Key Concepts
Friction, solids, liquids
Two marbles going down two glasses containing liquids with different viscosity.

Introduction

Have you ever tried to squeeze honey or syrup out of a bottle at breakfast on a cold winter morning? Do you notice that it's harder to do that than on a hot summer day? As the liquid gets colder, its viscosity, or resistance to flow, increases. Viscosity is a properly of liquids that can be hugely important in wildly different applications – from how the syrup flows out of your bottle, to how blood flows through the human body, to how lava flows out of a volcano. In this project you will learn a little bit about viscosity using some liquids you already have in your kitchen.

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

You experience friction every day. Friction is what allows your shoes to grip the floor so you don't slip, and it's what makes your bike come to a stop when you squeeze the brakes. This type of friction is a force that resists motion between two solid objects. However, liquids have friction too-not just against solids (for example, water against a drinking glass)-but internal friction, the liquid against itself. This internal friction is called viscosity. Different liquids have different viscosities, which means some liquids flow more easily than others. You will notice this if you think about squirting water out of a bottle or squirt gun. Imagine how much harder that would be to do with syrup!

There are several different ways scientists can measure the viscosity of a liquid. One method is called a falling sphere viscometer, where you drop a sphere (like a marble) through a tube filled with liquid. By measuring how long it takes the marble to fall and how far it travels, you can calculate the viscosity of the liquid. You won't need to do any calculations in this experiment – but you will get to "race" marbles by dropping them in different glasses of liquid. Will viscosity affect how fast the marbles fall? Try this project to find out!

Materials

  • About a dozen equal-size marbles
  • At least two equal-size tall drinking glasses (the taller the better)
  • Assorted liquids from around your kitchen, like water, syrup, honey, molasses, olive oil, vegetable oil, etc.
  • Strainer or colander

Optional: extra bowls/containers, and/or a funnel (for storing and re-using the liquids you use for the experiment, if you do not want to throw them away)

Optional: volunteer to help you see which marble hits the bottom first.

Preparation

  1. If you want to save and re-use the liquids you use for the experiment, then make sure you thoroughly wash your marbles and drinking glasses with soap and water, then dry them completely. This will make sure they are clean and you do not get your liquids dirty.

Procedure

  1. Fill your two (or more) drinking glasses with each of your different liquids, to the same height. To avoid spilling when you drop the marbles in, do not fill them all the way to the brim.
  2. Which liquid do you think has a higher viscosity? Can you tell when you pour them into the glasses? Do you think the marble will fall faster through one of the liquids?
  3. Hold one marble in each hand, just above the surface of the liquid in each glass.
  4. Watch the glasses closely. Be prepared to watch the bottom to see which marble hits first. If you have a volunteer, have them look at the glasses too.
  5. Let the marbles go <i>at the same time</i>.
  6. Observe which marble hits the bottom of the glass first.
  7. Which marble won the "race"? Do your results match your prediction?
  8. Repeat the experiment with a few more marbles to see if you get the same results. If you have more than two different liquids, you can try racing marbles in other liquids to see what happens.
  9. Through which liquid do the marbles fall the fastest? The slowest?

Extra: what happens if you drop different types of marbles (for example, steel marbles instead of glass marbles), or different size marbles? Do the results of your races change?

Extra: what happens if you change the temperature of a liquid? Have an adult help you cook some syrup in the refrigerator and heat some on the stove or in the microwave. What happens if you do a race with cold syrup and hot syrup instead of room-temperature syrup? How does temperature affect the viscosity of the liquid? Is the temperature affect stronger on some liquids than it is for others?

Observations and Results

When pouring your liquids, you might have noticed that some of them were "thicker" or harder to pour. These are the more viscous liquids. You can also think about what these liquids are like when you use them every day. For example, what would happen if you poured water on pancakes? Would it flow slowly like syrup or spread out very quickly? What about if you tried to pour and drink a glass of syrup? Taste aside, would that be harder than drinking a glass of water?

You should have observed that the marbles fell more slowly through more viscous liquids (like syrup) than through less viscous liquids (like water). This is because the more viscous liquids have more resistance to flow, making it more difficult for the marble to fall through them. However, especially if your glasses are not very tall, it might be hard to tell the difference between the results for some liquids. This is why you should do multiple trials and have a volunteer help watch the marbles.

Cleanup

  1. If you want to store the remaining liquids for future use, have an adult help you pour them back into storage containers (use the strainer to strain out the marbles if necessary). Otherwise, have an adult help you dispose of the liquids properly. Be careful because pouring some viscous liquids like cooking oil down the sink can clog the drain.

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Credits

Ben Finio, PhD, Science Buddies

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Key Concepts
Friction, solids, liquids
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