Understanding Phase Changes

Study Notes

A phase change is a reversible physical change where a substance transitions from one state of matter to another.

The temperature of a substance remains constant during a phase change.
When a solid changes to a liquid, the temperature rises until the melting point, where it remains constant until melting is complete, then rises again.
When a liquid changes to a solid, the temperature drops to the freezing point and remains constant until freezing is complete, then drops again
The freezing and melting points of a substance are identical.
When a liquid changes to a gas, the temperature rises to the boiling point and remains constant until boiling is complete.

The arrangement of water molecules becomes less orderly during melting and more orderly during freezing.
In ice, attractions between water molecules hold them in fixed positions.
When ice is removed from the freezer, heat flows from the air to the ice.
As ice gains energy, the molecules vibrate more rapidly.
At the melting point, some molecules gain enough energy to move from their fixed positions.
Melting is complete when all molecules have gained this energy.
When liquid water is placed in a freezer, energy flows from the water to the air in the freezer
Freezing is the reverse of melting.

Vaporization is the phase change where a substance changes from a liquid into a gas.
A substance must absorb energy to change from a liquid to a gas.
The heat of vaporization for water is 2261 joules of energy.

Boiling and evaporation are two vaporization processes.
Evaporation takes place at the surface of a liquid.
Evaporation is the process that changes a substance from a liquid to a gas at temperatures below the boiling point.
Vapor is the gaseous phase of a substance.
Vapor pressure is the pressure caused by the collisions of vapor and the walls of a container.

Water boils when vapor pressure equals atmospheric pressure.
The boiling point of water is the temperature at which this occurs.
As temperature increase, water molecules move faster
When water reaches 100°C, some molecules below the surface gain enough kinetic energy to overcome attraction of neighboring molecules
Because water vapor is less dense than water, the bubbles rise to the surface.

Condensation is the phase change where a substance changes from a gas or vapor to a liquid.
Condensation is responsible for dew and condensation on bathroom mirrors.

Sublimation is the phase change in which a substance changes from a solid to a gas or vapor without first changing to a liquid.
At room temperature, dry ice can directly change from a solid to a colorless gas.
When dry ice sublimes, the cold carbon dioxide vapor causes water vapor in the air to condense and form clouds.

Deposition is the phase change in which a gas or vapor changes directly into a solid without first changing to a liquid.
Deposition causes frost to form on windows.