Introduction
No one is exempt from an occasional “bout with the blues.” Whether it is because “your favorite team just lost the big game” or “your favorite TV show was canceled”—we are all susceptible to a temporary period of brief sadness. Still, in time, we recover and return to our normal state of mind.
The following demonstration provides a visual perception of how the “blues” come and go when our lives get a little shaken up.
Background
The blue solution becomes colorless upon standing due to the reduction of the methylene blue. By shaking the flask, oxygen dissolves in the colorless solution which oxidizes the methylene blue to its blue form.
The observation of the blue color at the interface between the gas (oxygen in the flask) and the solution demonstrates the fact that the gas is entering the solution and causing the blue color.
In a series of fast reactions, alkaline glucose is converted to glucoside, oxygen in the flask goes into solution, and dissolved oxygen oxidizes the reduced colorless methylene blue to the blue oxidized form of methylene blue. In a slow reaction, the blue oxidized form is reduced by the glucoside, and it is not until this occurs that a completely colorless solution can be observed.
The reaction mechanism (Campbell, 1963) occurs in four steps:
{14119_Background_Equation_1}
Materials
Dextrose (glucose) solution*
Methylene blue solution*
Potassium hydroxide solution*
Flask, 500-mL with cap or stopper
Graduated cylinder, 100-mL
*Materials included in kit.
Safety Precautions
Potassium hydroxide solution is corrosive; skin contact causes severe blisters. It is very harmful if swallowed and extremely dangerous to eyes. Wear a chemical-resistant apron, chemical-resistant gloves and chemical splash goggles. Please review 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. The resulting solution may be neutralized according to Flinn Suggested Disposal Method #10.
Procedure
- Using a 100-mL graduated cylinder, measure out 100 mL of potassium hydroxide solution and transfer it to a 500-mL flask.
- Using the graduated cylinder, measure out 100 mL of dextrose solution and transfer it to the flask. Swirl the flask to mix the solutions.
- Add 3–4 drops of methylene blue solution to the flask. Stopper the flask and swirl to mix the solutions.
- Allow the solution to stand undisturbed until it turns colorless. This may take a few minutes.
- Shake the flask gently to obtain the blue color again.
- Repeat steps 4 and 5. The process of shaking to obtain the blue color can be repeated about 10 times.
Teacher Tips
- The Feeling Blue Demonstration can be used to develop your students’ observational and scientific skills. Allow them to make observations, suggestions and ask questions. Make sure your students have a chance to observe the blue interface between the gas in the flask and the solution.
- Campbell’s article in the Journal of Chemical Education, (1963, 40, 578) is a well written explanation of kinetics as well as the specific kinetics of this demonstration.
- The length of time the solution stays blue is directly proportional to the amount of shaking.
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
Obtaining, evaluation, and communicating information
Engaging in argument from evidence
Disciplinary Core Ideas
MS-PS1.B: Chemical Reactions
HS-PS1.B: Chemical Reactions
HS-PS1.A: Structure and Properties of Matter
Crosscutting Concepts
Cause and effect
Systems and system models
Stability and change
Performance Expectations
MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
MS-PS1-6. Undertake a design project to construct, test, and modify a device that either releases or absorbs thermal energy by chemical processes.
HS-PS1-2. Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
Answers to Questions
- Describe what happened in this demonstration.
Methylene blue was added to a solution containing potassium hydroxide and dextrose solution. After sitting for several minutes, the solution that was originally blue is colorless. However, upon shaking the flask the solution turns blue again.
- Methylene blue is reduced when it is allowed to stand undisturbed. However, dissolved oxygen will oxidize methylene blue. Knowing this, what color is methylene blue in its oxidized form and what color is it in its reduced form?
Methylene blue is blue in its oxidized form and colorless in its reduced form.
- Why is there a hint of blue color at the interface between the gas and the solution in the flask?
The blue color at the interface of the gas and the solution is caused by the oxygen that is slowly entering the solution and oxidizing the methylene blue.
- What is oxidation/reduction?
An oxidation/reduction (or “redox”) reaction occurs when one or more electrons are transferred between molecules. Oxidation refers to a loss of electrons (and rise in oxidation state), and reduction refers to a gain of electrons (and subsequent decrease in oxidation state).
References
Special thanks to Jim and Julie Ealy, The Peddie School, Hightstown, NJ, who provided us with the instructions for this activity.
Abbott, G. E., J. Chem. Educ. 1947, 2, 45.
Abbott, G. E., J. Chem. Educ. 1948, 26, 100.
Campbell, J. A., J. Chem. Educ. 1963, 40, 578.
Dutton, F. B., J. Chem. Educ. 1960, 37, A799.
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