Phase changes occur when matter transitions between solid, liquid, and gas states. During a phase change, molecules either absorb or release heat energy as they speed up or slow down. There are several types of phase changes, including melting (solid to liquid), freezing (liquid to solid), vaporization/boiling (liquid to gas), evaporation, condensation, and sublimation. A key characteristic of phase changes is that the temperature remains constant despite an exchange of heat energy, as the molecules rearrange their structure.

1. Phase Changes

2. Review: 3 Phases of Matter
• Solid
• Liquid
• Gas

3. What is a Phase Change?
• Is a change from one state of matter
(solid, liquid, gas) to another.
• Phase changes are physical changes because:
- It only affects physical appearance, not
chemical make-up.
- Reversible

4. What happens during a phase change?
• During a phase change,
heat energy is either
absorbed or released.
• Heat energy is released as
molecules slow down and
move closer together.
• Heat energy is absorbed
as molecules speed up
and expand.

5. Types of Phase Changes

6. Melting
• Phase change from a
solid to a liquid
• Molecules speed up,
move farther apart, and
absorb heat energy

7. Freezing
• Phase Change from a
liquid to a solid
• Molecule slow down,
move closer together
and release heat
energy.

8. Vaporization (Boiling)
• Phase change from a
liquid to gas. It occurs at
the boiling point of
matter.
• Molecules speed up,
move farther apart, and
absorb heat energy.

9. Evaporation
• Phase change from a
liquid to a gas on the
surface of a liquid
(occurs at all
temperatures).
• Molecules speed up,
move farther apart, and
absorb heat energy.

10. Condensation
• Phase change from a
gas to a liquid.
• Molecule slow down,
move closer together
and release heat
energy.

11. Sublimation
• Phase change from a
solid to a gas.
• Molecules speed up,
move farther apart, and
absorb heat energy.

15. Graphing a Phase Change

17. Melting & Boiling Points
• Melting Point: The temperature at which a
solid changes into a liquid.
• Boiling Point: The temperature at which a
liquid changes into a gas.
• Freezing point: the temperature at which a
liquid turns into a solid when cooled.

19. Summary

20. Graphing a Phase Change
– Why is there no change
in temperature during a
phase change?
– Define melting and
boiling points.
– What is the melting
point and boiling point
of water?
– At what temperature
does water freeze and
become a solid?