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.