1. Unit 3 Topic 3

2. In order for a substance to undergo a phase change (a change in physical state) the individual particles must either speed up or slow down.

3. Particles speed up when heat is added, this process is considered endothermic because heat is absorbed.
Particles slow down when heat is removed, this process is considered exothermic because heat is lost.
These processes can be shown graphically.

4. These phase changes can be represented by a chemical equation.
Melting
H2O(s) + Heat  H2O(l)
Endothermic (heat is required)
Freezing
H2O(l)  H2O(s) + Heat
Exothermic (heat is removed/given off)
H2O(s)  solid
H2O(l)  liquid
H2O(g)  gas

5. Note that melting and freezing occur at the same temperature.
This makes sense because each change occurs between the same two states of matter.
Whether heat is added or removed (endo vs exothermic) determines which phase change occurs at that temperature.

6. These phase changes can be represented by a chemical equation.
Boiling
H2O(l) + Heat  H2O(g)
Endothermic (heat is required)
Condensing
H2O(g)  H2O(l) + Heat
Exothermic (heat is removed/given off)

7. Note that condensing and boiling occur at the same temperature.
This makes sense because each change occurs between the same two states of matter.
Whether heat is added or removed (endo vs exothermic) determines which phase change occurs at that temperature.

8. Two other changes of state which can occur are evaporation and sublimation.
Both of these changes are endothermic and require heat because they involve particles in a slower moving state (solid or liquid) to enter a faster moving state (gas).

9. Sublimation H2O(s) + Heat  H2O(g) Endothermic (heat is required)
Substances that sublime, go directly from a solid to a gas w/o melting. Ex: CO2 carbon dioxide (dry ice)

10. Evaporation H2O(l) + Heat  H2O(g) Endothermic (heat is required)
The difference between evaporation and boiling is that evaporation occurs at all temperatures while boiling occurs at a set temperature.