1. Vapor Pressure

2. Evaporation u When fastest particles in a sample of liquid escape from the surface of the liquid u During evaporation the slower particles are left behind u What happens to the temperature of a liquid as it evaporates?

3. / When first sealed the molecules gradually escape the surface of the liquid / The vapor above the liquid has a pressure / As the molecules build up above the liquid some condense back to a liquid. Closed Container

4. Condensation u Slower particles of gas feel effects of intermolecular forces and become liquid u Leaves only the faster particles in the gas u What happens to the temperature of a the gas during condensation?

5. Rate of Vaporization = Rate of Condensation / Molecules are constantly changing phase “Dynamic”  The total amount of liquid and vapor remains constant “Equilibrium” Dynamic equilibrium

6. u The pressure of the gas at equilibrium is called the equilibrium vapor pressure u As the temperature goes up, more molecules vaporize u The equilibrium vapor pressure increases as temperature increases

8. Vapor Pressure Graph u Vapor pressure can be plotted vs the temperature u The curve obtained is the equilibrium vapor pressure u Since different substances all evaporate at different rates when heat is added, they all have different curves

9. Which of these substances vaporizes most easily? Which least? liquid gas

10. Boiling u A liquid boils when the vapor pressure = the external pressure u Vapor pressure great enough to form a bubble in the liquid u Normal Boiling point is the temperature a substance boils at 1 atm pressure. u The temperature of a liquid can never rise above it’s boiling point.

11. What is the boiling point of each liquid at 101.3 kPa?

12. Changing the Boiling Point u Lower the pressure (going up into the mountains). u Lower external pressure requires lower vapor pressure u Lower vapor pressure means lower boiling point. u Food cooks slower.

13. On a mountain the pressure is 90 kPa, what is the boiling point of water there?

14. u Raise the external pressure (Use a pressure cooker). u Raises the vapor pressure needed. u Raises the boiling point. u Food cooks faster. Changing the Boiling Point