1. Section 4: Changes in State
Objective:
explain that thermal energy flows from a warmer substance to a cooler substance
identify examples of changes in state and explain how thermal energy is involved in each example

2. I. Energy and Changes in State
A. Thermal energy always flows
from a warmer substance to a
cooler substance.
1 . A substance changes state
when its thermal energy
increases or decreases.

3. II. Changes Between Liquid and Solid
A. The change in state from a solid to a liquid is melting

4. B. Melting Point 1. The temperature when substance
changes from solid phase liquid.
a. Each has own melting
point.
b. physical property
1. used to identify the
substance.

5. C. Freezing Freezing change liquid to solid.
a. water freezes, the temperature stays at 0oC
until freezing is complete.

6. D. Changes between Liquid and Gas
1. Vaporization-
a. from liquid to gaseous
b. Two types:
1. Evaporation
a. from surface
2. Boiling
b. at surface and inside
liquid

7. Boiling Point and Air Pressure Boiling point of a substance
Depends upon air pressure above
The liquid.
The lower the air pressure the
Less energy the liquid molecules
Need to escape into the air.
At higher elevations liquids boil at
lower temperature because of less air pressure.
Food will take longer to cook because of the lower temp.

8. E. Condensation 1. Gas particles lose thermal energy
a. become a liquid.

9. F. Changes between Solids and Gas
1. Sublimation
Iodine crystals
Dry ice
solid directly to gas
Dry ice becomes gaseous carbon dioxide
iodine crystals become gaseous iodine.

10. FYI: Identifying Substance Through Changes of State
Comparing melting points and boiling points can be
important steps in identifying an unknown material.
Example: chloroform melts at -64 oC boils at 61oC
ethanol melts at -117oC and boils at 79oC
water melts at 0oC and boils at 100oC
All are clear, colorless liquids.

11. G. Relationship between temperature, energy and particle motion

12. As a sample of ice is warmed and the temperature is measured
every 30 seconds, the particles of solid water absorb the applied
heat energy. What effect does this energy have on the motion of
the particles?

13. As heat energy continues to be supplied, you may notice that the
solid water now begins to appear as a liquid, but the temperature is
not rising. To change a solid to a liquid, many forces must be
overcome. For each kilogram of ice you want to melt, 334 kJ of
energy must be supplied.

14. Once the work of melting is accomplished, the applied heat energy
Causes the water particles making up the liquid to move faster.
As you can see on the graph, the temperature begins to rise.

15. Once the boiling point has been reached, the liquid becomes a gas, and
The temperature again remains constant. At the heat of vaporization,
The forces of attraction between particles are totally overcome.
This requires 2260 kJ of energy.