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Lab / experiment Grades 7–8 Science

Coagulation and Sedimentation: A Jar Test for Cloudy Water

A two-session lab where teams make standardized muddy water, dose one jar with a coagulant, and graph clarity over time as treatment operators do in a daily jar test.

Objectives

  1. Design and run a controlled jar test comparing settling with and without a coagulant.
  2. Collect and graph clarity data at timed intervals to compare two conditions.
  3. Explain how coagulation and sedimentation remove turbidity from source water.
  4. Connect a classroom procedure to the jar test water treatment operators run daily.

Vocabulary

Turbidity
Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.
Floc
Floc is the fluffy clumps of dirt and treatment chemicals that form in the water during cleaning at a treatment plant. Floc looks like tiny snowflakes drifting in the water.
Sedimentation
Sedimentation is the step where water sits almost still in a big basin so the heavy clumps of dirt can sink to the bottom. The cleaner water on top then moves on to the filters.
Jar test
A jar test is a small-scale experiment operators run in the lab before treating a whole tank of water, testing different amounts of chemical in separate jars to see which dose clears the water best. It helps operators avoid wasting chemicals or under-treating the water.

Also: coagulant, supernatant

Teacher overview

Cloudy water drawn from a farm field, a construction site, or an eroding streambank must become clear water before it reaches a household tap, and this lab recreates the first major step in that process at lab-bench scale. Teams build two jars of standardized muddy water from an identical stock, treat one with a coagulant, and then observe, timing how fast each jar clears and how much settled material collects at the bottom. The procedure is simple by design: it closely resembles the actual test, called a jar test, that water treatment operators run most mornings to decide how much coagulant to feed into the whole plant that day.

Watch for this misconception: students who see Jar B turn from muddy to clear often conclude, incorrectly, that the water is now safe to drink. Settling only removes suspended particles, the sediment that causes turbidity, and does nothing to dissolved chemicals or to bacteria and viruses that were never visible in the first place. Question 8 in the analysis section confronts this directly, asking students to name the filtration and disinfection steps that must still happen afterward. Address it explicitly before students leave the room with the wrong idea, since it is a common assumption after any activity that visibly clears dirty water.

Both jars must start from one shared, teacher-premixed stock rather than each team mixing its own soil and water, because this is the fair-test control that makes the whole comparison meaningful, the same reason a real jar test always splits one raw-water sample into several jars rather than comparing water drawn at different times. Budget the full 90 minutes across two sessions: Session 1 covers setup, dosing, and short-interval readings; Session 2, held the next class period, covers the overnight reading, graphing, and analysis. Because the lab uses only soil, water, and either alum or a food-grade gelatin substitute, no jar contents are hazardous, but they are still treated as non-potable throughout, matching real lab practice.

Coagulation and Sedimentation: A Jar Test for Cloudy Water — Grades 7–8 Science
Name: ____________________________ Date: ______________

Procedure

Materials (per team of three or four; scale up the stock recipe for more teams).

  • 2 clear quart-size (32 oz) jars or wide-mouth clear containers with lids, labeled A – No Coagulant and B – Coagulant
  • about 2 cups (470 mL) per jar of the standardized muddy-water stock (teacher-premixed; recipe below), poured to a marked fill line so every jar starts identical
  • Option 1: alum (aluminum sulfate from hardware, garden, or pool-supply stores, or the potassium alum sold in grocery spice aisles for pickling; either works), 1/4 teaspoon (about 1 gram) per jar
  • Option 2, a safe substitute if alum is unavailable: 1/2 teaspoon (about 2 grams) unflavored powdered gelatin, dissolved ahead of time in 2 tablespoons of warm (not boiling) tap water and cooled to room temperature before use
  • 2 stirring sticks or spoons per team
  • a stopwatch or phone timer
  • a clarity card: a sheet of paper with an ordinary line of printed text, held behind the jar to check readability
  • safety glasses
  • masking tape and a marker for labels
  • one data recording sheet and pencil per team (table in Data & Analysis)

Teacher prep, the day of the lab (about 15–20 minutes before students arrive). Make one large batch of stock so every team starts from identical turbidity, the same principle that makes any real jar test meaningful. Mix 4 level tablespoons of sifted potting soil or topsoil per liter of tap water, stir vigorously for one minute, let it rest exactly 60 seconds so the heaviest grit drops out, then pour off (decant) the cloudy water into a large pitcher or bucket, leaving the grit behind. Repeat until you have about 4 cups (1 liter) of stock per team. If using the gelatin substitute, dissolve and cool it now.

Safety. Alum is a mild eye and skin irritant: students measuring or stirring it should wear safety glasses and wash hands afterward. Warm water for the gelatin should be hand-warm, never boiling; an adult should handle a stove or kettle if one is used. No one tastes or drinks jar contents at any point in the lab — treat every jar as non-potable throughout. Wipe up spills immediately, since a wet floor is a slip hazard. At the end of the lab, pour jar contents down a sink drain, never a storm drain, or onto an outdoor garden bed, since the contents are only soil, water, and a small amount of alum or gelatin.

Session 1 steps (about 50 minutes).

Step 1. Open with a short discussion: where does the cloudiness (turbidity) in a river or reservoir come from — rain, farm runoff, construction, eroding stream banks? Explain that operators run this same jar test most mornings before deciding how much coagulant to feed the whole plant.

Step 2. Label two jars per team A and B with tape and marker.

Step 3. Fill both jars to the marked line with the same muddy-water stock, stirring the stock pitcher between pours so every team gets a fair, evenly mixed sample.

Step 4. Record "0 minutes" and each jar's starting clarity score (scale in Data & Analysis). Jar A and Jar B should match closely here, since neither has been treated yet.

Step 5. Add the coagulant to Jar B only — 1/4 teaspoon alum or 1/2 teaspoon of the cooled dissolved gelatin — and stir Jar B briskly for 30 seconds (a "rapid mix," matching the fast stir operators use to spread coagulant through incoming water). Do not stir or disturb either jar again except to observe.

Step 6. Observe and record clarity scores for both jars at 5, 10, 20, and 30 minutes using the clarity card. Between readings, have students sketch what they see and predict the next reading.

Step 7. At the end of Session 1, move both undisturbed jars somewhere safe to sit for about 20 to 24 hours.

Session 2 steps (about 40 minutes, the next class period).

Step 8. Take a final clarity reading and note the depth and appearance of settled material at the bottom of each jar.

Step 9. Graph the complete data set from both sessions and complete the Data & Analysis questions.

Data & Analysis

Each team keeps one data table across both sessions, rating the clarity of the water sitting above the settled floc — called the supernatant — on a simple 1–5 scale that needs no special equipment:

  • 1 – Opaque. The clarity card cannot be seen at all through the jar.
  • 2 – Very cloudy. The card is a blur; no letters are visible.
  • 3 – Hazy. Large letters or shapes are visible but blurry.
  • 4 – Mostly clear. Most of the printed text can be read.
  • 5 – Clear. The full line of text reads easily through the jar.

Record a score for Jar A and Jar B at each time point:

Time Jar A (no coagulant) clarity score Jar B (with coagulant) clarity score Notes
0 min
5 min
10 min
20 min
30 min (end of Session 1)
~24 hours (Session 2)

  1. Explain why Jar A and Jar B were filled from the same premixed stock rather than each team mixing its own soil and water separately. What variable does this control?

  2. At 0 minutes, right after stirring, compare the clarity scores of Jar A and Jar B. Should they be similar or different at this point? Explain your reasoning.

  3. Using your data table, make a line graph with time on the horizontal axis and clarity score (1–5) on the vertical axis, plotting one line for Jar A and one for Jar B on the same graph. Describe the shape of each line.

  4. At which time interval did Jar B first show a clearly higher clarity score than Jar A? Describe what you saw happening inside Jar B at that point.

  5. Compare the layer of settled material (floc, once the clumps have sunk) at the bottom of Jar A and Jar B at your final reading. Which jar has more settled material, and why does that make sense given what the coagulant does?

  6. Water treatment operators run a test almost identical to this one, the jar test, before deciding how much coagulant to feed into a plant's incoming water. Based on your results, if Jar B's dose worked well, what would an operator likely do with that information the next morning?

  7. A real treatment plant's sedimentation basin typically holds water for 2 to 4 hours before it moves on to filtration — longer than your 30-minute Session 1 readings but shorter than your overnight reading. Describe one way your jar test is a fair, small-scale model of that step, and one way a real sedimentation basin differs from your jars.

  8. Turbidity, the cloudiness this lab measures, is also something environmental monitors track in real rivers and streams, where a spike after a storm can signal soil erosion from farms, construction sites, or collapsing stream banks — a human impact on the watershed. Jar B represents one method (coagulation and sedimentation) that treatment plants use to reduce that impact once the cloudy water reaches the plant. Does settling alone make Jar B's water safe to drink? Explain what treatment step or steps would still be needed, and why.

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: Using one shared stock controls the starting turbidity so the comparison is fair.
    If each team mixed its own soil and water, teams could not tell whether a clarity difference between jars came from the coagulant or from simply using more or less soil. A single shared stock isolates the coagulant as the one variable being tested, the same reason a real jar test always splits one raw-water sample across several test jars.

  2. Answer: Nearly identical — both jars should score close to 1, still opaque.
    Right after stirring, the coagulant has not yet had time to pull particles together into settleable floc, so Jar B should look just as cloudy as Jar A. A visible difference already at 0 minutes usually means the stock was not mixed evenly before pouring, not that the coagulant is working instantly.

  3. Answer: Jar A's line stays low and mostly flat; Jar B's line rises over the first several readings and then levels off.
    Expect Jar A to hover around a clarity score of 1 to 2 for the full 24 hours, since fine clay and silt particles can stay suspended for days without help. Jar B typically climbs from about 1 at time 0 toward 3 to 5 by the 24-hour reading, with most of the rise happening within the first 20 to 30 minutes and the overnight reading confirming or slightly improving on the 30-minute score.

  4. Answer: Most teams see a clear gap open somewhere between the 10-minute and 30-minute readings, though a faster or slower result is also plausible.
    Alum tends to form visible floc within the first 5 to 10 minutes; the gelatin substitute is usually slower, sometimes not showing a clear difference until 20 to 30 minutes. Accept any answer that correctly identifies a time within the data the team actually collected, along with a description of visible clumps forming and sinking in Jar B.

  5. Answer: Jar B should have a distinctly thicker layer of settled material than Jar A.
    The coagulant pulls fine, slow-settling particles into larger, heavier clumps (floc) that sink readily, concentrating material at the bottom of Jar B. Jar A's particles are very small and carry like electric charges that make them push one another apart, so without a coagulant to neutralize those charges they either remain suspended or settle into a much thinner layer over the same time.

  6. Answer: The operator would use that dose, or one close to it, as the plant's coagulant feed rate for the day, adjusting again if incoming water changes.
    Real jar tests are run precisely to find a working dose before committing an entire plant's chemical feed to it; a successful classroom result mirrors the decision an operator makes each morning based on that day's raw-water conditions.

  7. Answer: Answers vary; must name one real similarity and one real difference.
    A strong answer notes the similarity that both processes rely on gravity settling of coagulated floc out of still or slow-moving water over time, and the difference that a real basin runs continuously — new raw water enters while settled floc is removed from the bottom — rather than as one still, one-time batch like a classroom jar. Basin scale, mechanical mixing paddles, and sludge-removal equipment are also acceptable differences.

  8. Answer: No — settling alone does not make the water safe to drink; filtration and disinfection (typically chlorination) are still required.
    Sedimentation removes suspended solids, which is what causes turbidity, but it does not remove dissolved chemicals or kill bacteria and viruses, which were never visible to begin with. Real plants must bring finished-water turbidity down to 0.3 NTU or below in at least 95% of monthly samples under EPA's Surface Water Treatment Rules — clearer than settling alone typically achieves — which is exactly why sand filtration and a disinfectant dose always follow sedimentation before water reaches a tap.

Differentiation

Support (below grade level)

Provide the clarity scale as a printed card taped to each jar for quick reference, and reduce the data table to three readings (0, 30 minutes, and the overnight reading) instead of five. Pair students so one partner reads the clarity card while the other records, and allow the graph in question 3 to be drawn on pre-labeled axes with the scale already marked.

Extension (above grade level)

Have advanced teams run a third jar with a smaller coagulant dose (half the standard amount) to test whether less coagulant still produces a strong result, then write two or three sentences arguing whether a real plant should ever use less coagulant than the amount that gives the clearest water, considering both chemical cost and the sludge left over for disposal.

ELL supports

Pre-teach turbidity, coagulant, floc, and settle with the actual jars as visual references, pointing at the cloudy water and later at the clumps. Provide the clarity scale with both words and simple icons (a solid gray square for score 1, a clear square with text for score 5) so students can match observations to scores without relying on the English scale names alone. Accept data table entries and short answers in labeled numbers and drawings where full sentences are still developing.

Extensions

If a local stream, pond, or ditch is accessible, have an adult collect a jar of real surface water from the bank after a rain event, staying away from high or fast water and wearing gloves, and run the same jar test on it alongside the classroom stock, comparing how it settles and discussing what land uses nearby (farms, roads, construction) might explain its starting turbidity. Many utilities will also let a class visit the treatment plant's own jar-test bench or send an operator to demonstrate the real version of this test with the plant's actual raw water, turning the classroom procedure into a direct comparison with the full-scale process it was built to model.