Color Change Challenge
Student Laboratory Kit
Materials Included In Kit
Bleach (sodium hypochlorite, 5%) solution, 30 mL*
Potassium iodide, KI, 5 g*
Sodium thiosulfate solution, Na2S2O3, 1 M, 40 mL
Starch, 1 g*
Labels, 1 sheet of 80
Pipets, Beral-type, thin-stem, 60
Pipet holders, 15
Toothpicks, 1 box
*See Prelab Preparation.
Additional Materials Required
Water, distilled or deionized*
Beakers, 100-mL, 3*
Beaker, borosilicate glass, 100-mL*
Hot plate or microwave oven*
Spot plate or reaction plate, 12- to 15-well
Stirring rods, 3*
*for Prelab Preparation
Potassium iodide solution, 0.1 M: To make 0.1 M solution needed for the experiment, weigh out approximately 1.6 g of potassium iodide and dissolve in 100 mL of distilled or deionized water.
Bleach solution, 50%: Add 30 mL of 5% sodium hypochlorite solution to a 100-mL beaker. Stir in 30 mL of distilled water. Label.
Starch solution, 2%: (Best prepared one to three days before the lab.) Heat 50 mL of distilled water to boiling in a 100-mL borosilicate glass beaker. Add 1 g of starch to a clean 100-mL beaker. Add a few mL of the hot water to the starch in the beaker. Stir. Continue to add the remaining hot water slowly to the beaker, stirring until the mixture appears uniform. Allow the colloidal starch solution to cool slowly to room temperature before using. Label.
- Obtain 60 thin-stem Beral-type pipets and the sheet of labels. Fold one adhesive label around the stem of each pipet (see Figure 1). Label each set of four pipets A, B, C and D.
- Open each pipet holder (cassette case) as in Figure 1.
- Fill each pipet with approximately 2 mL of the appropriate solution, using the following chart. (The capacity of each pipet is approximately 4 mL, so fill each bulb about half way.)
- Place the pipets stem side up in the pipet cases for storage.
Sodium hypochlorite solution (bleach) is a corrosive liquid, reacts with acid to evolve poisonous chlorine gas and is moderately toxic by ingestion and inhalation. Avoid contact with organic material. Sodium thiosulfate pentahydrate is slightly toxic by ingestion and a body tissue irritant. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing the disposal of laboratory waste. All solutions used in this activity may be disposed of down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b. The small amounts in the spot plates may be blotted up with several thicknesses of paper towel and disposed of according to Flinn Suggested Disposal Method #26a.
- Enough materials are provided in this kit for 30 students working in pairs, or for 15 groups of students. This laboratory activity can reasonably be completed in one 45- to 50-minute class period. The prelaboratory assignment should be completed before coming to lab, and the questions may be completed the day after the lab.
- The solutions used in this activity have a poor shelf life and are best prepared one to three days before the lab. A 0.1 M potassium iodide solution and starch solution will grow mold. A 1 M sodium thiosulfate solution is subject to bacterial decomposition. The 50% bleach solution will keep for seven to ten days.
- Students may prepare their own pipets of each solution. Have identical beakers of each solution labeled A–D available in a central area. Advise students to fill the pipet bulbs only halfway.
- Only one sequence will yield the correct results. Avoid telling students this, however, so they will test all possible combinations.
- The intensity of the orange color depends on the number of drops used. When adding the 50% bleach solution to the 0.1 M potassium iodide solution, a yellow color will appear first. If the maximum of five drops is used, the solution will turn orange.
- Economical spot plates (Flinn Catalog No. AP6399) allow students to easily see the reaction taking place. Reaction plates (Flinn Catalog No. AP 1447) may also be used.
- Assure students that even though the solutions are unknown to them, the instructor knows what they are, and mixing them according to the general procedure is safe. Using trial and error to mix completely unknown solutions of course is not safe!
- Stirring too long may cause the orange solution to turn colorless.
- The following is an explanation of the reactions taking place.
Iodine ions, I–, from the potassium iodide solution react with the bleach to produce elemental neutral iodine, I2, and chloride ions. Iodine is an amber color.
Iodine reacts with starch to form the familiar dark blue starch–iodine complex.
Sodium thiosulfate reduces the iodine in the starch–iodine complex to regenerate colorless iodide ions, I–.
- Adding sodium thiosulfate solution to the starch–iodine complex may not reduce all of the iodine immediately. A few dark “flecks” may remain momentarily, but with stirring the solution should become completely clear.
- This is a great activity to develop problem-solving and critical thinking skills as well as to introduce chemical reactions.
- Review the students’ procedures or flow charts from Prelab Question 3 before they begin the lab.
- Offer a slightly greater challenge by adding a fifth mystery liquid, distilled or deionized water. More steps (minimum 15) would be required, but the basic outcome would be the same. The addition of a few drops of water will have no effect on the color of the solutions, other than a slight dilution in intensity (a physical change). The instructions for the challenge would be to determine which four out of the five solutions will product the correct sequence of color changes.
- The Think Tube Demonstration Kit (Flinn Catalog No. AP6149) and Observation and Experiment Laboratory Kit (Flinn Catalog No. AP6167) can be used to further explore the concept of problem solving.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems
Developing and using models
Planning and carrying out investigations
Analyzing and interpreting data
Disciplinary Core Ideas
MS-PS1.B: Chemical Reactions
MS-ETS1.A: Defining and Delimiting Engineering Problems
HS-PS1.B: Chemical Reactions
Cause and effect
Systems and system models
MS-PS2-1: Apply Newton’s Third Law to design a solution to a problem involving the motion of two colliding objects.
MS-PS2-2: Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object
MS-PS2-3: Ask questions about data to determine the factors that affect the strength of electric and magnetic forces
MS-PS2-5: Conduct an investigation and evaluate the experimental design to provide evidence that fields exist between objects exerting forces on each other even though the objects are not in contact
MS-ETS1-1: Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions.
HS-PS2-1: Analyze data to support the claim that Newton’s second law of motion describes the mathematical relationship among the net force on a macroscopic object, its mass, and its acceleration.
HS-PS2-4: Use mathematical representations of Newton’s Law of Gravitation and Coulomb’s Law to describe and predict the gravitational and electrostatic forces between objects.
HS-PS2-5: Plan and conduct an investigation to provide evidence that an electric current can produce a magnetic field and that a changing magnetic field can produce an electric current.
Answers to Prelab Questions
- Read through the Background section and the Procedure. Starting with four available solutions, how many different combinations of any two solutions are possible? Note: Adding solution A to solution B will produce the same result as adding solution B to solution A.
Six different combinations of two solutions are possible—A + B, A + C, A + D, B + C, B + D and C + D.
- What is the maximum number of drops that will be dispensed into any one well of the spot plate?
Since five drops of each solution may be placed in one well, the maximum number of drops would be 20.
- Working with a partner, write a general outline describing an “action plan” to test the possible combinations of solutions. This action plan can be written as specific numbered steps or as a graphic organizer such as a flow chart or other diagram. Allow for more than one possible correct sequence. Keep in mind you will have enough of each solution for approximately 50 drops.
Note: Ask to see the students’ plans. Students should have a step-by-step procedure or a chart showing the order in which they plan to test the different possible combinations of solutions.
Answers to Questions
- Write out the order in which the solutions were combined to produce the desired series of color changes—colorless to orange to dark blue and back to colorless. Was there more than one correct sequence?
The correct sequence is B + C + A + D (or C + B + A + D). Only one sequence produced the desired results.
- The number of wells used is an indication of the number of steps that were needed to discover the correct sequence. How many steps did it take your group to discover the correct sequence? Do you think the solution could have been discovered in fewer steps? Why or why not?
Student answers will vary. The minimum number of steps is nine, using eight wells. After testing the six possible combinations of two solutions, students will discover only B + C will produce orange. Using two more wells, for (B + C) + A and (B + C) + D, students will see that (B + C) + A yields bluish-black. They can then add D to the well containing (B + C + A) to produce a clear solution. (Students may or may not count this last addition as a separate step.)
- Would another problem-solving strategy have been better than trial and error for this activity? Why or why not?
Student answers will vary. Trial and error is most likely the best strategy. Since students have no way of knowing what the solutions are, they will have no prior knowledge to direct their steps.
- Adding solution A to solution B would produce the same result as adding solution B to solution A. Would reversing the entire order in which the four solutions were mixed (see Question 1) produce the desired series of color changes? Explain.
Adding B + C + A + D would not produce the same results as adding D + A + C + B. When two solutions are combined, a chemical reaction takes place, and a new substance is formed with properties that are different from the original reactants. Thus adding D to the product of B + C + A would yield different results than adding B to the product of D + A + C.
- Briefly describe an everyday situation in which trial and error might be used to solve a problem.
Student answers will vary. Solving mechanical puzzles, working through a maze, some types of math problems, creating a recipe, etc.
- Think of a situation in which trial and error would not be a good problem-solving strategy. Briefly describe the problem and suggest an alternative strategy for solving the problem.
Trial and error would not be appropriate for any situation that might be dangerous, where making an error could lead to disaster (e.g., mixing unknown chemicals, defusing an explosive device, eating unknown wild berries or mushrooms).