Materials

Flask, round bottom, 500-mL

Rubber stopper for flask, solid

Ring stand

Ring of appropriate size to hold inverted flask

2 or 3 Ice cubes

1 Oven mitt to fit hand

Safety goggles

Hot plate or burner

Safety and disposal

A round bottom flask is less likely to break under the reduced pressure produced in this demonstration. The boiling water and steam will cause burns. Wear the oven mitt for protection. At the end of the demonstration, gently pry the stopper off and pour the water down the sink. The stopper will be somewhat difficult to remove as the pressure on the outside of the flask is substantially greater than the pressure within the flask.

Procedure

Place 200 mL of water in the 500-mL round-bottom flask. Heat the water to a vigorous boil in full view of the class. Using the oven mitt, remove the flask and stopper it immediately. Let the water cool for (at most) a minute. Hold the flask with the mitt. Invert the flask into the ring on the ring stand. Have a student come up in front of the class. Have the student put on safety goggles and then take an ice cube in each hand and rub them on the rounded bottom of the flask. The water will soon (15 to 30 seconds) begin to boil vigorously. The demonstration can be repeated for up to 10 minutes.

Background

Placing a stopper on the flask shortly after the boiling water is removed from the heat source, most of the Òair spaceÓ above the water is filled with water vapor, not air. Ice placed on the top of this space causes water vapor at (relatively) high temperature to condense. When removed from the vapor state, the contribution of water vapor to the total pressure inside the flask is lowered. When the total pressure inside the flask goes below the vapor pressure of water (the warm water), that water begins to boil. So, even though the water is considerably below the normal boiling temperature, it does boil at the ambient pressure inside the flask.

Reference

Taken from Doing Chemistry, Experiment D24.