Lab / experiment Grade 9 Science

Clearing Muddy Water: A Coagulation and Settling Lab

A grade 9 lab where students compare how muddy water clears with and without a coagulant, rating cloudiness over time and linking their results to real treatment plants.

Grade: Grade 9
Subject: Science
Material: Lab / experiment
Time: 50 min
Prep: Moderate prep
Materials: Two clear jars or tall clear cups per group (about 1 liter total), Tap water, about 1 liter per group, Pitcher or 1-liter container for mixing the muddy water, Measuring spoons, Fine soil or silt, about 2 tablespoons per group, Alum (aluminum sulfate) or a garden-store coagulant, about a quarter teaspoon per group, Stir stick or spoon, Stopwatch or classroom clock, Cloudiness comparison card (1-5 scale), 1 per group, Safety goggles

Standards

HS-ESS2-5 NGSS

Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.

How this material aligns: Students plan and carry out an investigation of how suspended sediment settles out of water with and without a coagulant, the water-properties and surface-process investigation this standard requires.

All materials for HS-ESS2-5 →

HSN-Q.A.3 Common Core Math

Choose a level of accuracy appropriate to limitations on measurement when reporting quantities.

How this material aligns: Analysis question 4 asks students to judge why the whole-number clarity card cannot support a reading such as 3.5, the accuracy-to-measurement matching this standard requires.

All materials for HSN-Q.A.3 →

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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 compare how quickly muddy water clears with and without a coagulant.
Students will record cloudiness ratings over time and calculate the change.
Students will connect their results to settling in a real treatment plant.

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.
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.

Also: coagulant, settling

Teacher overview

Cloudy water is full of tiny particles that are too light to settle quickly on their own. Treatment plants add a coagulant, which makes the particles clump together into larger, heavier clumps that sink faster. This lab lets students see that effect by comparing two jars of muddy water, one with a coagulant and one without. Students rate the cloudiness over 15 minutes and connect what they see to how a real sedimentation basin works. Classrooms cannot measure true turbidity in NTU, so the 1-to-5 cloudiness card is a simple stand-in.

Clearing Muddy Water: A Coagulation and Settling Lab — Grade 9 Science

Name: ____________________________ Date: ______________

Procedure

Step 1. Put on safety goggles. Each group works with two clear jars, labeled Jar 1 and Jar 2.

Step 2. Make muddy water by stirring about 2 tablespoons of soil into roughly 1 liter of water. Split the muddy water evenly between Jar 1 and Jar 2 so both start about the same.

Step 3. Add about a quarter teaspoon of coagulant to Jar 2 only. Jar 1 gets no coagulant and is your control. Safety note: do not taste or drink any water in this lab, and wash your hands after handling the soil and coagulant.

Step 4. Stir both jars the same way for 30 seconds, then stop stirring at the same time.

Step 5. Right away, rate each jar's cloudiness using the 1-to-5 card, where 1 is clear and 5 is very cloudy. Record both ratings in the "0 minutes" row.

Step 6. Let both jars sit still. Without stirring, rate the cloudiness of each jar again at 5 minutes, at 10 minutes, and at 15 minutes. Record each rating in the data table.

Step 7. Once you have all your readings, compare your data table with another group's before you answer the analysis questions.

Data & Analysis

Time (minutes)	Jar 1 (no coagulant) cloudiness 1-5	Jar 2 (with coagulant) cloudiness 1-5
0
5
10
15

Suppose Jar 1 started at a cloudiness rating of 5 and read 2 after 15 minutes. How many rating points did it improve?
Using your own data, find the change in Jar 2's cloudiness rating from 0 minutes to 15 minutes.
Which jar cleared more over 15 minutes, and by how many rating points? Use your two changes to compare.
The cloudiness card only allows whole-number ratings from 1 to 5. Explain why it would not make sense to record a jar's cloudiness as 3.5 on this card.
A treatment plant lets water settle for 2 to 4 hours before filtering. Using your results, explain why more settling time usually gives clearer water.
Finished drinking water must reach 0.3 NTU, a very low cloudiness, in nearly all samples. Treating your jar as a simple model, could settling alone reach that standard? Explain why or why not.
Suppose you doubled the amount of coagulant in Jar 2. Predict whether the water would clear faster, slower, or about the same, and justify your prediction.

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: 3 rating points. 5 − 2 = 3 rating points of improvement.
Answer: Answers will vary with the student's data. It should equal the 0-minute rating minus the 15-minute rating for Jar 2, and it is usually a larger drop than Jar 1's.
Answer: Answers will vary. Most groups find Jar 2 (with coagulant) cleared more, and the difference equals Jar 2's change minus Jar 1's change; accept any answer consistent with the student's data.
Answer: The card only offers whole-number steps, so a rating of 3.5 claims a level of detail the card cannot support. A student should choose the nearest whole number, either 3 or 4.
Answer: Accept any answer noting that particles keep sinking as long as the water sits still, so more settling time lets more particles reach the bottom, leaving clearer water above.
Answer: No. Settling in a jar removes only the larger particles; a real plant follows coagulation and settling with filtration, the step that brings turbidity down to 0.3 NTU, and then disinfects the water. The jar is a simplified model, not a full treatment process.
Answer: Accept a prediction of faster clearing, reasoning that more coagulant forms more clumps that sink quickly. Accept an "about the same" answer if the student reasons that the first dose already clumped most of the particles. Also accept "slower" if the student explains that too much coagulant can keep particles from clumping well; plants run jar tests to find the best dose.

Differentiation

Support

Build one demonstration pair of jars as a class first, and give students a simplified data table with only the 0-minute and 15-minute rows so they compare a clear start and end.

Extension

Have groups test a third jar with a different soil amount or a different settling time, then write a short comparison of which setup produced the clearest water and why.

ELL supports

Provide picture cards for "coagulant," "settling," and "cloudiness," and a labeled 1-to-5 cloudiness scale with a shaded example for each number. Allow students to record ratings using the numbered card alone rather than written descriptions.