The Blue Bottle

The Blue Bottle

Description

One solution is divided between two containers (flasks) such that one container is nearly full and the other nearly empty. Shaking the filled container does not seem to produce a dramatic change, but shaking the partially-filled container does. Pouring the liquids from the filled container to the empty one and repeating the procedure leads to the same result -- only the nearly empty container gives a dramatic change.

Hazards

The solution is alkaline (about 0.2 M NaOH) and can cause blindness. Skin irritation is possible if exposure is prolonged. Methylene blue will stain clothing.

Precautions

Wear eye protection. Wash off spills. Wear old clothing or lab aprons.

Procedure

Place 1200 mL of distilled water in a 2-L beaker. Add to the water 10 g glucose, 10 g sodium hydroxide, and sufficient drops of 1% methylene blue to color the mixture blue.
Fill a 1-L Florence flask with this solution. Pour the remaining solution into a second 1-L Florence flask.
Swirl each flask vigorously, and note results.
Pour from the filled flask into the partially filled flask and swirl the contents of both flasks vigorously. Note observations.
Pour once again, shake once again.
Note results.

++Microscale:

Some teachers implement this as an experiment in which students are given test tubes and operate on the test tube scale. Great care must be taken that all students wear eye protection at all times during the microscale experiment. One way to present the experiment is to provide one filled and one half-filled 4-mL screw cap vial ask whether 'the two solutions are the same.'

Handout Makeup

Name ___________________________ Class ________

Teacher__________________________

DoChem 006 The Blue Bottle

The flasks contain glucose, NaOH, and a very small amount of methylene blue.

Watch the movie and carefully describe the conditions for each color change.
Account for your observations.
What reactant should be added to the list above?

Teachers Guide

Purpose

To study a heterogeneous chemical reaction.
To generate hypotheses.

Materials

2 1-L Florence flask
2-L beaker
40-cm stirring rod
10 g glucose (or fructose)
10 g sodium hydroxide
1% methylene blue, in dropping bottle (dissolve 1 g methylene blue in 100 mL of distilled water)
distilled water

Lab Hints

Sucrose (table sugar) is not a suitable substitute for glucose. Some other dyes can substitute for methylene blue.
Make a fresh glucose solution on the day of the experiment.
This experiment can be performed by students. The students may shake the solutions in stoppered test tubes. Emphasize the safety precautions. (Remember, every student in the class will be shaking a strongly alkaline solution in a glass container!)

Time

Teacher preparation: 20 minutes

Presentation: 10 minutes

Additional experiments, such as shaking in a sealed flask, take considerably longer.

Hazards

The solution is alkaline (about 0.2 M NaOH) and can cause blindness. Skin irritation is possible if exposure is prolonged. Methylene blue will stain clothing.

Precautions

Wear eye protection. Wash off spills. Wear old clothing or lab aprons. Check that the safety shower and eye wash fountain are in working condition.

Disposal

Neutralize the sodium hydroxide solution with vinegar; dispose of at the sink.

Closure

Ask the students to account for their observations. Once discussion slows down, assuming no one picks up on the role of air, suggest that one of the reactants, which they did not consciously add to the system, might be invisible. Ask, "What reactant should be added to the list?" Generate suggestions for testing their hypotheses.

Background

Oxygen oxidizes glucose well in a strongly alkaline solution. Methylene blue is also subject to oxidation. The oxidized form is blue; the reduced form (leuco methylene blue) is colorless. When oxygen is present, methylene blue is blue. When the solution is undisturbed, the glucose reduces the methylene blue to the colorless form.
Shaking the partially filled flask causes oxygen to dissolve and to oxidize the methylene blue. When there is no source of fresh air -- a filled flask, or one with very little surface -- the blue does not form. You will probably be able to see a bluish color at the top surface of the solution. This careful observation provides support for the mechanism suggested to account for these color changes. In a sealed flask, once all of the oxygen is used up, there is no longer a changing of the color. Care must be taken to use fresh solutions.
Readily oxidized sugars may be used to replace glucose. Several redox dyes may replace methylene blue.
This demonstration can be used as an effective model for lakes in which there has been eutrophication (enrichment). When the environment is very rich, algae blooms and excessive weeds lead to oxidation of organic matter and subsequent oxygen depletion.

Makeup Ans.

Each time the flask is filled the solutions turns colorless. Each time the flask is half-full and shaken vigorously the solution turns blue.
Air is mixed with the other reactants during shaking. The air is oxidizing the methylene blue. A reducing agent (Students may not know that glucose is a reducing agent.) in the solution is reducing the methylene blue on standing with out shaking.
air

Key Words

heterogeneous reaction
surface area
oxygen
dioxygen
eutrophication
rate of reaction
dissolved gas
discrepant event