Grades: Grades 5-8
Author: Sandy Van Natta
Source: Sarquis, Mickey and Jerry, editors, Fun With Chemistry, Institute for Chemical Education, U of Wisconsin-Madison, Madison, WI 1991, pg. 147-153
Three substances are mixed in a sealed plastic bag. A reaction occurs that causes the bag to get warm and expand while the color of the contents of the bag changes.
What should students know as a result of this lesson?
What should the students be able to do as a result of this lesson?
Per Group of Students:
Before beginning, remind students to exercise safe practices in conducting the activities in this lesson.
For the Teacher:
Discuss the students' observations with the class. Students should have noted a temperature increase, a color change from red to yellow, and the filling of the bag with a gas. A few students may have also noted a temporary temperature decrease. Ask the students what they think is happening in the bag.
Assessment: Assessment is informal at this stage. Make sure all students are following the proper procedure and contributing to the discussion. Make sure they are recording their observations on their worksheets.
Challenge the students to design additional experiments to determine the combination of chemicals responsible for each change that occurred. Remind them not to change the amount of each chemical used. Tell them that the phenol red solution was made by mixing phenol red powder in water. Water can be substituted for phenol red in some of the experiments. The following experiments might be tried:
Students should record their observations after each experiment.
Assessment: Again, see that students are following proper laboratory procedures. Monitor the groups and make sure they are recording their observations.
Have students report their findings.
Ask the students what evidence they might have that a chemical reaction took place in the bag. They can cite the production of a gas (the bag expanded), a change in temperature, and a change in color. Point out that these things MAY indicate a change has taken place. Any of these observations alone may represent something else.
Discuss with students, what components they think were responsible for each of their observations. Use only as much of the following information as you think is appropriate for your students. The solution gets warm because heat is given off when calcium chloride dissolves in water (not a chemical reaction). This process is exothermic. When sodium bicarbonate dissolves in water, heat is absorbed and the solution gets colder. This process is endothermic. In the first experiment, students may not have felt this lowering of temperature since twice as much calcium chloride was used as the sodium bicarbonate. (If the two solids are not mixed well, some of the regions of the bag may feel cold.) The phenol red turns yellow when CO 2 produced dissolves in water to produce carbonic acid.
Note: the addition of some solutes to a solvent will raise the temperature of a solution, while others will have no noticeable effect. This behavior depends on the heat of solution of the solute in a given solvent. The heat of solution, i.e., the amount of heat given off or absorbed during the process of solution, is equal to the difference between the energy that must be supplied to break up the crystals of the solute and the energy that is released when the solute particles are taken into solution by the solvent. If more energy is required to break up the crystals than is released in forming the solution, the temperature will decrease. If more energy is released in forming the solution than in breaking up the crystals, the temperature will increase.
The following is for your information and may be too detailed for your students:
The gas evolved is carbon dioxide. The bicarbonate ion (HCO3-) is a weak acid and partially ionizes in solution:
HCO3- ↔ H+ + CO32-
The calcium ion (Ca2+) from the calcium chloride reacts with the carbonate ion (CO32-) to form insoluble calcium carbonate:
Ca2+ + CO32- ↔ CaCO3
The hydrogen ions react with the bicarbonate ion to yield carbon dioxide:
H+ + HCO3- ↔ H2O + CO2
The indicator changes colors because the carbon dioxide dissolves in water to produce an acidic solution. Phenol red is red in basic solutions and yellow in acidic solutions.
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
Assessment: Collect and evaluate the students' worksheets for completeness and accuracy.
In this stage, students determine the amount of temperature change in the exothermic and endothermic processes.
Note: If a temperature probe and computer are available, use the probe to record the temperature changes and have the computer graph the data. Many more data points can be taken if a temperature probe is used. Time should be placed on the X-axis and temperature on the Y-axis.
Assessment: Collect and evaluate student graphs as well as their answers to questions asked in Step 6.
Knowledge of properties of matter as well as physical and chemical changes is helpful here.
Common Core Standards:
Students should be able to recognize the difference between a physical and a chemical change. Both are present in this activity. The dissolving of calcium chloride and baking soda in water are physical changes even though they release or absorb energy. (The solids could be reclaimed if the water were evaporated.) The formation of calcium carbonate, carbon dioxide and carbonic acid are chemical changes. (New substances have been produced with a new set of properties.) Please see content information found within the lesson procedures.
If temperature probes are used in the elaboration, students will see the ease at which the equipment can accurately record the changes taking place and graph the data in an easily interpretable form. Many more data points can be obtained if probes are used instead of thermometers.
Students witness physical and chemical changes daily. Melting of ice, sugar dissolving in tea, boiling of water, etc. are all physical changes. The substances involved retain their original properties. The frying of an egg, the burning of a candle, the digestion of food, the rusting of iron, etc. are all chemical in nature. New substances are produced with a new set of properties. Students should be aware of the differences in the two types of changes and look for indicators that one or the other type of change has taken place.
Ask students to write a paragraph describing the role of each chemical in the original Ziplock activity they observed. They should include any temperature changes noted. Next, they should state whether or not a chemical change took place and the evidence they have to support their claim.
Grouping Suggestions: Be aware of students' abilities and ethnic backgrounds when choosing groups. Try to place students of varying abilities and backgrounds in each group.
Pacing/Suggested Time: The Engagement through Explanation parts of this experiment should be able to be completed in a 45-minute period. The Elaboration may take an additional 20 minutes.