How to Measure Viscosity of Liquids

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Updated on Aug 06, 2013

The frozen waffle you put in the toaster just popped out, but your maple syrup is still in the fridge. You squeeze the syrup container, but it takes forever to flow out of the bottle! Why?

This sticky situation illustrates a property of liquids called viscosity. Viscosity is the measure of how resistant a substance is to flowing. The slower a liquid flows, the higher its viscosity. Remember—a liquid is a state of matter that has a definite volume, but not a definite shape. You probably have a qualitative understanding of relative viscosities of different liquids, meaning you can describe them pretty well just by watching them. Scientists like to be quantitative when they describe something’s behavior—they use numbers! Can we find a way to quantitatively how much faster water flows than syrup?

In this investigation, you are going to make your own tool for measuring viscosity, called a viscometer, and use it to compare the viscosities of liquids you find around your home.

Problem:

How can viscosity be measured?

Materials

  • Clear plastic dishwashing detergent bottle with a pull-out top
  • Sharp scissors
  • Adult helper
  • Ruler
  • Permanent marker
  • Modeling clay
  • Container with a top slightly smaller than the top of your dishwashing detergent bottle
  • Watch or timer that can measure seconds
  • Water
  • Assortment of liquids, including water, all at the same temperature
  • Calculator

Procedure

  1. Ask your adult helper to use the sharp scissors to cut off the bottom of the detergent bottle.
  2. Remove the label, and thoroughly rinse the inside of the bottle. Make sure to run some water through the pull-cap, too.
  3. Use your ruler to measure one inch below where you cut off the bottom. Draw a line, and label this line Start.
  4. Use your ruler to measure 4 inches below your first line. Draw a line, and label this line Stop.
  5. Close the pull-out top.
  6. Make a thick ring of modeling clay and place it around the top of the bottle.
  7. Place the detergent bottle upside down on the container. The bottle should rest comfortably on the clay. Do not seal the bottle against the clay. If you block all air from moving out of the container, your viscometer will not work correctly.
  8. Test water first.
  9. Fill your bottle to about 1/2 inch above your start line.
  10. Lift up the bottle, pull the top open, and immediately set back on top of the jar.
  11. Start timing when the water level has reached the Start line.
  12. Stop timing when the water level reaches the Stop line.
  13. Record your results in a data table. Perform multiple trials for each liquid tested and take the average time for each of them. Why do you think this helps us get a more accurate measurement? What factors do you think might cause you to get different times for the same liquid?
  14. Your data table might look like this:

Liquid

Trial 1 (seconds)

Trial 2 (seconds)

Trial 3 (seconds)

Average

Water

Maple syrup

Shampoo

  1. You calculate the average by adding up the time for the three trials and dividing by 3.
  2. Repeat this procedure for as many liquids as you like. Just make sure all of them are the same temperature. Why do you think keeping temperature the same is important? What happens to a liquid’s viscosity as its temperature rises?
  3. Once you have calculated average times for several liquids, you can calculate a viscosity index for each so that you can easily compare them with each other. The formula for viscosity index is:

Viscosity Index Equation

  1. For instance, if your average flow rate for water was 20 seconds and corn oil took 600 seconds (5 minutes) to flow out of the viscometer, than the viscosity index for corn oil is 30. That means it is 30 times more viscous than water. Why do you think the viscosity index uses the viscosity of water for comparison?
  2. You might make another table with the liquids and viscosity indexes, or you could add another column to your first table.

Liquid

Viscosity Index

Water

1

Maple Syrup

Shampoo

Results

The actual times for the liquids will depend on the size of your viscometer and the temperature of your liquids. Your viscosity index for maple syrup should be 150-200 times that of water. Most shampoos have 10-100 times the viscosity of water.

Why

Viscosity is caused by friction within the liquid. Friction is the resistance one object encounters when moving over another. You probably remember that all matter, including liquids, is made of molecules. Water is composed of small simple molecules (H20). These molecules can move past each other easily and quickly. Maple syrup, on the other hand, has a complex molecular structure—it’s composed of many different types of molecules, many of which are quite large. These big molecules do not move easily past one another, and they are often tangled together, giving syrup a much higher viscosity.

The viscosity of a liquid depends on the temperature. Increasing temperature usually decreases viscosity because the molecules have more energy to move past each other, and there is less intertwining. Water is used as the basis for the viscosity index because we are all familiar with how fast it moves.

You can test all kinds of liquids—but for some of them, like ketchup, be prepared to wait a very long time!

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