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 Gas Solid Sublimation Deposition Vaporization Melting Liquid Freezing Condensation

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 water under constant heat boiling point gas 100oC 0oC temperature melting point liquid solid time

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 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!

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