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Viscosity: A Difference in Stickiness

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Author: Janice VanCleave

Liquids move to fill up the space of their containers. This ability to move or flow is a very important physical property of liquids. Viscosity is the measurement of the resistance of fluids to flow.

In this project, you will use the viscosity of common household liquids to make a viscometer, an instrument that measures the flow rate of liquids. The flow rate will be used to calculate the viscosity index, the viscosity of a fluid relative to the viscosity of water, of each testing liquid. You will also determine the effect of temperature on viscosity and look at the binding forces between liquid molecules.

Getting Started

Purpose: To make and use a viscometer to determine the flow rate for a defined volume of water.

Materials

• scissors
• clear plastic dish detergent bottle with a pull top
• marking pen
• ruler
• modeling clay
• glass jar with a mouth slightly smaller than the upper part of the detergent bottle
• water
• timer

Procedure

1. Cut off the bottom of the detergent bottle.
2. Hold the bottle upside down. With the marking pen, make two straight lines, one about 1 inch (2.5 cm) below the cut-off bottom and the second 4 inches (10 cm) below the first line.
3. Label the first line "Start" and the second line "Stop."
4. Close the pull top.
5. Place a ring of clay around the top edge of the jar's mouth.
6. Stand the bottle upside down inside the jar. Mold the clay ring so that the bottle stands upright, but do not secure the bottle with the clay.
7. Fill the bottle to about 1/2 inch (1.3 cm) above the start line with cold tap water.
8. Lift the bottle and pull the top open.
9. Immediately set the bottle back on the jar (see Figure 18.1).
10. Start the Stop the timer when the water level reaches the stop line.
11. Repeat the procedure three times and average the flow rate of the cold water.

Results

The author's flow rate for cold water was 39.3 seconds. Note: The flow rate will vary depending on the bottle used.

Why?

The amount of time it takes a liquid to flow out of a container depends on its viscosity. The viscosity of a liquid is the resistance of the liquid to flowing, because of the friction between the molecules. Viscosity depends on the structure of the liquid molecules. If the molecules are small and simple in structure, as in water, they move past one another quickly. But if they are large and intertwined, as in oil for example, they move slowly past one another. Liquid molecules that slide quickly past one another have a low viscosity; liquid molecules that move more slowly have a high viscosity.

Try New Approaches

1. Does the temperature of water affect its viscosity? Repeat the experiment twice, first chilling the water in a freezer until its temperature is just above the freezing point, and then using warm tap water.
2. How does the viscosity of other liquids compare with the viscosity of water? The flow or viscosity of a liquid compared with the flow of water gives a relative viscosity for the liquid. A number measurement for the relative viscosity of a liquid is called its viscosity index. Any number less than 1 indicates a lower viscosity than water; a number greater than 1 indicates a higher viscosity than water. Repeat the original experiment using liquids such as oil, dishwashing liquid, honey, and/or syrup. Wipe out the viscometer after each test with a paper towel or prepare separate instruments for each liquid tested. Use the flow rate measurements and the following equation to compute the viscosity index for each liquid. See Appendix 10 for an example calculation.
3. Science Fair Hint: Label and display, in order of viscosity index, photographs of each liquid as it flows from the viscometer.

4.
1. The viscosity of motor oil is rated by the Society of Automotive Engineers (SAE). The numbers assigned are called ''weights" and are not exact viscosity values. Do higher weights of oil indicate a more viscous liquid? Repeat the original experiment using light, medium, and heavy weights of motor oil.
2. How does temperature affect the viscosity of oil? Vary the temperature of the motor oil previously tested and again use the viscometer to determine its flow rate. Place some samples in a freezer overnight to chill the oil. Heat other samples by placing containers of oil in hot tap water. Use a thermometer to determine the exact temperature of each sample tested.
3. Science Fair Hint: Construct and display a graph showing the relationship between temperature and flow rate.

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