Warmer temperatures generally decrease viscosity. This can be observed in liquids with high viscosity that flow more easily when heated. Next time your students eat pancakes, ask them to compare cold syrup from the refrigerator with syrup that's been heated. As the temperature increases, the molecules of the liquid move faster, and the attraction between the molecules decreases, meaning they don't spend as much time in contact with each another. Motor oil used in cars can decrease in viscosity in hot weather. If the viscosity becomes too low, the oil can allow friction to heat the engine and cause damage.
Cooler temperatures generally increase viscosity. Oil for used in cars can get too viscous in cold weather, making it difficult to start the car. Ask students if any of their parents use an oil heater for their car in cold weather. Manufacturers sometimes put additives into the oil to keep it at the correct viscosity for the weather.
Some materials are more sensitive to variations in temperature. A relatively small increase or decrease in temperature makes a difference in viscosity. Others liquids are relatively insensitive to temperature changes and require a greater temperature difference before their viscosity is affected. To demonstrate this principle to students, put a stick of butter into a glass bowl and heat it in the microwave on high. Watch carefully as the butter quickly loses its stick form and melts.
You can see the effect of temperature on viscosity using four glass, 100-ml graduated cylinders; 100 ml each of water, vegetable oil, honey and corn syrup; four equally sized metal ball bearings; four stopwatches; and access to a refrigerator and stove top.
Fill each graduated cylinder with one of the four liquids. Allow them to come to room temperature over two or three hours. Assign one student per cylinder to place a ball bearing into a cylinder and time how long it takes to reach the bottom. They should start their stopwatch immediately when they drop the bearing into the cylinder, and stop timing immediately when it reaches the bottom. Record the results. Retrieve the ball bearings. Place the cylinders into the refrigerator for two hours and repeat the experiment. Repeat the experiment a third time with an adult's help. Heat the liquids to 100 degrees Fahrenheit and try to observe how quickly they each become less viscous. Ask the students to analyze how temperature effects the viscosity of the different liquids.