Make Quicksand

Learning Objectives

Students will:

• Use cornstarch and water to make quicksand.
• Experiment with the quicksand to reach conclusions about the properties of a suspension.
• Describe how a suspension behaves.

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Materials

For each student pair:

• 1 cup water
• 1 to 2 cups cornstarch
• Mixing bowl
• Spoon
• (Optional) Food coloring

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Preparation

• Read and familiarize yourself with the activity.
• Watch the video linked in the Resources section to understand the steps of the activity.
• Make a bowl of the quicksand mixture in advance so students can see what the final product looks like as they make their own mixture.
• Gather the materials for the activity.

Resources:

YouTube Video: Science of cornstarch and water (MIT)

Activity Steps

1. Invite students to tell you what they know about quicksand. Discuss how quicksand is a generic term for loose, wet sand that does not support much weight. Movies sometimes present a patch of quicksand as a deadly trap, but in real life it is not especially dangerous.
2. Tell students that today, they will work with a partner to make quicksand. Have students work in pairs and distribute the materials.
3. Have pairs pour 1 cup of cornstarch into a bowl.
4. Have pairs add food coloring, if desired, to 1 cup of water. Then have them slowly pour the water into the bowl with the cornstarch. They should mix the two ingredients until the mixture becomes thick and hardens when students tap on it. If the mixture gets too thin, add more cornstarch. If it gets too thick, add a little more water.
5. For a model, show students the quicksand you prepared, if necessary.
6. Have pairs observe what happens when they work with the quicksand. Instruct students to move their fingers slowly and quickly through the mixture. Have them describe to their partner what happens.
7. Next have students experiment further. Ask: What happens when you push hard on the material? What if you squeeze it into a ball? Stir gently? Roll it? Encourage students to try each of these activities and observe the results, and then to try explaining their observations.
8. Reconvene the whole group. Explain that the particles of cornstarch are very small—100 times smaller than a grain of sand. Because the particles are so small, they behave in unusual ways. When you apply pressure, such as by pounding on the quicksand, friction between the particles causes them to behave like a solid. But when you move slowly, the particles slide past each other like a liquid.
9. Discuss these questions: 
   
   • What happens when you apply pressure to the cornstarch quicksand? What happens when you do not apply pressure? (When you apply pressure, the quicksand becomes solid. When you do not apply pressure, it behaves like a liquid.)
   • Real quicksand is made of sand and water. Using what you know about the cornstarch mixture, how do you think real quicksand behaves? (It is solid, but if you step on it, the mixture takes on the properties of a liquid.)
   • People sometimes think that matter is either a solid, a liquid, or a gas. How does this activity challenge that idea? (The cornstarch mixture has properties of both solids and liquids. It may not be useful to classify the mixture in either category.)

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Variations

1. Make a large batch of quicksand in a vat or tub. Use about 10 cups of water and 20 cups of cornstarch. Have students take off their shoes and socks to stand in the quicksand. Can they walk without sinking in? What happens when they stand still on the quicksand? Encourage students to sink down and then wiggle their toes. What happens?

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