LF-05 · TEACHER LESSON PLAN

Drops on a penny

How surface tension holds a dome of water on a coin

ChemistryGrades 5–830 minHazard low; small coin, slippery soapy waterNGSS MS-PS1-1

01 //

Driving question

START THE LESSON WITH THISHow does adding dish soap to water change how many drops a penny can hold?

02 //

Learning objectives

Students plan a repeated comparison of plain and soapy water using the same clean coin, dropper, drop height and level surface.
Students measure the whole-number count of drops held on the coin immediately before the next drop causes the first spill, for three trials per liquid.
Students explain how cohesion and surface tension help plain water form a dome, while soap can reduce the number of drops held.

03 //

Standards

Dimension	Alignment
Performance expectation	MS-PS1-1Develop models to describe the atomic composition of simple molecules and extended structures. (Partial alignment: this is the performance expectation the lab lists, but the activity investigates how many water drops stay on a coin surface (surface tension and cohesion) rather than building atomic models.)
Practices	SEP 3Plan and carry out investigations.
Core idea	PS1.AEach pure substance has characteristic physical and chemical properties (for any bulk quantity under given conditions) that can be used to identify it.
Crosscutting	Cause and effectCause and effect relationships are routinely identified, tested, and used to explain change.

Standards text paraphrased from nextgenscience.org. Alignment is partial; this hands-on activity does not cover every part of the performance expectation. CCC wording not independently verified.

03A //

State standards crosswalk

Use the row as a planning crosswalk, then confirm local district pacing and grade placement. “Partial” labels describe a skill connection, not full mastery of the complete standard.

State	Verified standard	How this lesson aligns	Official source
California	CA NGSS 5-PS1-3	Limited. Students measure a liquid-related outcome, but do not identify materials by the full set of properties in the expectation.	Open standard ↗
Texas	Science.8.6(C)	Partial. The standard names cohesion, adhesion and surface tension; this lab compares drop counts with and without soap but does not model the full water-property set.	Open standard ↗
Florida	SC.5.P.8.1	Limited. Students observe water behavior, but the benchmark's broader properties-of-matter expectations are not all assessed.	Open standard ↗
New York	5-PS1-3	Limited. The activity collects measurements, but surface tension is not one of the example identification properties in the grade-five standard.	Open standard ↗
Pennsylvania	3.2.5.B	Limited. Students measure a water response, but do not use a range of observations to identify unknown materials.	Open standard ↗

04 //

Materials and prep timeline

PER GROUP

One clean penny placed flat on a tray
One dropper, 30 mL plain water and 30 mL water with 1 drop of dish soap
Two small labeled cups for the liquids
Six paper-towel squares to dry the coin completely between all trials
Recording sheet and calculator

TEACHER ONLY

Check that coins are age-appropriate; keep them away from mouths and younger children because they are small parts.
Demonstrate a consistent dropper height and angle and a level penny. Use a tray to catch spills and wipe soapy water promptly.
Do not drink test water after soap is added. Provide clean water for handwashing and keep the penny clean and dry between every trial.

CLASS-SIZE KIT PLANNER

One per-group kit contains every item in the team materials list above. Groups are balanced around the listed 2–3 students target.

Students	Groups	Per-group kits	Student sheets	Suggested group split
20	7	7 complete kits	20 copies	6 groups of 3 and 1 group of 2
25	9	9 complete kits	25 copies	7 groups of 3 and 2 groups of 2
30	10	10 complete kits	30 copies	10 groups of 3

0–5 MINPredict and standardize

Predict which liquid may allow more drops. Agree to count drops that stay on the coin, excluding the drop that causes the first spill; standardize dropper height and review safety.

5–10 MINPrepare the clean coin

Use one clean penny, a level tray and separate cups of plain and soapy water. Practice steady, one-at-a-time drops from the same height.

10–20 MINRun plain-water trials

Add drops one at a time; when the next drop causes the first spill, record only the drops that stayed on the coin. Dry the penny fully and repeat until three plain-water trials are recorded.

20–25 MINRun soapy-water trials

Repeat three trials with water containing 1 drop of dish soap, drying the coin completely between each trial. Keep dropper height and angle unchanged.

25–30 MINAverage and explain

Calculate each three-trial mean and compare the liquids. Explain how soap changes surface tension; wipe spills and wash hands.

04A //

Lab figures and data visuals

Reviewed article images support the lesson flow. Use the tables and student sheet to record actual observations; pictured patterns are not target results.

A rounded dome of water sits on a copper penny as a dropper adds another drop. — **A water dome forms before it spills.**Water-water attractions contribute to surface tension; the coin surface, wetting, and gravity also influence the dome.

A simple penny-drop diagram labels cohesion between water molecules, adhesion where water meets the coin, and gravity pulling the drop downward. — **More than one interaction shapes the dome.**Cohesion, adhesion to the coin, and gravity all contribute to the visible shape; surface tension is part of this balance.

A neutral comparison setup shows matched plain-water and detergent-water trials, a consistent dropper height, and blank spaces for three student trials. — **Keep the technique steady and record your result.**The diagram leaves outcomes blank: drop volume, coin condition, and the spill rule affect the observed count, so students should report their own trials.

04B //

Printable shopping list

TEACHER PREP · Drops on a penny

Shopping and staging checklist

Multiply the per-group kit contents by the class-size planner. The teacher section includes teacher-provided materials and setup or safety notes.

PER-GROUP KITS

One clean penny placed flat on a tray
One dropper, 30 mL plain water and 30 mL water with 1 drop of dish soap
Two small labeled cups for the liquids
Six paper-towel squares to dry the coin completely between all trials
Recording sheet and calculator

TEACHER-PROVIDED MATERIALS + SETUP RULES

Check that coins are age-appropriate; keep them away from mouths and younger children because they are small parts.
Demonstrate a consistent dropper height and angle and a level penny. Use a tray to catch spills and wipe soapy water promptly.
Do not drink test water after soap is added. Provide clean water for handwashing and keep the penny clean and dry between every trial.

QUANTITIES BY CLASS SIZE

Material	Per group kit	20 students	25 students	30 students
Clean coin	1	7	9	10
Dropper and labeled cups	1 dropper and 2 cups	7 droppers and 14 cups	9 droppers and 18 cups	10 droppers and 20 cups
Plain water	30 mL	210 mL	270 mL	300 mL
Soapy water	30 mL with 1 drop dish soap	210 mL with 7 drops soap	270 mL with 9 drops soap	300 mL with 10 drops soap
Coin-drying towels	6 small squares	42 squares	54 squares	60 squares
Student recording sheets	1 per student	20 sheets	25 sheets	30 sheets

Prepare 7 kits for 20 · 9 kits for 25 · 10 kits for 30 students.

05 //

Lesson flow

Minutes	What happens
0–5 min	Engage Ask whether a penny will hold more drops of plain or soapy water. Set a common drop height and agree to exclude the drop that causes the first spill; review coin and soap precautions.
5–10 min	Explore Place one clean penny flat on a level tray. Prepare separate cups of plain water and water with 1 drop of dish soap and practice a steady dropper position.
10–20 min	Explore Add drops one at a time; when the next drop causes the first spill, record only the drops that stayed on the coin. Dry it fully and repeat for three plain-water trials.
20–25 min	Explore Repeat for three soapy-water trials using the same coin and dropper position. Dry the penny completely between trials.
25–30 min	Explain Calculate three-trial means and compare liquids. Explain how soap can reduce surface tension; wipe drips and wash hands.

06 //

Discussion questions

1. Why can plain water form a dome above the penny?

SAMPLE ANSWERCohesion attracts water molecules to one another at the surface. Surface tension lets the dome hold together until added drops make it spill.

2. Why does soapy water often spill sooner?

SAMPLE ANSWERSoap molecules can disrupt the attraction among water molecules at the surface, so the dome may hold fewer drops.

3. Why dry the same penny between all trials?

SAMPLE ANSWERLeftover water changes the surface and can affect how the next dome forms. A clean, dry coin makes trials more comparable.

4. Why compare three trials instead of one?

SAMPLE ANSWERDrop size, release and counting can vary from trial to trial. The mean summarizes repeated counts while the individual results show variation.

07 //

Common misconceptions

STUDENTS OFTEN SAY“The penny absorbs the water until it is full.”

BETTER IDEAThe coin is not absorbing the drops; cohesion and surface tension hold a dome above its surface.

STUDENTS OFTEN SAY“Soap makes water molecules stop attracting one another.”

BETTER IDEASoap can weaken surface tension, but it does not remove all attraction or make water stop behaving as a liquid.

STUDENTS OFTEN SAY“Twenty-five drops is the one correct answer.”

BETTER IDEADropper, height, coin cleanliness and technique affect counts. Compare matched trials and report the measured values.

08 //

Differentiation

SUPPORT

Use a marked stand or card to keep the dropper at the same height for every trial.
Assign one partner to release drops and another to count aloud and record.
Offer the frame: “Plain water held an average of __ drops and soapy water __, a difference of __ drops.”

EXTEND

Calculate the range of three counts for each liquid and compare it with the difference between the means.
Test another measured soap amount while keeping coin, dropper and height constant.
Compare two clean coins and explain how surface condition might affect results.

09 //

Assessment rubric

Criterion	Beginning	Developing	Secure
Controlled method	Changes coin or dropper method between trials.	Keeps some conditions consistent but does not dry the coin or level the surface.	Uses the same clean, dry penny, dropper, height, angle and level surface throughout.
Trial record	Records few counts or omits the endpoint.	Records trials but omits units or does not separate the two liquids.	Records three whole-number counts of drops held before the spilling drop for each liquid.
Mean and comparison	Does not calculate means or compare liquids.	Calculates one mean or compares without using all trials.	Calculates both three-trial means correctly and compares the measured counts.
Explanation and care	Says the coin absorbs water or drinks soapy water.	Names surface tension but omits repeatability or coin safety.	Explains cohesion/surface tension, does not taste samples and wipes spills to prevent slipping.

10 //

Vocabulary

Cohesion – Attraction among molecules of the same substance, such as water molecules attracting one another.
Surface tension – The effect of cohesive forces at a liquid surface that resists breaking or spreading.
Drop count – The number of separate drops added before the stated spill endpoint.

Trial – One run of a test under a specified method.
Mean – The sum of values divided by the number of values.
Surfactant – A substance such as soap that can change surface tension.

11 //

Safety, cleanup and troubleshooting

BEFORE STUDENTS TOUCH ANYTHING

Use a clean penny and keep coins away from mouths and younger children because they are small parts. Do not drink water after dish soap is added.
Keep soapy water away from eyes. Work over a tray and wipe drips promptly so the surface does not become slippery.
Wash hands after handling the coin and liquids. Dry the penny completely between every trial and keep it on the tray.

Troubleshooting: If the water spreads immediately, check that the penny is level, clean and fully dry and that the dropper is held at the same height. If drops spill before counting is clear, dry the coin and repeat. Wipe all soapy drips before moving the tray or walking nearby.