Phase Changes

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!

Names of Phase Changes

Solid

Liquid

Gas

Melting

Freezing

Vaporization

Condensation

Sublimation

Deposition

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).

So a graph would look like…

Time vs. Temperature of waterunder constant heat

temperature

100oC

0oC

time

meltingpoint liquid

boiling pointgas

solid

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.

Evaporation works like thisLiquids have molecules moving around in them

Temperature is the average kinetic energy (which depends on the speed) of these molecules.Some are movingfaster than others!

If they are moving fast enough, at just the right angle,some will escapethe surface of the liquid

and turn into a gas.These evaporated!

Molecules are held in by intermolecular forces.

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.

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.

Assignment

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