Blue Shaking Solution
Age
Elementary School, Middle School, High School
Format
Stage Show, Hands-on
Materials
Flask and stopper 8g Potassium Hydroxide [corrosive] 10g Glucose (AKA dextrose) 0.05g Methylene Blue [harmful] 50mL Water
Safety Precautions
Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Make sure the stopper fits tightly on the flask and that kids don't drop/break the flask. It might be a good idea to use a clear plastic pop bottle with a screw on lid. Also please check how to properly dispose of the solution.
Preparation
Make a solution of 0.05g o methylene blue in 50 mL of water Add 8g of potassium hydroxide into the flask Add 300mL water to the flask Add 10g glucose to the flask Swirl the flask until the solids are dissolved Add 5mL of the previously prepared methylene blue solution. The exact quantity used is not important.
The resultant blue solution should turn colorless after about a minute. Stopper the flask.
Demonstration
While holding the stopper securely in place, shake the flask so that air dissolves in the solution (you can let the kids try it out on their own, just make sure they wear goggles and are careful not to drop it). The color of the solution will change from clear to blue while shaking, then will go clear again after standing still for about 30 seconds. The more shaking, the longer the blue color will take to fade. Process can be repeated many times, after a while (several hours) the solution will turn yellow and the color changes won't happen anymore.
What to Say
A redox reaction is one in which oxidation and reduction are taking place. Oxidation is when an atom or molecule loses electrons (or gains oxygen)
Reduction is when an atom or molecule gains electrons (or loses oxygen)
A good way to remember the difference is LEO says GER where LEO stands for Losing Electrons=Oxidation and GER stands for Gaining Electrons=Reduction.
So what happens when we shake the solution? --- We dissolve air (and therefore oxygen!) into the solution. We just learned that when a molecule gains oxygen it is oxidized. So when we shake the solution, the glucose is oxidized.
An indicator is something that can be used to tell us what is going on in a solution. Methylene blue is a redox indicator that is colorless when under reducing conditions, and turns blue under oxidizing conditions. So when we shake the solution, we oxidize the glucose and the methylene blue turns blue!
What happens when we let the solution sit for a while? The solution is reduced (loses oxygen) and the methylene blue returns to a colorless state.
What happens if we shake the flask for a very long time? The time that the flask is shaken is directly proportional to the amount of time that it will take for the blue color to fade. So the longer you shake the flask, the longer it will take to return to it original (lack of) color. This is because the longer you shake the solution the more oxygen is dissolved, and the solution remains oxidized longer.
Why It Is
Methylene Blue is a redox indicator that is colorless under reducing conditions and blue when oxidized. The glucose in the solution causes the methylene blue to initially be colorless. When the solution is shaken, oxygen dissolves into it. This oxidizes the glucose, which gives the methylene blue its blue color, indicating the oxidative conditions. When the oxygen is no longer dissolved in the solution (after you let it sit for a while), the glucose returns to its reduced state and the solution loses its blue color. The longer you shake the solution, the longer it will take to return to its colorless state because more oxygen is initially dissolved.
Oxidation refers to the loss of electrons/hydrogen or the gain of oxygen/increase in oxidation state by a molecule, atom, or ion.
Reduction refers to the gain of electrons/hydrogen or the loss of oxygen/decrease in oxidation state by a molecule, atom, or ion.
The reaction:
HOCH2(CHOH)4CHO + 3 OH - → HOCH2(CHOH)4CO2 + 2 H2O + 2 e –
Real Life Examples
In the dairy industry the dye reduction test using methylene blue is used to determine whether milk is classified as “excellent”, “good”, “fair”, or “poor” based on the rate at which the solution turns colorless. The faster the methylene returns to its colorless state, the lower the rating. This is because the bacteria that sometimes live in milk use oxygen as fuel. The more bacteria in the milk, the less oxygen there is, and so the oxidized (blue) state of the methylene blue fades more quickly.