What explains the result?
Surface tension helps a water dome hold together; soap changes the surface behavior and usually lowers the drop count.
PHYSICAL SCIENCE · GRADES 4–8
Drops on a penny
INVESTIGATION QUESTIONHow does soap change drops held on a coin?
● ILLUSTRATIVE SCIENCE MODEL
ENGLISH CAPTIONS
Clean and dry the coin. A clean surface gives each trial a fair starting condition.
0.0 / 3.0 s
Where does this matter?
Surface forces affect droplets, cleaning, and how water spreads on leaves and materials.
What should you keep in mind?
Drop size, coin cleanliness, and the way the dropper is squeezed affect the count.
DEDICATED INVESTIGATION · GRADES 4–8
Penny-drop observatory
Add one measured drop at a time. Compare clean water with soapy water and watch the curved surface give way.
MACRO CAMERA · COIN LEVELH₂O
ONE COINTEST SURFACE
0DROP COUNT0
COUNTED DROPS00ADD ONE DROP SLOWLY
TRIAL LEDGER
Evidence before conclusion
PLAIN WATER MEAN—0 trials · drops
SOAPY WATER MEAN—0 trials · drops
plainsoapy
No runs recorded yet. Add drops until the surface breaks, then save the trial.
CONNECT TO EVERYDAY LIFE
Surfactants in soaps help water spread over greasy dishes and into fabric fibers. A penny dome is a visible model of surface behavior, not a direct measurement of surface tension in newtons per meter.
Research basis: Science Buddies — Measuring Surface Tension with a Penny ↗