How Many Drops of Water Can Fit on a Penny?

Materials

• penny
• dropper
• water in a container (small plastic/paper cup)
• plate/paper towel (optional)

Directions

• Have students observe a penny with a hand lens.
• Have students predict the number of drops of water that can fit on a penny.
• Use an eye dropper to place drops of water one at a time on the penny until the water runs over the edge of the penny.
• Record the data.
• Older children can repeat the experiment a few more times and calculate the average.

If it is their first time doing this experiment, children will probably be amazed at how many drops can fit on a penny. Most students will underestimate the number.  Water molecules are attracted to other water molecules (cohesion). Molecules near the surface of a liquid pull toward each other. The cohesion of water molecules forms a kind of "skin" on the surface.  A drop keeps its shape because of this surface tension.

Older students can brainstorm ways to change this experiment. Perhaps use a nickel or quarter. Try the back of the penny. What other ideas can students come up with?

Another variation of this experiment is to use a cup of water filled to the brim. Have the students predict how many drops of water can fit in the cup without the water spilling over.  Have them use a dropper to add drops of water to the cup. They should count the drops and record their data.


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Let's try another experiment with  cohesion and surface tension.

Materials

• water
• pennies
• cup
• plate/pan/paper towel to catch any overflow

Directions

• Place the cup on a desk or table with a pan/plate underneath to catch any overflowing water.
• Fill the cup with water to the rim.
• Predict how many pennies you can add to the full cup.
• Slowly and carefully drop a penny into the cup.
• Continue adding pennies in the same manner until the water spills/overflows.
• Record the data.

The water level actually rises above the top of the cup due to surface tension. The surface tension forms a kind of "skin" on the top of the water to hold the water together. Water drops are more elastic than we think!

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