Classroom activity Grade 7 Science

Pentworth's PFAS Stations: Bond Models and Carbon Filters

A four-station rotation where students build carbon-fluorine models, compare well results with the 4.0 ppt limit, and test activated carbon against sand.

Grade: Grade 7
Subject: Science
Material: Classroom activity
Time: 55 min
Prep: Moderate prep
Materials: Gumdrops or modeling clay in four colors, about 20 pieces per group, Toothpicks, about 25 per group, 2 clear plastic cups or cut bottles per group, 2 empty cups per group to catch the filtered water, Aquarium activated carbon, about half a cup per group, Clean play sand, about half a cup per group, Coffee filters or paper towel, Food coloring mixed into water, about 200 mL per group, Printed station cards and student handouts

Standards

MS-PS1-1 NGSS

Develop models to describe the atomic composition of simple molecules and extended structures.

How this material aligns: At Station 1 students build ball-and-stick models of a water molecule and a fluorinated carbon chain, describing the atomic composition of each.

All materials for MS-PS1-1 →

MS-ETS1-2 NGSS

Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.

How this material aligns: Station 4 and handout items 7 and 8 have students score three treatment options against removal, cost, and waste criteria using one shared table.

All materials for MS-ETS1-2 →

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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 model the atomic composition of a water molecule and a fluorinated carbon chain.
Students will compare well test results with the federal limit for PFOA.
Students will score three treatment options against removal, cost, and waste criteria.

Vocabulary

PFAS: A large family of human-made chemicals used in things like nonstick pans, waterproof fabric, and firefighting foam. They are called forever chemicals because they take a very long time to break down.
Parts per trillion: A unit for measuring extremely small amounts of a chemical in water. PFAS limits are set in parts per trillion because these chemicals matter even in tiny doses.
Activated carbon: Activated carbon is a special charcoal full of microscopic holes that grab and hold chemicals as water flows past. It is the same material used in many home water filter pitchers.

Also: adsorption, bond

Teacher overview

This station rotation runs in four groups of four or five students, each spending 10 minutes at a station before moving on. The stations map onto the problem in order: students first build the carbon-fluorine chain that makes PFAS so hard to break down, then read well results against the federal limit of 4.0 parts per trillion for PFOA and PFOS, then watch activated carbon hold color that sand lets straight through, and finally score three treatment options for the fictional town of Pentworth. Teaching note: students often say that boiling water removes contaminants, which is true for some microbes but not for PFAS. Boiling removes water as steam and leaves the PFAS behind in what remains, so the concentration goes up rather than down.

Activity Directions

Set up before class (moderate prep). Build four station areas for the Pentworth rotation. Station 1 needs gumdrops in four colors and toothpicks; use modeling clay instead if any student has a food allergy, and remind students not to eat the gumdrops. Station 2 needs the printed well results card. Station 3 needs two cups per group, one packed with activated carbon and one with clean sand, each with small holes poked in the bottom, lined with a coffee filter, and set over an empty cup, plus the colored water. Station 4 needs the printed treatment options card.
Introduce the problem (5 min). Tell students that the fictional town of Pentworth found PFAS in two of its wells and has to decide what to do. Explain that PFAS is a family of manufactured chemicals built from carbon chains with fluorine attached, used in coatings that resist grease and water, and that the federal limit for PFOA and for PFOS is 4.0 parts per trillion each.
Assign groups and hand out the recording sheet (3 min). Groups of four or five each start at a different station and carry the student handout with them.
Station 1, molecule models (10 min). Students build one water molecule using one oxygen gumdrop and two hydrogen gumdrops, then a three-carbon chain with two fluorine gumdrops on the middle carbon and three on each end carbon. The station card tells them the carbon-fluorine bond is one of the strongest bonds in organic chemistry, which is why these molecules last so long in the environment. Students answer handout items 1 and 2.
Station 2, reading results (10 min). Students read the Pentworth well results and compare each one with the 4.0 ppt limit, then answer handout items 3, 4, and 5.
Station 3, carbon against sand (10 min). Students pour equal amounts of colored water through the carbon cup and the sand cup and compare what comes out. Remind them that nobody tastes or drinks anything at this station, and that the dye stands in for a dissolved contaminant rather than being PFAS itself. Students answer handout item 6.
Station 4, choosing a treatment (10 min). Students read the three treatment options and score them against removal, cost, and waste, then answer handout items 7 and 8.
End the rotation (debrief, 7 min). The rotation ends when every group has completed all four stations. Ask two questions of the whole class: what did the carbon column do that the sand column did not, and why would a town pick a cheaper option that removes less. Close by pointing out that Pentworth has to handle the spent carbon or resin afterward, so the contaminant is moved rather than destroyed.

Pentworth's PFAS Stations: Bond Models and Carbon Filters — Grade 7 Science

Name: ____________________________ Date: ______________

Student Handout

Record your answers as you move through the stations for the fictional town of Pentworth.

Station 1 card, what to build

Build one water molecule from a single oxygen atom bonded to two hydrogen atoms. Then build a three-carbon chain, and attach two fluorine atoms to the middle carbon so that its other two bonds go to the carbons on either side. Then attach three fluorine atoms to each end carbon so that every carbon has four bonds. The carbon-fluorine bond is one of the strongest bonds in organic chemistry, which is why these molecules last so long in the environment.

Station 2 card, Pentworth well results

Well	PFOA measured
Well 2	3.2 ppt
Well 4	9.6 ppt
Well 4 after treatment	1.2 ppt

Station 4 card, treatment options for Pentworth

Option	PFAS removed	Cost to build	Waste to handle
Granular activated carbon	92 percent	$1,200,000	Spent carbon shipped out
Ion exchange resin	96 percent	$1,600,000	Spent resin shipped out
Reverse osmosis	99 percent	$2,900,000	Concentrated brine to dispose of

In the chain you built at Station 1, how many fluorine atoms are attached to the middle carbon?
In the water molecule you built, how many hydrogen atoms are attached to the one oxygen atom?
Well 4 measured 9.6 ppt of PFOA, and the federal limit is 4.0 ppt. How many times the limit is that result?
Well 2 measured 3.2 ppt of PFOA. Is that above or below the 4.0 ppt limit?
After treatment, Well 4 dropped from 9.6 ppt to 1.2 ppt. What percent of the PFOA was removed?
At Station 3, which column let more color pass through, the sand or the activated carbon? Write what you observed.
Using the Station 4 card, which option removes the highest percent of PFAS?
Which option would you recommend to the town of Pentworth? Name two pieces of evidence from the Station 4 card.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

These items key the numbered questions on the student handout, in the same order.

Answer: 2 fluorine atoms.
The chain built at Station 1 places two fluorine gumdrops on the middle carbon, with the remaining two bonds going to the neighboring carbons.
Answer: 2 hydrogen atoms.
A water molecule is one oxygen atom bonded to two hydrogen atoms.
Answer: 2.4 times.
9.6 ppt divided by 4.0 ppt equals 2.4, so the Well 4 result is 2.4 times the limit.
Answer: Below the limit.
3.2 ppt is less than 4.0 ppt, so Well 2 is below the federal limit for PFOA.
Answer: 87.5 percent.
The drop is 9.6 minus 1.2, which is 8.4 ppt. Then 8.4 divided by 9.6 equals 0.875, or 87.5 percent removed.
Answer: The sand column, in nearly every trial.
Accept the student's observation. Activated carbon holds dye molecules on its surface, so its water usually runs much clearer, while sand strains out only particles and lets dissolved color through.
Answer: Reverse osmosis.
The card lists reverse osmosis at 99 percent, compared with 96 percent for ion exchange and 92 percent for granular activated carbon.
Answer: Accept any option defended with two pieces of evidence from the card.
A strong carbon answer cites the lowest build cost and removal that still clears the limit. A strong reverse osmosis answer cites the highest removal, and should acknowledge the brine that Pentworth would then have to dispose of.

Differentiation

Support

Give these students a printed picture of the finished Station 1 models with the atoms labeled, so building the chain does not depend on reading the card alone. At Station 2, provide a number line from 0 to 10 ppt with the 4.0 ppt limit already marked so the Pentworth results can be placed on it.

Extension

Ask students to calculate what percent of the PFOA remained in Well 4 after treatment, then write a short recommendation on whether the town of Pentworth should keep using that well and what it should test next.

ELL supports

Preteach removed, remaining, and limit with the number line from the Support section. Allow students to answer item 6 by drawing the two cups from Station 3 and shading the water that came out of each, rather than writing a sentence.

Extensions

Groups can design a fifth station for the town of Pentworth that shows what happens to the spent carbon after it is replaced, using labeled drawings to trace where the captured PFAS goes next.