Viscosity: A Difference in Stickiness (page 2)
Design Your Own Experiment
- Compare the viscosity of liquids by dropping a glass marble into samples of the liquids. Fill identical slender jars with equal amounts of the liquids. Position the jars in front of a white background to increase your ability to clearly see the results. As you observe the contents of the jars, ask a helper to test two liquids at a time by holding a glass marble in each hand, and holding one hand over each jar. The helper should release both marbles from the same height and at the same time. Observe the movement of the marbles through the liquids. Continue to compare two liquids at a time until you can put the liquids in order of viscosity.
- Vary the temperature of the liquids. Prepare two jars of each testing sample. Chill one in a refrigerator and heat the second by setting it in a bowl of hot tap water. Measure and record the temperature of each liquid and then repeat the experiment You could display drawings or photographs of the jars, showing them in order of viscosity.
- Does the viscosity of a liquid affect its stickiness? Test the stickiness of each liquid by dipping your index finger into each liquid one at a time. Touch your thumb and wet index finger together and then slowly separate them. The more difficult it is for you to separate your fingers, the stickier the liquid.
- Lubricants should be able to be spread on a surface but not run off it Compare the viscosity of body lubricants such as baby oils, hand creams, and other body lotions by placing drops of the liquids at the top of a cookie sheet raised at one end by a clump of modeling clay. With the assistance of a helper, start the drops of liquid on their downhill flow at the same time (see Figure 18.2). Observe and compare the flow rates of the flowing liquids.
Get the Facts
- liquid molecules are free to slide over and around one another. The degree to which these molecules move is directly related to the forces of attraction between the molecules. These forces are called cohesive forces. Find out more about this binding force between liquid molecules.
- What happens to the kinetic energy (energy of motion) of molecules in liquids as the temperature is raised? How does this energy affect the binding forces between the molecules? Use this information to explain the effect of temperature on the viscosity of liquids.
- Find out more about how the Society of Automotive Engineers (SAE) grades motor oil. Is there a difference between oil labeled SAE 30 and oil labeled SAE 30W? What does the label10-W-30 mean? If temperature affects the viscosity of oil, why is it not necessary to change the oil in a car as the seasons change? This information can be found in some chemistry texts, but a good resource is an automotive mechanic.
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