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How Many Drops of Water Can Fit on a Penny Surface Tension Experiment

Surface tension is a force that pulls together the outermost molecules of a liquid.

It makes water form tiny beads on a leaf and hold a raindrop in the air.

It is also why bubbles can form and then burst.

Here’s a STEM challenge.

This challenge is to see how many drops of water will fit on a penny without spilling.

Besides penny, you may use a dime or quarter.

See who can put the most drops of water on the coin without spilling.

drops of water on top of penny form a dome shape

How Many Drops of Water Can Fit on a Penny Surface Tension Experiment

Active Time: 30 minutes
Total Time: 30 minutes

Here is a STEM challenge for you: how many drops of water can you put on a penny?

Materials

  • water
  • coins

Tools

  • a dropper or pipette
  • adult supervision

Instructions

  1. Using the dropper, carefully put a drop of water in the center of the coin.

    Using a dropper, drop water onto the penny
  2. Keep adding water one drop at a time and keep count.

    Closeup of dropper putting water onto penny
  3. You will see a dome shape start forming by the puddle of water.

    water on penny showing surface tension in this experiment
  4. Stop when the water spills over the coin.

    water drops overflow from the penny

Notes

  • Repeat the experiment using different types of liquid, such as cooking oil, water with dish washing liquid added, liquid sunscreen, etc.
  • Pay attention to the shape of the liquid puddle and keep count of the number of drops you can put before it spills over.

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Why

Water molecules attract each other and tend to stick together.

This cohesion property results in surface tension.

Because water molecules at the surface of the water puddle attract more to one other than they do to the air molecules above them, they cling together and form a dome shape on the coin.

Surface tension prevents the water molecules from falling out and spilling.

You can keep adding water drops until the surface tension is not strong enough to counter the gravitational pull on the water.

In the exploration part of the experiment, you see that the sizes of drops coming out of the dropper are different for different types of liquid.

That is because the molecules in each liquid are attracted to one another to a different degree.

So the surface tension of each liquid is different. The number of drops you can put on a coin without spilling is also different.

A drop of water on a pink surface, try this surface tension science project.

More Surface Tension Water Experiments

Surface tension is one of many interesting properties of water. Check out these other experiments on water properties.

Here are three science kits to help you explore more these amazing properties of water.

A dime with glob of water on top.
What is surface tension? STEM challenge, How much water can a coin hold?
a penny with a glob of water on.
What is surface tension? See how many drops of water you can put on a coin.
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