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Chapter 17 Changes of Phase

Chapter 17 Changes of Phase. Phases of Matter. Four Phases of Matter: Solid Liquid Gas Plasma Change of phase occurs when we pass from one phase to another, such as water (liquid) boiling to change into vapor (gas). Water. Plasma. Ice. Steam. Evaporation.

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Chapter 17 Changes of Phase

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  1. Chapter 17Changes of Phase Physics 1 (Garcia) SJSU

  2. Phases of Matter Four Phases of Matter: • Solid • Liquid • Gas • Plasma Change of phase occurs when we pass from one phase to another, such as water (liquid) boiling to change into vapor (gas). Water Plasma Ice Steam Physics 1 (Garcia) SJSU

  3. Evaporation Evaporation is a change of phase from liquid to gas that takes place at the surface of a liquid. A random molecule at the surface acquires enough energy to escape the attraction force among the molecules (which holds the liquid together). GAS LIQUID Physics 1 (Garcia) SJSU

  4. Evaporative Cooling Because only the most energetic molecules can escape the surface, evaporation removes internal energy from the liquid, that is, evaporation cools. Brr WET CLOTH HEAT WET BODY & TOWEL WET TONGUE HEAT HEAT Wet towel cools head Wetness cools person Wet tongue cools dog Physics 1 (Garcia) SJSU

  5. Condensation Condensation is the reverse of evaporation, a change of phase from gas to liquid that takes place at the surface of a liquid. A random molecule from the gas strikes the surface and sticks instead of bouncing back into the gas. Condensation heats. GAS LIQUID Physics 1 (Garcia) SJSU

  6. Hot and Humid A 90 degree day in a dry climate, like San Jose, is more comfortable than a 90 degree day in a humid place like New Orleans. In a dry climate you’re cooled by evaporation, in a wet climate you’re heated by condensation. Heat index is the apparent temperature a person feels for a given humidity. Physics 1 (Garcia) SJSU

  7. Demo: Wet/Dry Bulb Thermometer Pair of thermometers; one is kept dry while the other’s bulb is wrapped in wet cloth. Difference of their temperatures gives relative humidity. Wet bulb Dry bulb Large temperature difference indicates high or low humidity? Low humidity; evaporative cooling is significant. Physics 1 (Garcia) SJSU

  8. Fog & Clouds Warm air rises. As it rises, it expands. As it expands, it cools. As it cools, vapor molecules condense into water droplets. This forms a cloud (or fog if warm, moist air cools near the ground). As vapor expands, it cools and tiny, visible, water droplets (liquid) condense. Cool Water vapor (gas) is invisible Warm breath feels cool when it expands Warm Physics 1 (Garcia) SJSU

  9. Boiling When the temperature of a liquid is high enough that evaporation occurs everywhere, not just the surface, then the liquid boils. The temperature required depends on the pressure; lower the pressure, the lower the boiling temperature (boiling point). Tiny bubbles grow due to evaporation at their surface Physics 1 (Garcia) SJSU

  10. Liquid Nitrogen Liquid nitrogen boils at atmospheric pressure and room temperature. Boiling point is -320 ºF and freezes at -346 ºF. Physics 1 (Garcia) SJSU

  11. Demo: Slowing Air Molecules Balloon returns to its original state Air molecules slow down and lose kinetic energy Cool balloon using liquid nitrogen Balloon slowly warms up, restoring energy Physics 1 (Garcia) SJSU

  12. Demo: Low Pressure Boiling Water boils at room temperature if the pressure is low. Cooking at high altitudes is difficult due to this effect; coffee brewed in the mountains always tastes lukewarm. Physics 1 (Garcia) SJSU

  13. Melting Melting is the change of phase from solid to liquid. Melting is a cooling process; the solid must absorb heat to melt. Physics 1 (Garcia) SJSU

  14. Sublimation Sublimation is change of phase from solid to gas without passing through liquid phase. Solid carbon dioxide (dry ice) sublimates at a chilly -109 °F. Put dry ice into warm water to create dense fog of tiny water droplets. Physics 1 (Garcia) SJSU

  15. Demo: Carbon Dioxide Carbon dioxide, released when dry ice sublimates, is heavier than air. • Burning candle • Extinguished under CO2 layer Bubbles float on layer of dry ice. (c) Scoop out some CO2 in a cup (d) Pour it on candle to extinguish Physics 1 (Garcia) SJSU

  16. Freezing Freezing is the opposite of melting, that is, the change of phase from liquid to solid. Heat must be removed from a liquid in order to freeze it into a solid. Lava (liquid) freezes into rock (solid), heating the seawater. Seawater (liquid) boils into vapor (gas), cooling the lava. Physics 1 (Garcia) SJSU

  17. Demo: Freeze Solid Materials become brittle when frozen solid. Organic materials appear solid but cells contain large amounts of liquid water. Physics 1 (Garcia) SJSU

  18. Energy & Changes of Phase Physics 1 (Garcia) SJSU

  19. Heats of Fusion & Vaporization 100 cal 540 cal 720 cal 80 cal Heat of Fusion Heat of Vaporization Total Energy Heat Capacity Heating a gram of water Physics 1 (Garcia) SJSU

  20. Check Yourself Is boiling a cooling or a warming process? Boiling is a cooling process. So can you cool your hand by putting it in boiling water? NO! Ouch! So why is boiling a cooling process? Because when a liquid boils it cools by itself releasing its most energetic molecules, just as with cooling by evaporation. Physics 1 (Garcia) SJSU

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