Chapter 23 – change of phase

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. 23.5 - 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. 23.7 - 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