Lab #23.4 Heat: Cooling by Boiling

In this activity, students will see that water will boil when pressure is lowered.

Discussion
Whereas evaporation is a change of phase from liquid to gas at the surface of a liquid, boiling is a rapid change of phase at and below the surface of a liquid. The temperature at which water boils depends on atmospheric pressure. Have you ever noticed that water reaches its boiling point in a shorter time when camping up in the mountains? And have you noticed that at high altitude it takes longer to cook potatoes or other food in boiling water? That’s because water boils at a lower temperature when the pressure of the atmosphere on its surface is reduced. Let’s see!

Required Equipment

400-mL beaker, thermometer, vacuum pump with bell jar

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Student Worksheet   Teacher Notes