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B. Phase changes - vaporization and condensation













Liquid + heat





Why do molecules evaporate, vaporize?

->motion of the atoms or molecules overcome intermolecular forces

Vapor pressure - the pressure exerted by a vapor in equilibrium with its liquid

Vapor pressure is a strong function of temperature

For water

Temperature  (K)

Vapor pressure (mm Hg)

323 K


353 K


373 K

760 ( = 1 atm)

Boiling point - the temperature at which the vapor pressure of a liquid is equal to the external pressure. 

The pressure of the gas leaving the liquid is sufficient to push back the atmosphere and liquid is converted to solid throughout the liquid (thus bubbling), not just at the surface. 

The vapor pressure is a property of a particular substance whereas the boiling point depends on the atmospheric pressure.  The higher the altitude, the lower the atmospheric pressure, the lower the boiling point.   On top of Mt. Everest (28,000ft), the boiling point of water is 76.5oC.

Normal boiling point - the boiling point of a liquid at 1 atm pressure (see level)


boiling point

butane (non-polar)


Ethyl alcohol (polar)


water (polar, hydrogen bonds)



Heat of vaporization (endothermic) - the amount of heat required to convert 1 g of a liquid to a gas at 1 atm


heat of vaporization

butane (non-polar)

34.6 cal / g

Ethyl alcohol (polar)

78.5 cal / g

water (polar, hydrogen bonds)

540 cal / g


Energy input to a liquid at its boiling point goes into vaporizing the liquid as opposed to raising the temperature.  Very difficult to raise a liquid to a temperature higher than its boiling point. 

Vaporization is a cooling process!  The evaporation of our sweat helps cool us off.  Heat from our bodies goes into vaporization.

Condensation is exothermic, exact opposite of vaporization.

III. Solids -

Atoms, molecules or ions that are held in an ordered arrangment

As a solid is heated, the particles vibrate more rapidly.













solid + heat





melting point - temperature that a solid changes to a liquid

Heat of fusion (endothermic) - heat needed to completely transform 1 g of a solid at its melting point to 1 g of liquid.


heat of fusion


19.2 cal / g

Ethyl alcohol

26.1 cal / g


79.7 cal / g



60-70% of the mass of our bodies

pdb model

Hydrogen bonding - a special type of dipole-dipole intermolecular interaction that is particularly important in water.  Hydrogen bonding in water leads to many of its unique properties (notice high values of heats of vaporization of fusion above)


Properties of water:

1. Lower than expected vapor pressure - Hydrogen bonding makes it difficult for molecules to leave.

a. relatively high boiling point

b. relatively high freezing point

2. Relatively high heat capacity and heats of vaporization and condensation. 

-Good coolant (Steam towers at nuclear power plants, you car radiator)

-Good at helping to maintain a particular temperature

-Lake effect - large bodies of water exert a temperature modulating effect

-Spraying orange crops with water - As H2O freezes on orange, it releases heat to keep fruit at a higher T than air (freezing is an exothermic process)

-Human body - important in regulating temperature

3. Very uncommon temperature-density relation


-Ice floats providing insulating barrier over lakes and other bodies of water.

-Water at 4oC sinks to the bottom helping to stir nutrients

4. Higher than expected surface tension

surface tension - a measure of the inward force on the surface of a liquid - caused by unbalanced intermolecular forces

Demo: Tacks in water.