1. Boiling Liquid At A Reduced Pressure
A liquid boils under reduced pressure without
being heated

2. Purpose To observe the effect of reduced pressure on a liquid
To cause a liquid to boil without heating it

3. 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

4. Concepts 1. Intermolecular Attractions 2. Vapor Pressure
3. Boiling Point

5. 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

6. 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

7. Vapor Pressure Curve for Acetone
760 nm, Standard Atmospheric Pressure
100
200
300
400
500
600
700
800 10
900 20 30 40 50 60 70 80 90
120
140
Vapor Pressure nm
Temperature, Degrees Celsius
At atmospheric pressure, acetone will boil at 57 degrees!

8. 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

9. 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

10. 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