Materials Included In Kit

Additional Materials Required

Safety Precautions

Calcium chloride is slightly toxic. It readily absorbs moisture from the air. Keep the container tightly closed when not in use. Sodium bicarbonate slowly decomposes in moist air. Store in a tightly closed container. Phenol red is a dye solution and will stain skin and clothing. Avoid contact of all chemicals with skin and eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Be careful to mix the chemicals in the amounts listed in the procedure. Adding too much of the solids may result in excessive release of gases that are difficult to contain and may cause chemical splashing. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All solutions may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs, or for 15 groups of students, to complete at least six different control experiments. Encourage students to brainstorm and pool results so that all nine possible controlled interactions are accounted for. Challenge students to determine what combinations of chemicals are responsible for the observed changes, using the least number of tests possible. The experimental work for this lab can reasonably be completed in one 50-minute lab period.
• If teaspoon and half-teaspoon measuring spoons are not available, plastic teaspoons will work. Do not increase the amounts of reagents used.
• It is not necessary to understand the precise chemical events that take place in this reaction in order to appreciate the essence of this activity, which is the nature of scientific inquiry.
• This lab is a fun and easy way to introduce students to the study of chemistry—students can actually hold an experiment in their hands to see and feel what chemistry is all about. The experiment allows students to practice key skills that are the backbone of curriculum standards for chemistry and science education. Students learn to (1) observe phenomena and ask questions, (2) design a scientific investigation to answer their questions, (3) collect data and (4) analyze and summarize their data.
• The following information summarizes the chemistry behind the observations. Do not present this information to the students prior to the activity! Calcium chloride produces heat when it dissolves in water, while baking soda absorbs heat as it dissolves. Calcium chloride, baking soda, and water combine to produce carbon dioxide gas. Phenol red is an acid–base indicator that is yellow in acid solution (pH < 6.8) and red in basic solution (pH > 8.4). Baking soda is a base, so it maintains the bright red (basic) color of phenol red. As sodium bicarbonate reacts with calcium chloride in water, acidic substances are produced and the color of the phenol red indicator changes from the basic form to its yellow, acidic form. Carbon dioxide is one of the acids produced—it dissolves in water to form carbonic acid, H₂CO₃. As carbon dioxide is allowed to escape, the color may revert to an intermediate color, orange.
• The products of the overall reaction include sodium chloride (NaCl), table salt; calcium carbonate (CaCO₃), the main component of chalk; and carbon dioxide (CO₂), the metabolic “waste” gas that we exhale during respiration.
• The events that take place in the zipper-lock bag are part of a dynamic and complex reaction. Intermediate products may be formed and then react further to produce the final composition of the product mixture. The following (almost certainly over-simplified) equation provides a snapshot of the initial and final composition and does not show all of the possible intermediate products.
  {11989_Tips_Reaction_1}
  This experiment is designed as a first experiment for the chemistry lab and does not require any knowledge of chemistry content beyond the scientific method. Teachers who have already covered the composition of matter and physical and chemical changes, however, may find it natural to use this experiment as a springboard to discuss these additional concepts as well. Which observations are due to physical changes and which to chemical changes? Challenge your students to propose additional experiments to prove that dissolving a solid in water represents a physical change, but that reacting the two solids together in water produces a chemical change.

Sample Data

Answers to Questions

1. Based on the results of the control experiments, what interaction among the substances seems to be responsible for the observed temperature change in the overall reaction?

   Mixing calcium chloride with water (either alone or in a solution of phenol red) produced the same temperature change as was observed for the overall reaction. A great deal of heat was produced and the mixture felt very hot to the touch.

2. Was there a temperature effect observed in any of the individual control experiments that was NOT observed in the overall reaction of the chemical substances? Explain.

   When sodium bicarbonate was mixed with water (either alone or in a solution of phenol red), the solid dissolved partially and the solution felt slightly cold to the touch. This temperature decrease, due to the solution absorbing heat energy, was not observed in the overall reaction, which was dominated by the temperature increase discussed in response to Question 1.

3. What color change was observed in the overall reaction of the substances? Do the control experiments provide any evidence concerning the interaction(s) responsible for the observed color change?

   In the overall reaction in Part A, the color changed suddenly from red to yellow. Mixing either of the solids with phenol red did not produce a color change. A slight color change was noted when phenol red was mixed with water. The red color faded to a paler shade of red-orange. The control experiments suggest, but do not prove, that the color change to yellow in the overall reaction is due to reaction of calcium chloride and baking soda and the production of a new substance with new acid–base properties.

4. Does the formation of gas bubbles occur independently of the observed temperature and color changes? Explain.

   The formation of gas bubbles occurs independently of the color change. Bubbles were observed in a controlled experiment with three substances—calcium chloride, baking soda, and water—in the absence of the phenol red indicator. It is possible, therefore, that the gas bubbles make the solution turn yellow. Gas bubbles were always observed along with the large temperature increase, which was also observed when calcium chloride alone reacted (dissolved) with water. It is not possible, therefore, to conclude whether a temperature increase was also associated with the production of the gas.

5. What control experiments were done to evaluate if a liquid is necessary for the observed effects in Part A? Does any reaction occur in the absence of water?

   The two solids were mixed without any liquid (phenol red or water) present. No evidence of any reaction was observed. The two solids seemed to retain their individual appearance in the mixture of the two.

6. Is there any evidence that a new chemical substance is produced in the overall reaction of the substances mixed in Part A? Explain. What interaction among the components must be responsible for the new substance?

   The strongest evidence for the production of a new chemical substance comes from the formation and release of a large amount of gas bubbles when calcium chloride and baking soda are mixed together in a liquid. Since the fizzing is not observed when either solid alone dissolves in water, even though the temperature changes, there is a good likelihood that the gas represents a new chemical substance. In order to answer this question precisely, however, it would be necessary to isolate the gas and study its properties. If it has different properties than any of the other substances, then it is a new chemical substance. The color change to yellow also suggests, however, that a new substance is produced (it may be the gas!) which has different acid–base properties than the reactants. The new substance must be formed from “rearranging” the elements in the two solids, because either solid alone with water or phenol red failed to give either gas bubbles or a color change.

7. Let’s assume that the chemical identity of calcium chloride is not changed when it is mixed with water. Suggest an experiment that could be done to prove or disprove this hypothesis.

   If calcium chloride is still present in solution after the solid has dissolved in water, it should be possible to isolate it after all of the water has been removed by evaporation. Heat the solution until the water has evaporated and then test its physical and chemical properties (for example, would it still react with baking soda in water?).

8. Temperature changes are sometimes used as evidence to indicate that a chemical reaction, which produces a new chemical substance, has occurred. Comment on the suitability of this observation as a “test” of a chemical reaction.

   Temperature changes alone do not provide clear evidence that a chemical reaction has occurred. Temperature changes can also accompany a purely physical change, such as when a solid dissolves in water.

Introduction

“The world is full of obvious things, which nobody by any chance ever observes.”

This quote is from Sherlock Holmes, talking about the importance of careful observation in detection. Careful observation is also the foundation of chemistry as an experimental science, leading us to question what we have observed. How, what, why? The answers to these questions are sought in experiments, which may be described as observations made under controlled conditions. Observation and experiment—twin pillars of the scientific method.

Concepts

• Scientific method
• Chemistry
• Observation
• Experiment

Background

The scientific way of knowing, often called the scientific method, is sometimes presented as a rigid and linear sequence of events. The scientific method, however, is not a rigid path, it is a dynamic and somewhat unpredictable process—a process of discovery! Discovery begins when we make observations and then try to understand what we have observed by asking key questions and proposing possible answers. The process of discovery continues when we design and conduct experiments to test whether our answers to these questions are valid.

Chemistry is defined as the study of matter—what a substance is made of, its structure and properties and the changes that it undergoes. Observations of the properties of matter are often complex, arising from the interaction of many different factors or variables. Experiments need to be designed in such a way that the effects of different variables on the behavior of a substance can be studied independently. This is done by making observations under controlled conditions, where only one variable at a time is changed. Controlled experiments make it possible to separate or isolate the factors that are responsible for a given observation in a complex series of events.

In this laboratory experiment, three substances—solid sodium bicarbonate, solid calcium chloride and a solution of phenol red in water—are placed in a closed container. Upon mixing, a complex series of changes is observed. Observations include temperature changes, color changes and changes in state. How can we learn more about the individual interactions responsible for each observation? Imagine that each substance in the mixture represents a variable. By changing only one variable at a time, it should be possible to determine the contribution of each substance to the changes observed for the overall reaction.

The three substances that are used in this experiment are all common chemicals. Sodium bicarbonate, or baking soda, is used as a food additive in baking. It is also used as a natural deodorant to absorb and remove odor-causing chemicals in refrigerators and in carpets. Calcium chloride (“road salt”) is a salt-like compound that is used as a de-icer for sidewalks and roads. Phenol red is a naturally occurring dye that is used as an indicator—it changes color under different conditions. Note: Because phenol red is a solution of the dye compound dissolved in water, it is actually composed of two substances. Thus, water must be considered as a fourth substance (variable) in the overall reaction, and its effect should also be examined!

The purpose of this experiment is to make observations about the changes that occur when several substances are mixed and to design controlled experiments to identify the substances responsible for each of the observed changes.

Materials

Safety Precautions

Calcium chloride is slightly toxic by ingestion. Phenol red is a dye solution and will stain skin and clothing. Avoid contact of all chemicals with skin and eyes. Be careful to mix the chemicals in the amounts listed in the procedure. Adding too much of the solids may result in excessive release of gases that are difficult to contain and may cause chemical splashing. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part A. The Overall Reaction

1. Label two 100-mL beakers “calcium chloride” and “sodium bicarbonate,” respectively. Fill each beaker about one-half full with the appropriate solid.
2. Obtain both a teaspoon and half-teaspoon scoop to dispense the calcium chloride and sodium bicarbonate, respectively. Store the scoops in their respective beakers and do not mix them up.
3. Obtain 30 mL each of phenol red solution and water in separate, labeled 50-mL beakers. Use a graduated cylinder to dispense the liquids as needed in the following steps.
4. Observe and describe the appearance of calcium chloride, sodium bicarbonate, and phenol red solution. Record the observations in Data Table A.

5. Lay a zipper-lock plastic bag flat and place the following three substances in separate locations in the bag.
   1. 1 teaspoon of calcium chloride
   2. ½-teaspoon of sodium bicarbonate
   3. 5 mL of phenol red indicator solution
6. Squeeze out as much air as possible from the zipper-lock bag and seal it. Allow the contents to mix thoroughly.
7. Carefully observe (by means of sight and touch) the changes that take place in the zipper-lock bag. Record all observations in Data Table A. Note: If the bag gets too tight due to the build-up of gas pressure, open the bag and then reseal it. Do NOT open the bag near your face or the face of any of your labmates!
8. After all reactions have subsided, the contents of the bag can be rinsed down the drain under running water. Rinse out the bag with water and dispose of the bag in the trash.
   What questions arise concerning the changes that were observed in the bag?
9. Think of at least four questions that could be investigated to determine the individual interactions that are responsible for the observed changes. For example, is water or a liquid necessary for the reaction to occur? Write your questions down in the space provided in Data Table A.

Part B. Control Experiments

10. Design and carry out a series of controlled experiments to determine which combinations of substances are responsible for each of the observed changes. Use the same quantities of chemicals as in Part A and carry out the reactions in separate zipper-lock plastic bags. If water is tested in a controlled experiment, use the same amount of water as of phenol red solution in Part A.
11. Fill out Data Table B to indicate the substances used in each control experiment and the resulting observations. Space has been provided for nine control experiments. Do as many experiments as needed to identify the substances responsible for the observed changes—it may not be necessary to conduct nine different tests!

Student Worksheet PDF

11989_Student1.pdf