1. Phase Changes

2. Phase Changes
Change of state or a phase change is a conversion of a substance from one phase (solid, liquid, or gas) to another.
Heating or cooling a substance can cause it to change phases.
Heating is adding (heat) energy.
Cooling is removing (heat) energy
There is no such thing as cold energy or coldness you can add!

3. Names of Phase Changes Gas Solid Sublimation Deposition Vaporization
Melting
Liquid
Freezing
Condensation

4. Phase changes and temperature
Normally when heat is added the temperature goes up.
However, at a phase change point (melting/freezing, boiling/condensation)…
The temperature stays constant when heat is added (until the phase change is complete).

5. So a graph would look like…
Time vs. Temperature of water
under constant heat
boiling point
gas
100oC
0oC
temperature
melting
point
liquid
solid
time

6. Evaporation
Evaporation is a change in phase from liquid to gas, but is not the same as vaporization!
Vaporization requires you to heat the substance to its boiling point.
Evaporation can happen at much lower temperatures.
Volatility- A measure of how easily a liquid evaporates.

7. Evaporation works like this
Liquids have molecules moving around in them
Temperature is the average kinetic energy (which depends on the speed) of these molecules.
If they are moving fast enough, at just the right angle,
some will escape
the surface of the
liquid
Some are moving
faster than others!
Molecules are held in by intermolecular forces.
and turn into a gas.
These evaporated!

8. Why are they a “gas”?
Intermolecular forces determine whether something is a solid, liquid or gas.
The ones that escaped aren’t next to any other particles so they have almost no intermolecular forces.
They have to be a gas.

9. Where did they go? They are in the air around the liquid.
They are called vapors, anything that naturally is a solid or liquid under standard conditions that is currently a gas at standard conditions.
If enough of them get together they will condense and reform a liquid.

10. Assignment Read sections 8.4, 8.5 and 8.6 Pg 282-285
Questions 15-18, 27, 76-80