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Ziplock Chemistry

Grades: 5-8
Author: Sandy Van Natta
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Abstract

Module Description

This module provides participants with an activity that can be used to teach their students both physical and chemical changes as well as demonstrate exothermic and endothermic processes. This simple experiment yields many interesting results. As baking soda and calcium chloride dissolve in a water-based indicator, temperature changes occur. As the dissolved chemicals mix in solution, carbon dioxide gas is created and the Ziplock bag swells. The gas mixes with water to produce a weak acid that changes the indicator color from red to yellow.

Experiments are designed, careful observations are made, and data recorded. Conclusions are drawn based upon the data.


Objectives

  • Participants will explain the difference between a physical and a chemical change.
  • Participants will list the factors observed that may indicate a chemical change is taking place.
  • Participants will identify the chemicals needed to produce each change in this lesson.
  • Participants will develop and write a simple lesson implementation plan based on physical and chemical changes.

Materials

Per Group:

  • Small box of sodium bicarbonate (NaHCO3) - commonly known as baking soda
  • 3/4 cup of calcium chloride (CaCl2) pellets or chunks about 2 to 3 mm in diameter
  • 60 - 70 mL of phenol red solution (available from scientific supply houses)
  • 100 mL tap water
  • 2 small plastic teaspoons
  • 2 small plastic bottles
  • 2 10 mL containers - calibrated pill vials work well
  • Plastic cup
  • 8 quart sized Ziplock bags
  • 8 small plastic bags - 2in x 3in (approximately 5 cm x 7.5 cm)
  • Goggles for each student
  • Thermometer or temperature probe and computer
  • Stopwatch or clock with a second hand

Procedures

Engagement

The presenter will want to engage the participants in an initial discussion of physical and chemical changes. Have them make a class list of all the factors they can think of that might indicate a chemical change is taking place. The discussion may lead to the difficulty participants have in demonstrating these factors safely to their students. Tell the participants that they are going to perform the experiment described in Engagement phase of the student directions. Ask them to make all possible observations and compare their observations to the list of factors they made as a class.

Assessment: The evaluation is informal. Monitor the participants' discussions for understanding of physical and chemical changes. Encourage all members of the group to participate.

Exploration

Remind participants to exercise safe practices when conducting the activities. Participants will be performing the activities described in Exploration phase of the student directions. Participants should be asked to design additional experiments to determine the combination of chemicals responsible for each change that occurred. If needed, 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. Make sure the group lists the following experiments to try:

  • Mixing calcium chloride with phenol red solution
  • Mixing calcium chloride with water
  • Mixing sodium bicarbonate and phenol red solution
  • Mixing sodium bicarbonate and water
  • Mixing sodium bicarbonate, calcium chloride, and water

In addition, the group may wish to allow the water to evaporate from the calcium chloride and water or the sodium bicarbonate and water. (Evaporation will take more than a day. You may want to set bags of sodium bicarbonate and water and calcium chloride and water up several days in advance and allow them to evaporate before starting your session.) These two experiments will help confirm that a physical change was taking place when a temperature change was noted.

Assessment: Monitor the groups' work. Make sure all safety precautions and procedures are followed. Check to see that groups are recording their observations.

Explanation

Have participants report their findings. Involve the entire group in a discussion of the physical and chemical changes observed. Participants may need help in identifying the temperature changes with the formation of solutions of calcium chloride and sodium bicarbonate as physical changes rather than chemical. If the evaporation step was not performed earlier, it could be suggested here. Following evaporation, reusing the chemicals in the first Ziplock experiment would show the chemicals were not altered during the solution and evaporation processes. The formation of a gas, and the color change produced by the indicator; help identify the chemical changes that took place.

Assessment: Monitor the discussion. Ask participants to give examples from their own teaching relating to ways to teach physical and chemical changes or exothermic and endothermic processes.

Elaboration

This part of the module allows for a quantitative collection of data as well as further investigation of the energy changes in exothermic and endothermic processes. A thermometer will work if temperatures are recorded every minute for several minutes. However, a better data set and graph will be produced if a temperature probe is used. Accurate temperature readings can be taken over shorter intervals of time. After completing data tables and a graph, ask participants to label each solution process as endothermic or exothermic.

Assessment: Look at, and compare, the participants' graphs. Make sure they have identified each process correctly as exothermic or endothermic. (Calcium chloride solution – exothermic; baking soda solution – endothermic) Monitor any discussion and encourage everyone to participate. Analyzing the graphs is important here. If enough data points are taken, the calcium chloride graph will show an initial temperature increase, then a decrease as the contents cool back down to room temperature. The graph will flatten out once room temperature has been reached again. The sodium bicarbonate graph will show an initial decrease in temperature followed by a gradual increase as the contents return to room temperature. Again, once room temperature has been reached, the graph will flatten out. Have participants discuss what is happening in each part of the graph.

Build in some time for participants to develop their implementation plans focused on teaching chemical/physical changes to their students.


Rationale

This lesson allows participants to distinguish between both physical and chemical changes in matter as well as determine the role each chemical plays in the changes. Inquiry is used as participants design their own experiments to determine what is happening when different components are mixed. Participants identify temperature and color changes as well as gas production. However, they will learn that each of these alone may, or may not, indicate that a chemical change has taken place.


Science Standards

Content, Technology, and Professional Development:

NSES Standard A: Science as Inquiry: As a result of activities in grades 5–8, all students should develop abilities necessary to do scientific inquiry:

  • Abilities necessary to do scientific inquiry

NSES Content Standard B: Physical Science: As a result of their activities in grades 5-8, all students should develop an understanding of:

  • Properties and changes of properties in matter

NSES PROFESSIONAL DEVELOPMENT STANDARD A: Professional development for teachers of science requires learning essential science content through the perspectives and methods of inquiry. Science learning experiences for teachers must:

  • Involve teachers in actively investigating phenomena that can be studied scientifically, interpreting results, and making sense of findings consistent with currently accepted scientific understanding.

Best Teaching Practices

  • Learning Cycle
  • Discussion
  • Questioning
  • Inquiry

Time Frame

This module can be conducted in a 45-minute period.


Preparation

Label one spoon sodium bicarbonate and the other calcium chloride.

Measure 10 mL of water into one of the vials. Make a mark at the 10 mL level with a permanent marker. Repeat for each of the vials to be used.

If powdered phenol red solution is purchased, prepare a solution by mixing a pea-sized amount of phenol red powder in 2L of tap water. The solution should be cherry red. If the solution is orange or yellow, add a small amount of dilute basic solution such as ammonia or NaOH in water.

Place the phenol red solution in plastic bottles and label.


Safety

Goggles should be worn during this experiment.

Avoid contacting the eyes with any chemical used in this experiment. Should contact occur, rinse the affected area with water for 15 minutes, and seek medical attention.

All solutions in this activity can be diluted with water and flushed down the drain. Unused indicator solution can be saved for future use.

Please see the MSDS sheets for each chemical for more information. Download a copy of the MSDS at http://www.flinnsci.com


Assessment

Have each group of participants suggest an additional activity that could be used to teach physical and chemical changes or exothermic and endothermic processes in their classrooms.


Explanation of Science

Evidence that might indicate that a chemical reaction took place in the bag may be 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 participants, what component they think was responsible for each of their observations. Use only as much of the following information as you think is appropriate for your participants. 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, participants 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 the CO2 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 or may not be needed for the discussion with participants:

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-

Participants that teach the lower grade levels may simply want to know that carbon dioxide is the gas produced. When carbon dioxide reacts with water, it forms a weak solution of carbonic acid. The acid is responsible for the color change of the indicator.


Handouts

None available for this module.


Extensions

Have participants identify physical and chemical changes in their daily lives. Try and relate some of these changes to an earth science or biological science topic.

Have participants hypothesize what causes the temperature changes in commercial chemical hot and cold packs (that can be purchased inexpensively at drug and sporting goods stores).


Lesson Implementation Template

Download Lesson Implementation Template: Word Document or PDF File


Equity

Seat everyone in groups with diversity in mind. Make sure each person participates in the discussions.


Resources

None available for this module


References

Sarquis, Mickey and Jerry, editors, Fun With Chemistry, Institute for Chemical Education, U of Wisconsin-Madison, Madison, WI 1991, pg. 147-153