Drops of Water on Penny (Erango)

Author

Erango Kelbisow

Principles

Hydrogen bonds and surface tension

Water molecules stick to other water molecules. If you are a water molecule on the top of the pile, your connection to the other water molecules is weaker because there are none to attach to on top. This weaker surface tension gets overcome by gravity and then the water overflows.

Standards

Materials needed

• pennies

• eyedropper

• water

• petri dish

Procedure

1. Have students predict how many drops of water can fit on the head and tail side of a penny, without any spillage.

2. Run trials on the head side.

3. Run trials on the tails side.

Explanation

Provide a solid scientific explanation for the principles illustrated by the demonstration equipment.

This explanation comes from pbskids. Water molecules stick to other water molecules. If you are a water molecule on the top of the pile, your connection to the other water molecules is weaker because there are none to attach to on top. This weaker surface tension gets overcome by gravity and then the water overflows.

Questions

1. How close to the results was your estimate? Explain

2. Why did the drops stay on the penny?

3. Was there a difference between how many drops could the head and tail side hold? If so, why do you think that difference existed?

Everyday examples of the principles illustrated

• water striders walking on top of water

• dew on the surface of leaves

Photos

Surface tension was not enough to hold this water on top on the penny.

Surface tension is still holding the drops of water on top of the penny.

Movies

References

http://www.stevespanglerscience.com/lab/experiments/penny-drops

http://www-tc.pbskids.org/zoom/printables/activities/pdfs/penny.pdf