Physical Science

Experiment 3-2

Background Information

In Experiment 3-1 we looked for patterns in the temperature of pure water cooling over time.  There are several patterns worth noting.

But first a caution about vocabulary:  There is a common misunderstanding about the term pure water.  Pure is not a synonym for good for you but instead a pure substance is composed of a single chemical material.  Thus poisons such as arsenic and cyanide can be both pure and deadly.  This concept of pure will be developed extensively later.

Most people now view cooling to be a process of energy transfer from a warmer object to a cooler one.  The greater the temperature difference, the faster the exchange of energy.  If this is true, we would expect the exchange to slow as the materials approach uniform temperature.  A graph of the temperature of a cooling material over time would look like Diagram 1.  Compare your graph to determine if freezing water followed this pattern with the temperature reaching a plateau at the temperature of the ice and salt mixture.

Presumably you noticed an additional feature on your graph that is not present on Diagram 1.  When pure water is cooled, there is a period of time when the temperature does not change although the temperature has not gotten to the temperature of the cooler ice and salt mixture.  The temperature at which this occurred is called the freezing point.  Where there any observable changes occurring to the water during this intermission?  Since there was nothing to block the transfer of energy, presumably energy continued to be transferred from the water to the salt and ice mixture.  But since the water did not change temperature during this intermission, the freezing process likely supplied the energy for transfer.

The steepness of the slope on your graph depends of many factors.  But if all other conditions such at the apparatus and amounts of material are the same, the slope is an indication of the rate of energy transfer.  Compare the slope of the temperature verses time just before and just after freezing.

Experiment

In this experiment we want to compare the pattern in temperature as ice warms and melts. 

1. Freeze some pure water with the thermometer embedded.
2. Remove the apparatus to a warmer environment.
3. Record the temperature at a set interval, perhaps every 20 or 30 seconds until the temperature has nearly reached room temperature.
4. Construct a line graph of the temperature with time as independent variable.
5. Compare the freezing point from Experiment 3-1 with the similar plateau of the melting point.  How do the two temperatures compare?
6. Compare the slope of the temperature verses time just before and just after melting.  Presuming heat can flow equally into both the liquid and solid water, which requires more heat (and therefore has a lesser slope) to warm?
7. Compare the time needed to melt all the ice with the time needed to warm the water 10°C.  Then compare a similar measure with freezing.  Is the apparent amount of energy needed to melt ice roughly the same as that water releases when it freezes?  Should it be?