How Many Water Drops?

Grades: 2 | Nonfiction Mini Book | 4 pages

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How Many Water Drops?

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Guided Reading Level K, Grade Level Equivalent 2.5. Children explore the surface tension of water as they predict and test how many water drops will fit on a coin.

Generate discussion about how many water drops might fit on a penny. Two? Thirty? One hundred? Have children make and record predictions, test them, and record results.

Continue with pages 3 through 5 in the same manner. Children can use the data they gather in each test to predict how many drops will fit on each type of coin. Have children organize their data on pages 6 and 7 of the mini-book, and then complete page 8 to make predictions based on their data.

TEACHING TIPS

- The Science Behind the Experiments: Inside a water drop, the water molecules are strongly attracted to each other. This attraction is called cohesion. As children drop water on a coin, drop by drop, cohesion tugs at the molecules on the surface, pulling them in from the sides and downward. This creates surface tension, which acts like a skin to hold the drop together as it gets bigger and bigger. (You might invite children to recall observations of bugs sitting on top of water and explain that it is the skinlike property of water that allows them to do this.) As more drops of water are added, the force of gravity becomes stronger than the force of attraction among the water molecules, causing the water to roll off the coin or surface. Larger coins have more surface area than smaller coins. Coins with lots of detail provide more surface area than coins with little detail. Hence, the larger the coin, and the more detailed, the more water drops it will hold. Reduce Surface Tension: Invite students to try the experiments in the minibook again, but this time with soapy water. (Adding soap reduces the surface tension; therefore, students may find that coins hold fewer drops of soapy water.)

- Making More Inferences: After students have had a chance to see how water drops react on a coin, invite them to test their inferences in different ways. Have students experiment with water drops on waxed paper, aluminum foil, sandpaper, and paper towels. Do the water drops act differently on different surfaces? On which surface do the water drops hold together best?