Continued
Teacher-Student Interaction
During the session it is important to circulate and ask questions about freezing and thawing phenomena.
Anticipated Student Results
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Figure 10. Sample data table.
Answers to Data Analysis and Concept Development
Post-Laboratory Discussion
1. Compare student answers. Have students write their answers and percentage error on the chalk board for later discussion or have the results placed into a spreadsheet on the class computer.
2. Have a student look up the heat of vaporization of water. Lead a discussion of what this heat of vaporization indicates about the attractive forces in liquid water compared to the attractive forces in solid water. [Students will find the heat of vaporization to be 40.68 kJ/mole for water. The ratio of the heat of vaporization to the heat of fusion is about 6.75:1. The attractive forces in the liquid state must be stronger than the attractive forces in the solid state as much more energy is required to separate the liquid molecules into gas molecules.]
Assessing Laboratory Learning
1. What would be the effect of replacing the Styrofoam Ò cup with a cup made of thin copper metal? [The system would no longer be a closed system of only the warm water and the ice. The copper cup is an excellent conductor of heat (unlike Styrofoam Ò ) and heat flow will continue into or out of the copper cup into the warm water and ice, according to whether the temperature is above or below the air temperature.]
2. Why is it important that there be ice in the system at the end of the experiment, in other words, why is it important to have the system at 0 °C at the end of the experiment. [If the temperature moves above 0 °C then the ice has all melted and some of the heat energy from the hot water has been used to actually warm the cold water which came from the ice. The arithmetic problem of solving this system is somewhat more complicated than for the case used.]
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