Viscosity of Liquids

The viscosities of three different liquids are determined by measuring the time required for a set amount of each liquid to drain from a modified Beral pipet. Conclusions about the nature of viscosity are drown from the results.

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• Obtain from the supply table the following items:
• modified Beral pipet
• toothpick
• 25 mL water in beaker with Beral pipet
• 25 mL ethylene glycol in beaker with Beral pipet
• 25 mL methanol in beaker with Beral pipet
• 1 one-hole stopper
• 1 utility clamp
• 1 ring stand
  

Place the modified Beral pipet (Figure A) in a one hole stopper. Clamp to a support stand. Fill the bulb with water by adding water from a Beral pipet through the opening at the top of the bulb. Plug the bottom with the toothpick once the long drain tube is filled (Figure B). Make certain that the tip is filled with water. Fill to the top with water
(Figure C).
1. Remove the toothpick, and measure the time required for the bulb to empty down to the top part of the drain tube (Figure D). Record the draining time.
2. Rinse the modified Beral (viscometer) with methanol. Repeat the procedure, this time measuring the time required for the methanol to drain.
3. Allow the viscometer to drain. Rinse with ethylene glycol. Repeat the procedure with ethylene glycol.
   

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Calculations

Determine the densities of the liquids tested by consulting a handbook.

Viscosity has dimensions of mass x length^-1 x time^-1. It is usually expressed as force x time per area. One unit that has the correct dimensions is the poise. The viscosity of water at 25 °C is 0.008904 poise, or 0.89 centipoise.

Using the value for the viscosity of water as 0.89 centipoise, calculate the viscosity of your sample by applying the follow formula:

viss = (dens ) (times ) (visw ) / (denw ) (timew)

Where:

viss = viscosity of sample

dens = density of sample

times = flow time for sample

visw = viscosity of water = 0.89 centipoise (25 °C)

denw = density of water = 1 g/mL

timew = flow time for water

Obtain values for the molar masses and dielectric constants from appropriate reference tables.

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Wear safety goggles and aprons in the lab at all times. Do not ingest the liquids. Keep methanol away from possible sources of ignition. Wash your hands before leaving the laboratory.

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Viscosity is defined as the resistance to flow of a fluid, either liquid or gas, in the minds of many students it is often thought of as "thickness". Viscosity is primarily due to intermolecular forces which result from asymmetrical distribution of electrons around molecules.

This exercise is intended to be multi-leveled; that is, it has applications appropriate for both beginning and advanced classes. At the most elementary level, student objectives include manipulation of apparatus, skills in calculation, and graphing. At the advanced level topics for discussion can also include the nature of intermolecular forces, polarity of bonds, structure of molecules and functional groups. For advanced students an additional consideration can be the effect of temperature on viscosity, this can measured by cooling the samples in an ice water bath before conducting the measurements.

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The Woodrow Wilson microscale version of this lesson was developed by:

Michael Mayfield
Downers Grove South HS
Downers Grove, IL

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