Drops on a penny
01 // Question
How does adding dish soap to water change how many drops a penny can hold?
Water molecules pull on each other at the surface, forming a stretchy "skin" called surface tension.
02 // Variables
Independent
Liquid dropped on the penny- Plain water
- Water with 1 drop of dish soap
Dependent
Number of drops before spilling- Drop count, whole number
- Average of 3 trials
03 // Hypothesis
IF dish soap is added to the water, THEN the penny will hold fewer drops before it spills, BECAUSE soap molecules weaken the attraction between water molecules, lowering surface tension.
04 // Procedure
1. Dry the penny
Place a penny flat on a level surface and make sure it is completely dry.
2. Add drops one at a time
Using a dropper held at the same height, add water one drop at a time.
3. Count until it spills
Keep counting until water spills over the edge of the penny, then record the count.
4. Repeat 3 times
Dry the penny and repeat two more times with plain water, then average the results.
5. Repeat with soapy water
Mix water with 1 drop of dish soap and repeat the same steps.
05 // Data table
| Liquid | Trial 1 | Trial 2 | Trial 3 | Average |
|---|---|---|---|---|
| Plain water | ____ | ____ | ____ | ____ |
| Water + dish soap | ____ | ____ | ____ | ____ |
06 // What to expect
Plain water typically holds somewhere around 20 to 30 drops before spilling, forming a visible dome above the penny's surface. Soapy water spills much sooner, often after only a handful of drops.
Dish soap molecules get between water molecules at the surface and weaken the pull that holds the dome together, which is also why soap makes bubbles and helps water spread out instead of beading up.
07 // Lab notes
- Dry the penny completely between every trial — leftover water changes the surface and skews the count.
- Hold the dropper at the same height and angle every time so each drop is a similar size.
- Watch closely near the end — the dome gets very tall right before it collapses, and it is easy to miscount the final drop.
- Safety. This lab uses only water, dish soap, and a penny — no real chemical hazard, but avoid tasting the soapy water.
08 // Remember
Surface tension is water holding its own hand. Soap breaks the grip.
09 // Questions people ask
Why does water form a dome on a penny instead of spreading out flat?
Water molecules are strongly attracted to each other, pulling the surface into the smallest possible shape it can hold — that inward pull, called surface tension, is strong enough to let water pile up above the flat edge of the penny before spilling.
How does soap change the number of drops a penny holds?
Soap molecules insert themselves between water molecules at the surface, weakening the attraction that creates surface tension. With less surface tension, the water dome collapses much sooner, so a penny holds far fewer soapy drops than plain water drops.
How many drops of water can a penny hold?
It varies by penny and drop size, but plain water on a clean, dry penny commonly holds somewhere around 20 to 30 drops before it spills, thanks to surface tension holding the dome together.