Blue bottle

Change a solution’s color with a shake of your hand!

Difficulty:
Danger:
Duration:
20 minutes
Blue bottle

Reagents

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

How warm should the water be?

The water can be anywhere from lukewarm to hot. Its temperature only influences the reaction rate. The warmer the water you use, the faster the solution will pale.

I added more than 5 drops of methylene blue. Should I start the experiment over?

No, don’t worry! Continue the experiment.

Can I use the stopper with one hole?

No – you need to be able to shake the solution without spilling the contents. Moreover, in closing the flask with the stopper without a hole, you’re cutting off the solution’s ambient air supply so oxygen from the air won't permeate into the flask. This is what allows you to control the color recharge by shaking the flask.

How long will the liquid be able to change colors?

The liquid will “work” as long as the flask contains oxygen and glucose supply.

My solution isn’t turning blue anymore. Is there anything I can try?

The solution stops turning blue when there is no more oxygen in the flask to oxidize the methylene blue. To refresh the flask’s oxygen supply, remove the stopper for a few seconds, then put it snugly back in the flask. As long as the solution still contains glucose, it should continue changing colors.

Step-by-step instructions

First, make a solution containing a reductant (glucose) and methylene blue.

oxygen_blue_bottle_en-astm963_iks-01

Now add some NaOH to make the solution basic.

oxygen_blue_bottle_en-astm963_iks-02

Methylene blue takes some electrons from the reductant, glucose, and turns colorless. You didn't add it intentionally, but the solution contains a powerful oxidant—oxygen. Oxygen can take electrons from methylene blue, making it blue again. But once all the oxygen in the solution is used up, the methylene blue stays colorless.

oxygen_blue_bottle_en-astm963_iks-03

Even though the solution is now devoid of oxygen, the air in the flask still contains some. Just shake the flask to dissolve it and see what happens.

oxygen_blue_bottle_en-astm963_iks-04

Expected result

Blue solution in the flask becomes colorless. Shaking the flask turns the solution blue again!

Disposal

Dispose of solid waste along with household garbage. Pour solutions down the sink. Wash with an excess of water.

Scientific description

In a chemical reaction, a reductant is the substance that gives (donates) electrons, while an oxidizer is the substance that gains (accepts) electrons. For example, in the reaction between glucose and methylene blue, glucose is a reductant, while methylene blue is an oxidizer. In the reaction between the colorless form of methylene blue and oxygen, oxygen is accepting the electrons that the colorless form of methylene blue is donating.

But why doesn’t glucose donate its electrons directly to oxygen? Sometimes, one substance cannot transfer its electrons directly to another. In this case, they need an assistant that can interact with both substances. In the experiment, this assistant is methylene blue. Methylene blue can transfer electrons from glucose to oxygen, but only if the solution medium is basic.

Why does the solution become colorless?

Initially, the solution contains the components for a potential chemical reaction. Glucose itself is more than happy to surrender its electrons. The oxygen dissolved in the water would be delighted to accept these electrons. Interestingly enough, though, oxygen isn't that willing to interact with glucose. And methylene blue can help: this colored compound acts as a carrier in our experiment, taking electrons from glucose and passing them to oxygen. However, at a certain point, the oxygen in the solution runs out, leaving methylene blue in an awkward position: it’s taken electrons from glucose, but has nowhere to pass them on to. When this happens, methylene blue cannot turn blue anymore and has no choice but to stay colorless.

Methylene blue:

Methylene blue

Why does the solution turn blue again?

We can saturate the solution again with oxygen from the air above the solution. When the flask is shaken, oxygen from the air dissolves in the solution. The reaction can then proceed until all the oxygen available in the solution is spent again. However, this trick cannot be repeated endlessly. Since the flask is tightly sealed, sooner or later all the oxygen from the air will be depleted, and the solution will then remain colorless even when shaken. Nevertheless, the process can be reactivated by opening the flask to let some more air in.

Why did we add an alkali to the glucose aqueous solution?

By adding sodium hydroxide NaOH aqueous solution, we created an alkaline environment. Methylene blue needs an alkaline environment in order to accept electrons from glucose; otherwise, the reaction will not proceed, and the solution will remain blue. You can check this condition by conducting the experiment without NaOH.

Sodium hydroxide

Sodium hydroxide

Why is it so important to seal the flask tightly?

First and foremost, you’ll be able to shake the flask without sending any liquid flying.

Moreover, in sealing the flask we are preventing ambient air from entering, and ensuring that the oxygen in the ambient air will not have access to our solution either. This is why the color can only be restored by shaking the flask (see Why does the solution turn blue again?). The most diligent observers may notice that the blue tint doesn’t disappear completely after the first shake, but remains at the border between the solution and air in the flask (along the so-called meniscus) and forms a nice blue fringe. The same would happen if the flask were left open. This is caused by a high concentration of oxygen present in the air above the solution. The oxygen permeates the liquid-gas interface and converts methylene blue to its colored form. However, as the oxygen supply in the flask is gradually depleted, this border gets increasingly thinner and finally disappears.