Changes of State

Chapter 4-2

Changes of State• A change of state is the conversion of

a substance from one physical form

to another.

• All the changes

are physical

changes.

•The identity of the substance does not change.

During a change of state, the energy of a substance changes.

• The energy of a substance is caused by the motion of the particles.

MELTING: Solid to Liquid• Melting point is the

temperature at which a substance melts.

• The melting point is different for all substances.

• For a solid to melt, energy must be absorbed to make the molecules move faster.

FREEZING: Liquid to Solid• Freezing point is

the temperature that a liquid becomes a solid.

• Energy is removed during freezing.

EVAPORATION: Liquid to Gas

• Evaporation is the change of state from liquid to a gas.

• It happens at the surface.

• Sweat evaporating cools your body.

BOILING: Liquid to Gas• Boiling is also a change

from a liquid to a gas at 100 degrees celsius.

• Large groups of molecules escape rather than just a few at a time.

• Water will boil at a lower temp. at higher altitudes because the atmospheric pressure is less.

CONDENSATION: Gas to Liquid

• The condensation point is the temp. at which a gas becomes a liquid.

• Beads of water form when water vapor in the air contacts a cool surface.

SUBLIMATION: Solid to Gas

• Sublimation is the change of state in which a solid changes directly to a gas.

• Dry ice goes through sublimation instead of melting into a liquid.

The ice, liquid water, and steam are all the same substance.

Temperature and Changes of State

• When substances gain or lose energy, its temp. changes or its state changes.

• When the temp. changes so does the particle movement.

• Higher temp. = faster movement

• Lower temp. = slower movement

The temperature remains the same at the horizontal lines while

energy is added constantly.

Stop here and do chart.