Lab / experiment Grade 10 Science

Counting Water Drops on a Coin: A Surface-Tension Lab

A grade 10 lab where students count how many drops of plain and soapy water a coin holds, then use the results as evidence of water's hydrogen bonding.

Grade: Grade 10
Subject: Science
Material: Lab / experiment
Time: 45 min
Prep: Low prep
Materials: Clean coins or pennies (1 per group), Medicine dropper or fine pipette (1 per group), Cup of tap water, Cup of soapy water (a few drops of dish soap stirred into water), Paper towels

Standards

HS-PS1-3 NGSS

Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.

How this material aligns: Students investigate surface tension by counting the drops a coin holds and compare plain to soapy water to infer the strength of water's hydrogen-bonding forces, this standard's bulk-property investigation.

All materials for HS-PS1-3 →

HSN-Q.A.1 Common Core Math

Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays.

How this material aligns: Analysis questions 1, 2, and 4 have students average trial drop counts and compute a percent change, keeping the count units consistent across each calculation.

All materials for HSN-Q.A.1 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will measure water's surface tension by counting the drops a coin holds.
Students will calculate an average and a percent change from their trial data.
Students will explain surface tension as evidence of strong attractions between water molecules.

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.

Also: cohesion, hydrogen bonding, surfactant

Teacher overview

Water molecules pull on one another strongly enough that a surface of water behaves as though it has a thin skin, which is why a coin can hold a surprising number of drops in a rounded dome before it spills. That skin is surface tension, and it comes from hydrogen bonding between water molecules. In this lab, students count how many drops of plain water a coin holds, repeat with soapy water, and use the difference as evidence about the strength of those attractions. The investigation is a simple stand-in for the molecular idea, not a precise measurement of surface tension in scientific units.

Counting Water Drops on a Coin: A Surface-Tension Lab — Grade 10 Science

Name: ____________________________ Date: ______________

Procedure

Step 1 (set up): Place a clean, dry coin flat on a paper towel. Fill the dropper with plain tap water. Decide as a group which face of the coin you will use, and use that same face every time.

Step 2 (first plain-water trial): Hold the dropper just above the coin and add water one drop at a time, counting each drop, until the water bulges over the edge and spills. Record the number of drops the coin held before spilling as Trial 1.

Step 3 (repeat plain water): Dry the coin completely and run two more plain-water trials the same way, recording each count. You will have three plain-water trials in all.

Step 4 (soapy-water trials): Empty and rinse the dropper, then fill it with the soapy water. Dry the coin and run three trials with soapy water, counting drops until it spills each time, and record each count.

Step 5 (safety and cleanup): Do not drink any of the water used in this lab, and wipe up spills right away so the floor and table stay dry and no one slips. Return materials as your teacher directs.

Step 6 (record): Enter all six counts in the data table before starting the analysis.

Data & Analysis

Trial	Plain water (drops held)	Soapy water (drops held)
1
2
3

Calculate the average number of drops the coin held for plain water across your three trials.
Calculate the average number of drops the coin held for soapy water across your three trials.
Find the difference between your plain-water and soapy-water averages. What does that difference show about how soap changed the water's surface tension?
Calculate the percent change from the plain-water average to the soapy-water average, using (plain average minus soapy average) divided by plain average, times 100.
Plain water forms a tall, rounded dome on the coin because its molecules attract one another through hydrogen bonding. Using your results, explain what the soap did to those attractions.
Surface tension is a bulk property that reveals strong forces between water molecules. Explain how a surfactant that lowers surface tension could help a treatment or cleaning process spread water across a surface.

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: Accept the student's own average; there is no single correct value, because the count is measured rather than calculated from given data. It should equal the sum of the three plain-water counts divided by three. Typical results fall in a range of about 20 to 30 drops, though the exact number depends on the coin, the dropper, and how steadily the drops are added.
Answer: Accept the student's own average; there is no single correct value, because the count is measured. It should equal the sum of the three soapy-water counts divided by three and should be clearly lower than the plain-water average, because the soap reduces the surface tension.
Answer: The difference equals the plain average minus the soapy average and should be positive, showing that soapy water held fewer drops. That means the soap lowered the surface tension, so the water spilled sooner. Accept any answer consistent with the student's own data and a clear explanation.
Answer: Accept the value calculated from the student's own averages; there is no single correct percentage, because it depends on the measured counts. It should be a positive percentage found with (plain average minus soapy average) divided by plain average, times 100. For example, a group whose plain-water average was 25 and soapy-water average was 10 would report (25 - 10) / 25 x 100 = 60 percent, but the exact figure varies from group to group.
Answer: The soap acts as a surfactant that gets between water molecules and weakens the hydrogen-bonding attractions at the surface. With weaker attractions, the surface cannot support as tall a dome, so the coin holds fewer drops before spilling.
Answer: Lower surface tension lets water spread out and wet a surface instead of beading up, so a surfactant can help water reach into small spaces, carry cleaning agents across a surface, or spread evenly during a treatment step. Accept any reasonable application tied to easier wetting or spreading.

Differentiation

Support

Run one plain-water trial as a class demonstration first, counting drops aloud, so students know what "spilling" looks like before they collect their own data. Provide a partly filled data table that already lists the three trial rows.

Extension

Have students test a third liquid, such as salt water, predict whether it will hold more or fewer drops than plain water, and explain their prediction using the idea of attractions between molecules before they run the trials.

ELL supports

Pair "surface tension," "cohesion," and "surfactant" with simple pictures of a water dome on a coin and a flattened soapy drop, labeled in English and, where possible, students' home languages. Allow students to record counts as numbers and answer items 5 and 6 with a labeled sketch plus a short phrase.