Phase Changes

Melting, freezing, boiling or evaporation and condensation (the reverse of evaporation) can be described in terms of heat transfer. These processes, in which one substance changes from a solid to a liquid, liquid to gas, etc., are called phase transitions. Ice, water and water vapor are said to be different phases of water. For interactions involving phase changes, the basic description in terms of heat transfer remains the same. The NET heat exchanged is zero. An alternate way of describing the interaction is that the heat lost by one part of the system equals that gained by the other. Phase transitions complicate the situations slightly. The following experiment and exercise illustrate the transfer of thermal energy when phase transitions are involved.

Experiment

Acquire the data.
Start Logger Pro™ by double clicking on the "Phase Change Lab" icon in the PHY 184 folder.  Calibrate the temperature sensor as you did last week.
Place 20g of room temperature water in a Styrofoam cup and determine the temperature of the water by collecting a set of data and then using the Statistics option to find the best estimate of the starting temperature. Record the temperature with its uncertainty.
Place ice at 0oC in the cup. Dry the ice with paper towels before placing it in the cup. Use a lot of ice -- about one or two times the volume of the water.
Stir continuously with a stirring rod, while measuring the temperature with the temperature sensor.
Notice that the temperature is going down. When the temperature STOPS going down (at approximately 0oC ), immediately pour the water into a graduated cylinder, leaving the ice behind. It is convenient to use a funnel when pouring the water into the cylinder -- any ice that spills will be trapped in the funnel.
Record the volume of water in the graduated cylinder and determine how much ice melted.
Repeat the experiment with 40, 60, 80 and 100 g of room temperature water. Be sure there is always plenty of ice in the cup during melting.
For each experiment, calculate the heat lost by the room temperature water and the heat gained by the ice.
Plot a graph of the heat gained by the ice versus the mass of ice melted using Graphical Analysis™.
The graph should be approximately a straight line. Find the best-fit straight line and determine the slope of that line. What does the fact that the line is straight indicate? How do you interpret the slope?

The amount of heat required to melt one gram of ice is called the latent heat of fusion of ice. Careful measurements give 80 cal/g for the latent heat of ice.

Thought Experiment

The effect of heat transfer on objects undergoing a phase transition is different from its effect on objects undergoing a temperature change. Transfer of heat is still thought of as the cause of what happens, but the effect of the heat transfer is different than usually expected. Instead of raising the temperature of the ice, the heat melts the ice without changing its temperature. There is another difference as well, In interactions with no phase change, the heat received by the cold object is evenly distributed -- shared equally among all of the grams (mass) of the cold substance. In the case of melting, all of the heat goes to just the ice that melts. No heat goes to the unmelted ice. The following exercise is a "thought experiment" that has you track how ice is melted by following quanta of heat. Exercise: A 25 g ice cube is put in 200 g of water at 20oC . What will be the final temperature? Reason out the solution instead of calculating it directly. To do this imagine sending heat from the hot material to the cold material in small amounts until equilibrium is reached. Since ice and water can only coexist in equilibrium at 0oC , all of the ice must be melted before the temperature of the cold material can be raised above 0oC. At the outset, it is not obvious whether all of the ice will melt.

Keep track of your calculations by making a table like the following.

Heat Transferred Temp of added water Mass of unmelted ice Mass of melted ice Temperature of melted water
0 cal 20oC 25 g 0 g 0oC
80 cal 19.60oC 24 g 1 g etc.

Continue the process until you reach equilibrium. What is the final temperature?



Web Author: Barney Taylor, MUH