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Boiling Liquid At A
Reduced Pressure
A liquid boils under reduced pressure without
being heated
Purpose
To observe the effect of reduced
pressure on a liquid
To cause a liquid to boil without
heating it
Demonstration
Rather than heat the liquid to make
it boil
Reducing the atmospheric
pressure above the liquid made the
liquid boil
The liquid boiled but then cooled
until boiling finally stopped
Concepts
1.
Intermolecular Attractions
2.
Vapor Pressure
3.
Boiling Point
1. Intermolecular Attractions
All liquids consist of particles which are
attracted to each other
These attractions are known as
“intermolecular attractions” because they
exist between separate molecules
When these attractions are overcome
the liquid will boil and be converted to a
gas
2. Vapor Pressure
The molecules of a liquid are in constant
motion and escape from the surface of
the liquid
The vapor above the liquid is in
equilibrium with the liquid
In a closed container, the escaped
molecules exert a pressure above the
liquid, known as the “equilibrium vapor
pressure”
The equilibrium vapor pressure depends
on the temperature
Vapor Pressure Curve
for Acetone
760 nm, Standard Atmospheric Pressure
900
800
Vapor Pressure 700
nm
600
500
400
300
200
100
10
20
30 40
50 60
70
80
90 100
120
140
Temperature, Degrees Celsius
At atmospheric pressure, acetone will boil at 57 degrees!
3. Boiling Point
The vapor pressure of a substance
such as acetone increases as the
temperature increases
When the vapor pressure equals
the atmospheric pressure
Intermolecular attractions are
overcome
The liquid boils
Conclusions
Boiling
Overcame the intermolecular
attractions between molecules
of a liquid
Occurred when the atmospheric
pressure above the liquid was
the same as the vapor pressure
of the liquid
Comments
The greater the vacuum over the
liquid the lower the temperature at
which it will boil
The intermolecular attractions
between acetone molecules are
weaker than those between water
molecules
Acetone boils at 57 degrees Celsius
at atmospheric pressure
Water boils at 100 degrees Celsius at
atmospheric pressure