Explain how a liquid can have a boiling point of 80 oC at sea level and a boiling point of 74 oC at an altitude of 1000 m?

Because there is a lower air pressure.

"The boiling point corresponds to the temperature at which the vapor pressure of the liquid equals the atmospheric pressure."*

Think about the energy needed to change from a liquid to a gas a you raising your arm up Now get a friend to push your arm down representing air pressure. The lighter the push (lower air pressure) the easier it is to raise your arm. You require less energy to overcome the 'air pressure'. For a liquid this corresponds as less energy (lower temperature) required to change state.

*Phase Changes: Vapor Pressure (http://www.chm.davidson.edu/chemistryapplets/PhaseChanges/VaporPressure.html)

Think about why your ears pop sometimes!