Summer Research Program for Science Teachers

Ben Stevens

Manhattan Center for Science and Math

2002

What factors affect when water boils?

Unit: Phases of matter, matter and energy

Materials: round bottom flask with stopper (half-filled with water), Bunsen burner, ring stand, thermal gloves, Ziploc bag with ice, duct tape, package of rice or noodles with special directions for high altitude cooking

Procedure:

Motivation: Elicit from students what makes water boil. The students will invariably respond you heat water to make it boil. Then tell the students that you can make water boil with ice, then perform the demonstration described below.

Demonstration: Bring the water in the round bottom flask to a rapid boil (no stopper). Remove heat and stopper the flask (secure stopper with duct tape). Flip round bottom flask upside-down, water is no longer boiling. Place bag of ice on top of round bottom flask and water will begin to boil again.

During Demonstration: Ask students to describe what is happening to the motion of the molecules while the water while it is being heated. Have the students diagram the process in their notebooks and record their observations.

After Demonstration: Elicit hypotheses from students as to why the water boiled again when ice was placed on top of the flask. The following may be used for clues to help students: Reference Table H of the chemistry reference tables (Vapor Pressure of Four Liquids) or a food package with directions to boil contents longer at high altitudes.

Key points for students: Substance boils when its vapor pressure equals the atmospheric pressure. Normal boiling point is the boiling point at standard pressure. Water boiled at less than normal point because the ice lowered the atmospheric pressure.

Homework and Extensions: Atmospheric pressure and weather; how does measuring atmospheric pressure to predict impending storms? Atmospheric pressure in extreme conditions; what dangers do mountain climbers and scuba divers face because of atmospheric pressure? How do they overcome these dangers?

National Science Standards Addressed: A2, understandings about scientific inquiry. B2, structure and properties of matter. B5, conservation of energy and increase in disorder.