Density of Methanol versus Density of Water

Description

A U-tube is filled with two different liquids under the illusion that only one liquid is being used. The liquids selected have different densities. A taller column of the less dense fluid is observed. Students are asked to predict the outcome of some operation under circumstances where they are likely to operate under an illusion or jump to a conclusion based upon their experience with everyday situations. The most usual case is to show the tube partly filled with identical levels in each arm and ask what will happen when more liquid is added to one arm.

Go to Top

• Alcohols are toxic and flammable.
• Avoid ingestion of the methanol.
• Keep away from open flames or other ignition sources. Use only with adequate ventilation.

Go to Top

1. Add a few drops of food coloring each to 60 mL of water and 60 mL of methanol. The food coloring is used to enhance the visibility of the demonstration, and has no effect on the outcome.
2. Set up a U-shaped apparatus. In the system illustrated, two burets are connected with a length of Tygon^® tubing.
3. Students predict what will happen when water (or the more dense liquid) is poured into one side of the apparatus.
4. Pour sufficient water into one arm of the apparatus to fill about 1/3 of the total tubing length, and note the heights of the liquid in each arm.
5. Students predict the outcome of adding methanol to the apparatus.
6. Pour in the methanol, and note the outcome.

Alternative Procedure:

1. Prepare the U-tube before the students arrive. Place a short length of latex tubing on the end of the U-tube that holds the lower liquid level. Use an aspirating bulb to raise this level; the level which will be lower at equilibrium now appears higher. Clamp the latex tube. With some groups of students, the most effective way to present the tube is to adjust the initial height so that the initially high side becomes even higher when the clamp is released.
2. In the presentation, students predict the outcome of removing the clamp. They usually predict that the two levels will become the same.
3.  Discuss the result.

Go to Top

Name ___________________________ Class _______

Teacher __________________________

DoChem 002 Density of Methanol versus Density of Water

1. View the computer movies. Write down your observations and sketch the apparatus as you are viewing the movies.
   • Observations
   • Sketch
2. Explain the result.
3. Predict the outcome of adding the less dense liquid first, and then adding the more dense liquid. Explain.

Go to Top

Purpose

To illustrate a consequence of the difference in density between two liquids.

Go to Top

• 2 50-mL buret, glass tube, or clear plastic tubing
• 1 40-cm length 1-cm i.d. Tygon^® tubing
• support stand, two-sided buret clamp
• 2 250-mL Erlenmeyer flask
• food coloring
• water
• 60-100 mL methanol (ethanol, or rubbing alcohol)

Go to Top

• Any U-shaped, see-through apparatus may work. A long length of large diameter Tygon^® tubing may be clamped in a two-sided buret clamp. A length of glass tubing may be shaped in a flame if the teacher has glass blowing skills. Aqueous solutions of different densities (e.g., salt, sugar) may be used.
• Have materials available so that the effect of connecting lengths of tubing with different diameters may be predicted and tested.

Go to Top

Teacher preparation: 15 minutes

Presentation: 10 minutes

Go to Top

Alcohols are toxic and flammable.

Go to Top

The chemicals used in this experiment may be safely disposed of at the sink. Flush alcohols down the drain with water; use at least 10 volumes of water per volume of alcohol.

Go to Top

This experiment is an example of a "discrepant" event. Students expect liquids of the same appearance to have the same composition and to reach the same level in each arm of the tube. The illusion is created that the liquids are the same. This is an excellent way to bring about a classroom discussion of how the "givens" of an experiment must be specified precisely.

Go to Top

Presentation Question:

• Predict the outcome of adding the less dense liquid first, and then adding the more dense liquid.
  • When the more dense liquid is added last, there is considerable mixing in the tube. When the less dense liquid is added last, it floats on top of the more dense liquid rather than mixing with it.
  • This is a way to add a second discrepant event to the same experiment; many students predict the outcome based upon levels in the tube, and overlook the possibility of mixing within one of the side arms.

Go to Top

The two liquids do not mix as a result of the way the experiment is performed; the less dense fluid floats on top of the more dense fluid. The U-shaped apparatus connects the liquids; each arm must contribute equally to the pressure. Since methanol is less dense than water, a larger height of methanol is required to provide an equal weight to maintain equilibrium in the U-tube.

Go to Top

• discrepant event
• density
• float
• sink
• U-tube

Go to Top