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Chapter 23 – change of phase

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1 Chapter 23 – change of phase

2 23.1 - evaporation Change of phase from liquid to gas
Constant colliding of molecules can give “extra” energy to surface allowing them to “escape” from liquid These are vapor molecules now The “extra” energy came at the expense of remaining liquid Thus, liquid is cooler Evaporation is a cooling process It leaves the remaining liquid cooler than before

3 23.2 - condensation Changing from gas to liquid
Collisions cause a slowing of gas molecules into liquid ones When they condense, they must give up energy Thus, they warm the area Condensation is a warming process Steam causes burns because of condensation

4 Relative Humidity Air is saturated when it can no longer “hold” water as a vapor Relative humidity – the amount a vapor in air compared to how much it could hold 100% = saturated Warm air holds more vapor than cold The slower the molecules move (ie colder) the more likely they are to condense Even when warm, some particles move slowly Rising air expands and cools, this condenses water vapor and creates a cloud Fog is a cloud at surface

5 23.3 – evaporation & condensation rates
Evaporation and condensation occur at same time When rate is the same = no change in temperature Evap > cond = cooler Evap < cond = warmer

6 23.4 - boiling Phase changes below a liquids surface – boiling
When a vaporous bubble forms, its pressure must counter the water pressure in order for it to surface This happens when it reaches a specific temperature Increasing outside pressure, increases the boiling temperature Thus, boiling is determined by temp at which there is sufficient vapor pressure

7 A pressure cooker increases the surrounding pressure, causing an increase in boiling temperature (food cooks faster) Lowering air pressure (at high elevation) reduces boiling temperature As water boils, it takes away heat energy from another source

8 freezing As energy is extracted from a liquid, molecules slow down and get closer to one another Eventually they vibrate about a fixed position – becoming a solid Adding impurities to a liquid lowers the freezing temperature

9 23.6 – Boiling & freezing at the same time
Lowing air pressure reduces the vapor pressure of a liquid Reduced enough, water will boil at room temperature As it boils, it takes away energy Given enough time, the remaining water will freeze Freeze dried coffee is made this way

10 Regelation Increasing pressure reduces the melting temperature because the crystals are crushed Once pressure subsides, refreezing occurs Only happens with substances that expand when freezing How snowballs can be formed

11 23.8 – energy & changes of phase

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