Transcript Collapsing Can

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