I. Phase Changes

Matter and Energy

Section 2.2

Phase Changes

Gas

Solid

Liquid

Amount of heat

When matter changes from one state to another, the substance itself does not change.

Water, ice, and steam (water vapor) are all the same basic substances.

The molecules themselves do not change. What changes is the

arrangement of the

molecules and the

space between them

Solids can become liquids, liquids can become solids

A. Melting - the process by which a solid becomes a liquid

1. Different solids melt at different temperatures.

2. Melting point - The lowest temperature

at which a substance begins to melt.

Solids can become liquids, liquids can become solids

3. Example: Snow melting to water at 0oC

REMEMBER: particles are always in motion, even in solids. Because the particles in a solid are bound together, they do not move from place to place-they vibrate.

As a solid heats up, the particles vibrate faster until they break loose and slide past each other-in other words turn into a liquid.

B. Freezing - the process by which a liquid becomes a solid.

A frozen substance does not have to have an extremely cold temperature, some substances are frozen at room temperatures.

(example: chocolate bars,

candles, soda cans)

1. Freezing point - the temperature at which a specific liquid becomes a solid.

2. The freezing point of a substance is the same as the melting point.

At temperatures below this point the substance is a solid,

above this point the substance is a liquid.

3. Example: Water freezes to make ice cubes at 0oC

C. Vaporization – change of matter from a liquid state to a gas.

1. Vaporization occurs at the boiling point or during evaporation.

2. Evaporation - the process by which a liquid becomes a gas

It occurs at the surface of a liquid

The fastest moving

particles at the surface

can break away from

the liquid and escape

to become gas particles.

Boiling3. Boiling is another process by which a liquid

becomes a gas. Unlike evaporation, boiling produces

bubbles that contain energetic molecules that have escaped from the liquid to form a gas

Boiling occurs when a liquid reaches a certain temperature called the boiling point, 100oC for

water.

When you coola gas it losesenergy. As theparticles move more slowly, the attractions among them cause droplets to form.

D. Condensation- the process by

which a gas becomes a liquid.

Sublimation Under certain conditions, solids can lose

particles through a process similar to evaporation.

E. Sublimation – when solids change directly to a gas.

Example: Dry ice (solid carbon dioxide) sublimates in normal

atmospheric conditions.

DepositionF. Deposition - When a gas changes

directly to a solid. When you cool a gas it loses energy. As

the particles move more slowly, the attractions among some them cause a solid to form.

When carbon dioxide is cooled, it forms a solid, not a liquid.