of vaporization:Q = m x 540 = 50 x 540 = 27000 calTotal Heat needed: 4000 + 5000 + 27000 = 36000 cal!Review of Phases:

A-B = Solid ice, temperature is increasing. Particles gain kinetic energy, vibration of particles increases.Heating and cooling curve for water heated at a constant rates.Ice

B-C = Solid starts to change state from solid to liquid. Temperature remains constant as energy is used to break inter-molecular bonds. H2O (s) H2O () energy required 80 cal/g0C

C-D = temperature starts to rise once all the solid has melted. Particles gain kinetic energy.Liquid water

D-E = Liquid starts to vaporize, turning from liquid to gas. The temperature remains constant as energy is used to break inter-molecular forces.H2O () H2O (g) energy required 540 calg100C

E-F = temperature starts to rise once all liquid is vaporized. Gas particles gain kinetic energy.steam

vaporization boiling meltingHeat & Changes of State condensation freezing

sublimation depositionsublimationWhen steam condenses to water, energy isabsorbed by the steam.released by the steam.conserved as the phase change occurs.changed to a different form.

58Answer: BWhen steam condenses to water, energy isabsorbed by the steam.released by the steam.conserved as the phase change occurs.changed to a different form.59Answer: BWhen water turns to ice, water moleculesabsorb energy.release energy.retain their energy.lose the quality of wetness.

60Answer: BWhen water turns to ice, water moleculesabsorb energy.release energy.retain their energy.lose the quality of wetness.61Answer: BWhen ice changes phase to become water, energy isabsorbed.given off.unchanged.in effect multiplied.

62Answer: AWhen ice changes phase to become water, energy isabsorbed.given off.unchanged.in effect multiplied.63Answer: AWhen a gas changes phase to become a solid, energy isabsorbed.given off.unchanged.in effect multiplied.

64Answer: BWhen a gas changes phase to become a solid, energy isabsorbed.given off.unchanged.in effect multiplied.Comment: This occurs when snowflakes form in the air.65Answer: B