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