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DETERMINATION OF THE RELATIVE VISCOSITIES OF LIQUIDS

* PURPOSE

The viscosity ranges of liquids, particularly common automotive oils, will be demonstrated in this experiment.

* DESCRIPTION

The viscosities of two grades of oil will be tested at room temperature and a higher temperature in order to study the relative meaning of the S.A.E. numbers. The experiment is intended for almost any level chemistry class and works especially well with a general level class. The activity can also be part of the study of matter by a first-year college-prep class. Variations may be utilized with a more advanced group or extensions of the experiment may be suggested as a research project for an ambitious student.

* TIME REQUIRED

Two lab periods are needed to test viscosity at more than one temperature.

* MATERIALS

Chemicals:
SAE 30 motor oil
gear oil
Equipment:
table tennis balls
PVC pipe (1.5 inch internal diameter)
rubber stoppers to fit PVC pipe
thermometers
stopwatches or watch with a timer function
Bunsen burners or hot plates
wire gauze
utility clamps
250-mL beakers
beaker tongs
glass tubing about 12" long or piece of metal rod for pushing table tennis balls to bottom of PVC tube
cafeteria trays

* HAZARDS

Extreme care is required when handling hot oil. The hot oil may fume and must be kept under the hood. Oil at 100°C may melt the table tennis balls. Oil should not be poured into the sink since it will clog the drain. Goggles must be worn throughout the experiment.

* PROCEDURE

  1. Obtain two 10" long, 1.5" internal diameter PVC pipes; 2 rubber stoppers; a 12" glass tube; two table tennis balls; a stop watch; a thermometer; and 250 mL of each grade of motor oil.
  2. Stand the PVC pipes, which have stoppers in the bottom, on a tray and clamp into position with a utility clamp attached to a ring stand.
  3. Record the temperature of the lower viscosity oil before it is carefully poured into the PVC pipe. Shove a table tennis ball into the oil, submerse it and force it to the bottom of the pipe with the 12" glass tube. Start the timing as soon as you quickly release the submersed ball. Stop the watch as soon as the ball reaches the surface and record the time. Notice the surface phenomena as the ball approaches the top.
  4. Repeat step 3 two more times for statistical purposes.
  5. Pour the oil from the pipe back into your beaker.
  6. PERFORM THE FOLLOWING PROCEDURE UNDER THE FUME HOOD. Place the beaker of oil on a hot plate. Slowly heat the oil in the beaker to 60°C.
  7. Carefully and quickly pour the hot oil into the pipe and perform the experiment in the same manner as for the room temperature oil. Measure the temperature of the oil before and after the experiment. Perform three trials.
  8. Using the second PVC pipe and a new table tennis ball, repeat the same procedure for the higher viscosity oil. Perform three trials at room temperature and three trials for a higher temperature.
  9. Return both grades of oil to the containers provided under the fume hood. Invert the beakers on a tray covered with newspaper to drain.
  10. Compute the average time for the ball to rise to the top of each grade of oil at each temperature.
  11. Compare the average times for the room temperature oils. Using your data, formulate a statement which will show the relationship between the SAE number and the viscosity (The higher the SAE number of an oil, the ..........)
  12. Compare the average times for the hot oils with the average times for the room temperature oils. From your data, formulate a statement which describes how temperature affects viscosity. (The hotter the oil, the........)
  13. Which oil has the greatest change in viscosity per degree change in temperature?
  14. Try to relate the differences in viscosity to the differences in attractive forces
  15. List the possible reasons for the variations in each of your data sets.

* DISPOSAL

Recycle the oil and keep the PVC pipe for use in this experiment only. Carefully wipe off the table tennis balls for reuse.

* DISCUSSION

Lubricating oil is a mixture of straight and branched chain hydrocarbons derived from the fractional distillation of crude oil. The fraction is collected above 300°C and contains hydrocarbons with from 18 to 22 carbon atoms. The higher viscosity oil has longer chains with a higher total number of electrons. This means that the dispersion forces are greater. The students should see the direct relationship of viscosity to the strength of the attractive forces. The oil mixtures usually consist of both straight chain and branched chain alkanes.

Lubricating oil behaves as a "Newtonian liquid". In his book Principia (1687), Newton discussed "lack of slipperiness" or resistance to flow as being "proportional to the velocity with which the parts of a liquid are separated from one another"(Whitten and Galley). This resistance to flow or fluid friction is referred to as viscosity.

In 1941, the concept of equilibrium was introduced into the theory of liquids. Liquids were described as structures with "holes" and the flow of the liquid was dependent on the transfer of molecules by way of these holes. For example, with two layers of molecules sliding past each other, the motion of the one layer with reference to the other involves the passage of a molecule from one position to another in the same layer. Scientists have a special interest in the measurement of the variation of viscosity with temperature (Blair).

Motor oil is generally labeled with an S.A.E. number. If only one number is stated (for example, an S.A.E. 20), this means that the oil was tested at only high temperatures (above 100°C). Most oils are multigrade and have two ratings (for example, S.A.E. l0W30). The lower rating was tested at -18°C and the higher at 100°C.

* TIPS

  1. All of the materials, with the exception of the stopwatch, can be purchased at a rather low cost. It is possible that many of the supplies can be borrowed or procured by donation.
  2. Have reliable students or the shop teacher cut the PVC pipe. Label pipes for high and low viscosity. For future years, store pipes in newspaper and/or plastic bags after draining.
  3. The viscosity will fall with a rise in temperature. The experiment will work best when there are extreme differences in the grades of the oils. SAE 30 (a normal lawn mower oil) and SAE 90 (a gear oil) were used with much success.
  4. If stopwatches are not available, the timer/stopwatch function on digital watches can be used.
  5. Further research is suggested in the following areas:
    1. Comparisons of multigrade oils with standard oils.
    2. Comparisons of average times to known viscosities.
    3. Literature research studies on the methods used to determine the viscosities of Newtonian liquids.
    4. Testing a series of oils and plotting the SAE number versus the time.

* REFERENCES

Whitten, K.W. and Galley, K.D. General Chemistry, Saunders College Publishing, Philadelphia, PA, 1984, p. 864. -- This work discusses Newtonian fluids and viscosity and its relationship to molecular structure.

Blair, Scott G. W., Elementary Rheology, Academic Press, New York, 1969. p. 24.

Submitted by Steve Cotton and John Hnatow


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