Energy and Changes of State

DeYoung, Robert
Lee, Lucile
Ferrell, Alisa

Objectives Students will: 1) gain an understanding about energy and changes of state; 2) read a thermometer; 3) collect and record data; 4) construct a graph; and 5) interpret graphed data. Equipment and Material Five grams acetamide (or another low melting point solid) per group 250 mL beakers (2/group) 18x150 mm test tube (1/group) test tube holder (1/group) hot plate (1/group) A Bunsen burner, ring stand, iron ring, and wire gauze may be used. Ice cream maker Ice cream mixture ingredients Two quarts whole milk Two cans condensed milk (Eagle brand) One cup sugar One half pint whipping cream Two tablespoons vanilla flavoring One quarter teaspoon salt Four eggs Ice Rock salt thermometers (2/group plus 2 for ice cream procedure) Graph paper (1/student) Recommended Strategies Prepare an ice cream mixture. Recipe: Combine eggs, cream, sugar, and vanilla in bowl and mix thoroughly with mixer. Pour into can, add condensed milk and stir well. Add dairy milk to fill line on can and stir. Makes approx. 4 quarts. Two thermometers will be used. One thermometer will measure the temperature of the ice cream mixture. The second thermometer will measure the temperature of the ice water. Select two students to collect and record the data on the chalk board. Construct a data table for recording these temperatures against time. Before setting up the ice cream maker, begin by measuring the temperature of both the ice cream mixture before placing it in the ice water and the ice before adding salt. Set up the ice cream maker and begin recording the time in one minute intervals of both temperatures. After the initial rapid temperature drop, the time interval may be increased to two or three minutes. While the data is being collected, the class will do the following activities: 1) Observations of changes of state. Procedure: a) Each group of students should obtain the following: one 250 mL beaker, one test tube with 5 grams of acetamide, one test tube holder, one hot plate. b) Add 150 mL of water to a 250 mL beaker. c) Heat water until hot. d) Place test tube with acetamide in the beaker of hot water. Record observations. e) When no further changes are observed, remove the test tube from the hot water. Let the test tube cool. Record observations. (Both procedures show changes of state. Discuss in terms of energy gained and lost and changes in particle motion.) 2) Effect of salt on ice. Procedure: a) Each group of students should obtain the following: two 250 mL beakers, two thermometers. b) Add equal amounts of ice to both beakers. c) Record the temperature of the ice in both beakers. d) Add salt to the ice in one beaker. e) Continue recording the temperature of the contents of both beakers until there are no changes in temperature. f) Organize the data in a data table. (The salt will cause the ice to melt faster. The temperature of the salt and ice water mixture will drop below zero degrees Celsius. Discuss in terms of heat transfer and particle motion.) Construct a graph from the data collected. Discuss the graphed data by relating it to the demonstration. Also use the class as a model to show energy involved in changes of state at different points on the graph. (Let the students represent particles of matter. Sitting in their chairs, the students represent the orderly arrangement of particles in a solid. An increase in the temperature of a solid will increase the vibrations of the particles and thus their kinetic energy. As energy is added at the melting point, only the potential energy of the particles is increased such that they will break loose from their fixed positions and become more disorganized. Use the students to illustrate this. In the liquid state energy added increases the kinetic energy. The particles are free to slip and slide over one another and they will move back and forth at a faster rate.) For a more advanced presentation, refer back to the demonstration and apply heat calculations to the melting of the ice and to the changes of state. Apply freezing point depression calculations to the salt and water combination.
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