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Classroom activity Grade 12 Science

Surface Tension, Foam, and PFAS at Ninebark Lake

A grade 12 four-station rotation comparing bulk properties of water and alcohol, then using surfactant behavior to explain why lake foam concentrates PFAS.

Objectives

  1. Students will compare surface tension, capillary rise, and evaporation rate across three liquids.
  2. Students will infer from those bulk properties which liquid has the stronger attractions between its particles.
  3. Students will explain why molecules with a water-attracted end and a water-repelling end gather at the surface.

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: capillary rise, surfactant, interface, volatility, cohesion

Teacher overview

This is a four-station rotation for groups of three to five students, run in one period. Three stations measure bulk properties of water, isopropyl alcohol, and oil, so that students can infer which liquid holds itself together more strongly. The fourth station adds a trace of dish soap to water and shows that a very small amount of the right kind of molecule changes the surface without changing the bulk. The rotation closes on a fictional lake, Ninebark Lake, where a foam line collects along the shore, and students use the surfactant idea to explain why that foam is treated as more hazardous than the water beneath it. The rotation also supplies the reason a contaminant need not spread evenly through a lake.

Activity Directions

  1. Set up before class (15 minutes of teacher time). Label four station trays. Station A gets droppers, pennies, paper clips, and three labeled squeeze bottles: water, isopropyl alcohol, and water treated with one drop of dish soap per 100 mL. Station B gets three capillary tubes, three shallow dishes of the same three liquids, and a ruler. Station C gets watch glasses, droppers, a small fan, a stopwatch, and water, alcohol, and vegetable oil. Station D gets six stoppered test tubes, three with plain water and three with the soap-treated water, plus a stopwatch. No PFAS chemicals are used anywhere in this activity; dish soap stands in for a surfactant because it has the same two-ended structure.
  2. Frame the question (5 minutes). Show the class a photograph or a sketch of a foam line collecting along a lake shore. Ask what has to be true about a molecule for it to end up in the foam rather than in the water. Hold the answers and tell students the rotation will supply the evidence. State the rule: 9 minutes per station, four stations, handout completed as they go. Students wear safety goggles, keep isopropyl alcohol away from any flame or heat source, taste nothing, and report broken glass to the teacher.
  3. Station A, surface tension (9 minutes). Groups count how many drops of each liquid a penny holds before the dome collapses, running three trials per liquid, and then test whether a paper clip floats on each liquid. They answer items 1 through 3.
  4. Station B, capillary rise (9 minutes). Groups stand a capillary tube in each liquid and measure how far the liquid climbs, in millimeters. They answer item 4.
  5. Station C, evaporation (9 minutes). Groups place one equal drop of water, alcohol, and oil on separate watch glasses at the same distance from the fan and time how long each takes to disappear, stopping at 4 minutes for any drop still present. They answer items 5 and 6.
  6. Station D, foam and the surface (9 minutes). Groups shake a stoppered tube of plain water and a stoppered tube of soap-treated water for the same count, then time how long any foam survives in each. They answer items 7 and 8.
  7. Debrief (8 minutes). Ask two questions of the whole class. First, which liquid, water or alcohol, held itself together more strongly, and which three measurements agree on that answer? Second, the soap-treated water in Station A gave a much lower drop count than plain water even though it was almost entirely water, so where in the sample were the soap molecules sitting? Use a show of hands on the second question as the formative check, and reteach if fewer than three quarters of the class places the soap at the surface rather than spread evenly through the tube.
  8. Close (6 minutes). Students complete items 9 and 10 individually and turn in the handout.
Surface Tension, Foam, and PFAS at Ninebark Lake — Grade 12 Science
Name: ____________________________ Date: ______________

Student Handout

Items 1, 4, 5, and 7 depend on what your group measures, so your numbers may differ from another group's.

Station A. Count the drops each liquid holds on a penny. Run three trials and average them.

  1. Record your average drop count for water, for isopropyl alcohol, and for the soap-treated water.

  2. Which liquid held the largest dome before collapsing?

  3. A paper clip can be floated on one of these liquids and not on the others. What does floating a metal clip on a liquid tell you about the forces acting at that liquid's surface?

Station B. Stand one capillary tube in each liquid and measure the climb.

  1. Record the rise, in millimeters, for water and for isopropyl alcohol.

Station C. Place one drop of each liquid on a watch glass at the same distance from the fan.

  1. Record the time for the water drop and the time for the alcohol drop to disappear, or write "over 4 minutes" if a drop is still present at 4 minutes.

  2. Rank water, alcohol, and oil from the liquid whose particles are hardest to pull away from each other to the liquid whose particles are easiest to pull away, and give the evidence from this station for your ranking.

Station D. Shake each stoppered tube for the same count and time the foam.

  1. Record how long the foam lasted in the plain water tube and in the soap-treated tube.

  2. The soap-treated tube is more than 99 percent water. Explain how so little soap changes what happens at the surface so much.

Closing items.

  1. Ninebark Lake is a fictional lake with a stable foam line along its downwind shore. PFAS molecules have one end that is attracted to water and one end that is repelled by it, the same two-ended structure as the soap you used. Predict whether a sample of that foam or a sample of the water beneath it would contain the higher PFAS concentration, and give your reason.

  2. Three of your measurements point to the same conclusion about attractions between water molecules. Name the three measurements and state the conclusion in one sentence.

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

Answer Key

  1. Answer: accept any set of averages in which water is highest.
    Typical results run about 25 to 40 drops for water, about 8 to 15 for isopropyl alcohol, and about 10 to 18 for the soap-treated water. Absolute counts depend on dropper size and on the coin, so judge the ordering rather than the values.

  2. Answer: water.
    Plain water holds the largest dome, because its surface resists spreading more than the alcohol or the soap-treated water does.

  3. Answer: the surface behaves like a stretched film that resists being pushed apart. A steel paper clip is denser than water, so it is not floating by buoyancy alone. It rests in a dimple in the surface because the water molecules at the surface pull toward each other strongly enough to support it. That points to strong attractions between water molecules.

  4. Answer: accept any pair in which water climbs higher than alcohol.
    Water commonly climbs roughly twice as far as isopropyl alcohol in the same tube. Groups should record both values in millimeters, not just the comparison.

  5. Answer: accept any pair in which the alcohol drop disappears first.
    Alcohol usually clears in well under a minute with the fan running, while the water drop often runs past 4 minutes.

  6. Answer: oil, then water, then alcohol.
    Alcohol evaporated fastest, so its particles are easiest to pull away from each other. Water evaporated much more slowly. The oil drop did not measurably shrink in 4 minutes, so it is hardest to pull apart. Accept an answer that ranks by evaporation time and states that a slower drop indicates stronger attractions holding particles in the liquid.

  7. Answer: accept any pair in which the soap-treated foam lasts far longer.
    Foam in plain water usually collapses within a few seconds. Foam in the soap-treated tube commonly persists for a minute or more.

  8. Answer: the soap molecules gather at the surface instead of spreading through the tube. A soap molecule has one end attracted to water and one end repelled by it. The arrangement that satisfies both ends is to sit at the boundary between water and air, with the water-repelled end sticking out. Because the molecules concentrate in that thin layer, a trace amount is enough to change the surface, while the bulk of the tube is still ordinary water.

  9. Answer: the foam sample.
    PFAS molecules have the same two-ended structure as the soap, so they also collect at the air-water boundary rather than distributing evenly. Foam is almost entirely air-water boundary, so it concentrates such molecules, and foam samples can carry far higher concentrations than the water below. This matters because the federal limits for two of these compounds are set at 4.0 parts per trillion each, so a process that concentrates them is significant at levels far too low to see.

  10. Answer: the drop count, the capillary rise, and the evaporation time.
    Water held the most drops on a penny, climbed highest in the capillary tube, and evaporated more slowly than alcohol. All three indicate that the attractions between water molecules are stronger than those between alcohol molecules.

Differentiation

Support

Give these students a prepared results table with the liquids listed down the side and the four measurements across the top, so recording does not compete with observing. Provide a two-part sentence frame for item 8 that names the water-attracted end and the water-repelled end and asks only where such a molecule would sit.

Extension

Ask students to design a test that would distinguish a surfactant from a substance that merely dissolves in water, naming the measurement, the expected result for each case, and the control they would run. Have them explain why the capillary tube alone would not settle the question.

ELL supports

Pre-teach surface tension, surfactant, and interface with a labeled drawing of a water surface showing molecules attracted in all directions in the bulk and attracted mostly sideways and downward at the surface. Provide the handout with the measurement units already written in, and allow groups to give item 9 orally before writing it.

Extensions

Have students look up their own state's public advice about foam on lakes and rivers, then write a short note explaining the chemistry behind the advice using the results from Station D. Ask them to identify what the advisory assumes about where such molecules collect and whether their station results support that assumption.