Drops on a penny
01 // Question
How does adding dish soap to water change how many drops a penny can hold?
Water-water attraction contributes to surface tension; wetting on the coin and gravity also shape the dome.
02 // Variables
Independent
Liquid dropped on the penny- 30 mL plain water
- 30 mL water + 1 drop of dish soap
Dependent
Drops before the first visible spill- Drop count for each trial
- Mean and range across 3 trials
03 // Hypothesis
IF dish soap is added to the water, THEN the dome may spill at a different drop count under the same method, BECAUSE a surfactant changes the liquid's surface behavior and may make the dome spread or spill sooner under the same drop method.
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
Hold the same dropper vertically at a marked height and add drops near the center, at a steady pace.
3. Use one spill rule
Count each drop that stays on the coin. When the next drop causes the first visible spill, do not count that spilling drop; record the last stable drop 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 1 measured drop of dish soap into 30 mL of water and stir gently. Run three trials, drying the penny between each, with the same coin face and drop method.
Materials // Prepare the bench
- Clean pennies
- water (two 30 mL portions per setup)
- graduated cup
- dropper or pipette
- tray
- paper towels
- dish soap
- optional marked 1 mL syringe
05 // Data table
| Liquid | Trial 1 (drops) | Trial 2 | Trial 3 | Mean | Range |
|---|---|---|---|---|---|
| Plain water | ____ | ____ | ____ | ____ | ____ |
| Water + dish soap | ____ | ____ | ____ | ____ | ____ |
06 // What to expect
Plain water may form a visible dome above the penny. A detergent solution often spreads or spills sooner, but the count is not guaranteed: drop volume, coin wetting and cleanliness, leveling, detergent concentration, and the spill rule can all change the result.
Surfactants such as dish detergent alter the balance of interactions at a liquid's surface and can lower water's surface tension. The drop-count test is a classroom comparison, not a direct numerical measurement of surface tension; use a marked syringe to compare delivered volume when available.
07 // Lab notes
- Dry the coin completely between trials; water, soap residue, skin oil, and scratches can change how the liquid spreads.
- Use the same dropper vertically at a marked height and pace. Different drop sizes make raw counts hard to compare.
- Define the endpoint before starting: count the last drop that stays on the coin, not the next drop that causes the first visible spill. Use that same rule for both liquids.
08 // Remember
Drop counts depend on the method. Surface tension helps shape the dome; technique sets the count.
09 // Make drop counts comparable
A drop is not a fixed unit unless the dropper, angle, and squeeze are kept consistent. Hold the pipette vertically at a marked height and release one drop at a time near the center of a level penny. Count each drop only after it lands, and stop when the dome spills over the edge. Use the same dropper throughout a set, because two pipettes can form different-sized drops.
Clean and dry the coin between trials. Skin oil, detergent residue, scratches, and coin orientation can change how water spreads across the surface. Repeat each condition at least three times on the same coin face and keep every count. Calculate the mean and range (highest count minus lowest count) rather than replacing an inconvenient result. If you have a marked 1 mL syringe, fill it to 1.0 mL, add liquid until the first spill, and record what remains. Subtract the remaining volume from 1.0 mL to estimate the delivered volume; repeat for both liquids. This is a more comparable measure than drop count because drops are not equal-sized units.
10 // Connect the dome to cohesion and surface tension
Water molecules attract one another, so the drop can hold a curved shape for a time before gravity and added water make it spill. The metal surface and any residue affect how strongly the water spreads. A tall dome is a visible result of competing forces, not a direct measurement of surface tension. If comparing plain water with soap solution, the treatment changes the liquid's properties and may make the dome collapse at a different count.
Graph the number of drops for each liquid or surface treatment. If the question is about soap, keep coin type and dropper technique constant and use several trials for both liquids. State the exact operational definition of 'holds': for example, the count just before the first visible spill. Two groups that use different stopping rules cannot fairly combine their results.
11 // Improve the test
A common problem is changing drop size midway through a trial. Practice a steady squeeze on a spare surface before collecting data, and have one person operate the pipette for all trials. If the penny is tilted, the dome will run toward the low edge; use a level tray. Replace a coin only when it is damaged or contaminated, and document that change.
For a more advanced version, compare several liquids or coated surfaces and photograph the dome from the side against a simple grid. The image can help compare height and contact angle, but lighting and camera position must remain fixed. A careful conclusion says which condition held more drops under this procedure. It does not claim that pennies always hold a fixed number of drops, because drop volume and surface condition vary.
12 // Why technique changes the drop count
The dome that forms on a coin comes from cohesion among water molecules and the surface interaction with the coin. A detergent changes those interactions, so a detergent solution may spread or break apart sooner. The number of drops is only a classroom proxy: drop size, dropper angle, release height, coin cleanliness, and the experimenter's definition of failure all affect the count.
Use one dropper, hold it at a marked height, and release drops at a steady pace near the center. Clean and dry the coin between trials, then repeat each liquid several times and compare the spread as well as the average. Keep detergent concentration consistent; otherwise the comparison changes both liquid identity and concentration at once.
13 // Questions people ask
Why can water form a dome on a penny?
Attractions among water molecules contribute to surface tension and help the drop resist increasing its surface area. Adhesion to the coin, the coin's cleanliness and texture, and gravity also affect the shape, so the dome is not caused by surface tension alone.
How can dish soap change the result?
Dish detergent contains surfactants that alter interactions at the water surface and can lower surface tension. They can also change wetting and drop formation, so compare several controlled trials and report what happened with your dropper and coin instead of promising a fixed count.
How many drops can a penny hold?
There is no universal count. Dropper opening and angle, squeeze, drop height, coin face and condition, leveling, and the definition of a spill all affect the result. Record your own repeated trials and their spread; use a marked syringe to compare water volume if available.
14 // Keep going
Can you explain the investigation?
Drops on a penny · Student Data Sheet
Question: How does adding dish soap to water change how many drops a penny can hold?
Hypothesis: If dish soap is added to the water, then the dome may spill at a different drop count under the same method, because a surfactant changes the liquid's surface behavior and may make the dome spread or spill sooner under the same drop method.
Materials
- Clean pennies
- water (two 30 mL portions per setup)
- graduated cup
- dropper or pipette
- tray
- paper towels
- dish soap
- optional marked 1 mL syringe
Safety before starting
- Coins and small parts are choking hazards for young children. Keep them out of mouths, and do not drink the test water after dish soap has been added.
- Keep soapy water away from eyes, work over a tray, and wipe up drips so the surface does not become slippery.
Lab notes
- Dry the coin completely between trials; water, soap residue, skin oil, and scratches can change how the liquid spreads.
- Use the same dropper vertically at a marked height and pace. Different drop sizes make raw counts hard to compare.
- Define the endpoint before starting: count the last drop that stays on the coin, not the next drop that causes the first visible spill. Use that same rule for both liquids.
Variables
Independent: Liquid dropped on the penny · Dependent: Drops before the first visible spill
Keep constant: same 30 mL water volume · same measured soap dose · same dropper and height · clean, dry coin · same spill rule
Procedure
- Dry the penny: Place a penny flat on a level surface and make sure it is completely dry.
- Add drops one at a time: Hold the same dropper vertically at a marked height and add drops near the center, at a steady pace.
- Use one spill rule: Count each drop that stays on the coin. When the next drop causes the first visible spill, do not count that spilling drop; record the last stable drop count.
- Repeat 3 times: Dry the penny and repeat two more times with plain water, then average the results.
- Repeat with soapy water: Mix 1 measured drop of dish soap into 30 mL of water and stir gently. Run three trials, drying the penny between each, with the same coin face and drop method.
Record your observations
| Liquid | Trial 1 (drops) | Trial 2 | Trial 3 | Mean | Range |
|---|---|---|---|---|---|
Answer key and expected pattern
Plain water may form a visible dome above the penny. A detergent solution often spreads or spills sooner, but the count is not guaranteed: drop volume, coin wetting and cleanliness, leveling, detergent concentration, and the spill rule can all change the result.
Surfactants such as dish detergent alter the balance of interactions at a liquid's surface and can lower water's surface tension. The drop-count test is a classroom comparison, not a direct numerical measurement of surface tension; use a marked syringe to compare delivered volume when available.
Remember: Drop counts depend on the method. Surface tension helps shape the dome; technique sets the count.