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 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!

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