The Idea
In this project, you use a piece of ice to cause a container of very warm water to start boiling. This is definitely not what most people would expect.
What You Need
- Pyrex Erlenmeyer flask (or a Florence flask with a partially flat bottom)
- rubber stopper (without holes)
- beaker tongs (or oven mitt)
- water
- few ice cubes
- hotplate
- ring stand with a ring small enough to support the flask upside down
- optional: bell jar and vacuum pump, beaker
Method
- Partially fill the flask with water. There should be a gap of an inch or two above the water level when it is upside down.
- Place the flask on the hotplate, as shown in Figure 94-1.
- Keep the flask on the hotplate until the water boils.
- Remove the flask from the hotplate.
- Without delay, put the rubber stopper (snuggly) in the flask, carefully turn it upside down, and place it in the ring. Use an oven mitt or tongs to handle the flask.
- The water (having cooled slightly) should now be still quite hot, but no longer boiling.
- Position a few ice cubes on the flat of the flask and observe.
Expected Results
Shortly after the ice cubes are placed on the bottom of the flask, bubbles start to emerge from the top (near the stopper). These bubbles continue and the water in the flask boils for a short time. Careful observation should convince anyone watching that the bubbles are coming from the liquid itself and are not a leak in the rubber stopper. See Figure 94-2.
Why It Works
When a vapor (such as the air/water vapor mixture) is cooled, it contracts. As the volume of gas above the hot water decreases, the pressure also decreases. Water boils at 100°C (212°F) at standard atmospheric pressure, but at a slightly lower temperature when the pressure above the liquid is reduced.
Other Things to Try
You can try this another way:
- Fill a beaker with water.
- Place it on a hotplate and bring it to a boil.
- Remove the beaker from the hotplate and let it cool until the boiling just stops.
- Place the beaker in a vacuum chamber (bell jar on a vacuum plate).
- Attach and turn on the vacuum pump to evacuate the chamber.
- Compare the effect of directly applying a vacuum to the reduced pressure caused by the ice.
The Point
Water boils at a lower temperature when the pressure of the air above it is lowered.
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