THE RACE TO FREEZE

Objective: Explore the exponential nature of cooling liquid.

Apply extrapolation methods to answer a question.

Supplies: CBL temperature probe

Equal containers of very hot and room temperature water.

A sub-freezing cooler. (Outside on a cold winter day is best.)

According to folklore, a pan of boiling water will freeze faster than a pan of room temperature water when placed outside on a cold winter day. Is this really true? If so, why?

Instructions:

1. Prepare two CBL temperature probes to take one reading per minute.
2. Fill two equal size and shape container. One with room temperature water and one with boiling water. Start taking measurements.
3. After 30 minutes take retrieve data.
4. Create an exponential model to each set of data.

Questions:

1. Use your models to extrapolate which one will reach the freezing point first.
2. If the boiling water is projected to reach the freezing point first explain how this could happen.
3. If the other water is projected to reach the freezing point first does the guarantee it will freeze first?
4. Based on your observations provide some possible ideas for further exploration.

To the instructor:

• If you wish to use extrapolation then choose a time interval that does not allow the water to freeze
• The colder the environment the better.
• Container shapes may change the results. Flat, shallow pans will probably work the best.
• Possible extension: Allow the water to sit until frozen solid. You will have to adjust the sampling rate. Students will see the exponential cooling, but will also see the phase change. In this case the first water to be less than 32 degrees would be the first to freeze solid.

David Anderson 2002-08-01.