Download Boiling Point and Air Pressure

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
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.
II. Changes Between Liquid and
Solid
A. The change in state from a solid to a liquid is melting
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.
C. Freezing
1. Freezing change liquid to solid.
a. water freezes, the temperature stays at 0oC
until freezing is complete.
D. Changes between Liquid and Gas
1. Vaporizationa. from liquid to gaseous
b. Two types:
1. Evaporation
a. from surface
2. Boiling
b. at surface and inside
liquid
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.
E. Condensation
1. Gas particles lose thermal
energy
a. become a liquid.
F. Changes between Solids and Gas
1. Sublimation
Dry ice
Iodine crystals
a. solid directly to gas
a. Dry ice becomes gaseous carbon dioxide
b. iodine crystals become gaseous iodine.
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.
G. Relationship between
temperature, energy and particle
motion
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?
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.
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.
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.