Lab / experiment Grade 11 Science
Surface Tension and Hydrogen Bonding: A Coin-and-Droplet Lab
A grade 11 chemistry lab where students measure surface tension with a penny-and-dropper method, comparing plain water to a surfactant solution to study hydrogen bonding.
Objectives
- Students will measure and compare the surface tension of plain water and soap-treated water using a coin-and-droplet method.
- Students will explain, in terms of hydrogen bonding, why adding a surfactant changes water's bulk surface behavior.
- Students will calculate a percent change in a measured bulk property and apply it to a water-treatment scenario.
Vocabulary
- Surface tension
- Surface tension is the way water molecules pull together at the surface, making the surface act a little like a stretchy skin. It is why some insects can walk on water and why water forms rounded drops instead of flat puddles.
Teacher overview
Water's surface behaves like a thin, stretched skin, a property called surface tension that comes directly from hydrogen bonding between water molecules. Every molecule in the interior of a sample is pulled equally in all directions by its neighbors, but molecules at the surface are pulled only sideways and downward, which draws the surface into the tightest possible shape and lets water dome slightly above a rim before it finally spills. This lab uses a plain penny and a dropper to measure that bulk property directly, then compares plain water to water with a small amount of surfactant added, the same category of chemical that causes unwanted foam in wastewater aeration basins. A common misconception to correct directly: many students assume soap works by somehow "cutting" or dissolving water itself, when in fact soap molecules insert themselves at the water's surface and disrupt the hydrogen bonds holding that surface together.
Procedure
Step 1. Safety: this lab uses only water and a small amount of dish soap, so there is no chemical hazard beyond ordinary lab housekeeping. Wipe up any spilled water promptly, since a wet floor or desk is a slip hazard, and do not taste or drink either solution.
Step 2. Prepare two solutions in labeled cups: Solution A is 50 mL of distilled water with no additive, and Solution B is 50 mL of distilled water with exactly 1 drop of dish soap stirred in gently, avoiding foam.
Step 3. Place a clean, dry penny heads-up on a flat, level surface with a paper towel underneath to catch overflow.
Step 4. Using a pipette filled with Solution A, add one drop at a time to the center of the penny, counting each drop aloud, until the water visibly overflows the rim. Record the total number of drops in the data table.
Step 5. Dry the penny completely and repeat Step 4 with Solution A two more times, for three trials total, to check how consistent the result is.
Step 6. Dry the penny and repeat Steps 4 and 5 with Solution B (the soap solution), again for three trials total, recording each count.
Step 7. Before moving to analysis, sketch or describe the shape of the water dome you observed just before overflow for Solution A and for Solution B, noting any visible difference in height or shape.
Data & Analysis
| Trial | Drops held: Solution A (plain water) | Drops held: Solution B (soap water) |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 |
- Calculate the average number of drops the penny held for the three Solution A trials.
- Calculate the average number of drops the penny held for the three Solution B trials.
- Calculate the percent decrease from your Solution A average to your Solution B average, using the formula (A average − B average) ÷ A average × 100.
- Explain, using the concept of hydrogen bonding, why plain water can dome above the rim of the penny before it finally overflows.
- Explain why adding a small amount of soap reduced the number of drops the penny could hold, connecting your answer to the strength of the forces between water molecules at the surface.
- Foam in a wastewater aeration basin is often caused by detergents in the incoming sewage. Based on your results, do detergents raise or lower the surface tension of water? Explain why lower surface tension makes it easier for a lasting foam to form.
- Predict whether a penny would hold more, fewer, or about the same number of drops of salt water compared to plain water, and explain your reasoning in terms of how dissolved ions interact with water's hydrogen-bonded structure.
- Challenge. A plant operator says detergent in the influent has lowered its surface tension by at least 50 percent. Based on your percent decrease in question 3, could one drop of dish soap per 50 mL (the concentration you tested) cause a change that large? Give one reason a drop-count change may not equal the same percent change in surface tension.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: about 25 drops.
This is an example value; accept any number equal to the average of the student's own three Solution A trial counts, typically in the range of 20 to 30 drops.Answer: about 11 drops.
This is an example value; accept any number equal to the average of the student's own three Solution B trial counts, typically in the range of 8 to 15 drops, and noticeably lower than the Solution A average.Answer: about 56 percent.
Using the example averages above, (25 − 11) ÷ 25 × 100 ≈ 56%. Accept any percent consistent with the student's own two averages and the stated formula.Answer: hydrogen bonds hold the surface together.
Hydrogen bonds pull surface water molecules sideways and downward toward their neighbors rather than outward into the air above, creating a taut surface layer strong enough to hold a dome of water above the penny's rim before gravity finally overcomes it.Answer: soap disrupts hydrogen bonding at the surface.
Soap molecules have one water-attracting end and one water-repelling end, so they position themselves at the surface and wedge between water molecules there, breaking up the hydrogen-bond network that gave the surface its strength; a weaker surface holds a shorter, flatter dome and therefore fewer drops.Answer: detergents lower surface tension.
The soap trials held fewer drops, showing that soap lowers surface tension. Less energy is needed to stretch the water into thin bubble films, and the soap molecules lining each film keep the bubbles from bursting, so detergent-laden water foams.Answer: about the same or slightly more.
Dissolved sodium and chloride ions are strongly attracted to water molecules and are pulled into the bulk, away from the surface, which slightly raises surface tension, the opposite of what a surfactant does. This is counterintuitive, so accept any well-reasoned prediction for partial credit, but full credit requires an explanation based on how ions interact with water molecules.Answer: it depends on the student's own data.
If the percent decrease calculated in question 3 is 50 percent or greater, the student's data are consistent with a change that large; if it is below 50 percent, the data suggest a larger dose would be needed. For the second part, accept a reason such as: soap also changes the size of each drop leaving the pipette and how the water spreads on the coin, so the drop count does not scale directly with surface tension.
Differentiation
Support (below grade level)
Provide a printed data table with the three trial rows already labeled, and allow students to complete Solution A fully before starting Solution B, rather than alternating between the two mid-lab. Give the percent-decrease formula from question 3 already written out, so students only need to substitute their own two averages.
Extension (above grade level)
Have students test a third solution, such as water with two drops of soap instead of one, and graph drop count against number of soap drops added to see whether the relationship between surfactant concentration and surface tension appears linear or levels off after the first drop.
ELL supports
Pair the terms surface tension, hydrogen bond, and surfactant with a simple labeled diagram showing water molecules linked at the surface, before and after a soap molecule is added. Allow students to record drop counts and complete the data table with numbers alone, saving full-sentence explanations for question 4 onward for sentence starters such as "The soap solution held fewer drops because ___."