Lab / experiment Grade 6 Science

Does a Coagulant Help Sediment Settle? A Mabry Plant Test

A grade 6 science lab comparing muddy water with and without a coagulant, rating clarity over time to test a fictional plant's settling step.

Grade: Grade 6
Subject: Science
Material: Lab / experiment
Time: 50 min
Prep: Moderate prep
Materials: 2 clear jars with lids per group, Soil to make muddy water (about 2 cups per group), Alum (pickling alum, which is potassium aluminum sulfate, or garden aluminum sulfate), about 1/4 teaspoon per group, Measuring spoons, Stirring stick per group, Cloudiness comparison card (1-5 scale), Timer or clock, Measuring cup, Large container for mixing the muddy water, Masking tape and marker for labeling jars, Safety goggles

Standards

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: Students compare two settling designs, plain water and water with a coagulant, using clarity data over time, matching this standard's systematic evaluation of competing design solutions.

All materials for MS-ETS1-2 →

6.RP.A.3 Common Core Math

Use ratio and rate reasoning to solve real-world and mathematical problems, e.g., by reasoning about tables of equivalent ratios, tape diagrams, double number line diagrams, or equations, including finding a percent of a quantity and converting measurement units.

How this material aligns: Analysis items 3 and 4 have students compare clarity drops and find one jar's ending rating as a percent of its start, the ratio and percent reasoning this standard targets.

All materials for 6.RP.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 run a fair test comparing how muddy water settles with and without a coagulant.
Students will use their clarity data to recommend one settling design and support it with evidence.

Vocabulary

Key words: sediment, coagulant, settle, clarity

Teacher overview

Before water is filtered at a treatment plant, it sits in a large basin so heavy bits of dirt can sink to the bottom. Plants often add a coagulant, a chemical that makes tiny bits clump together so they settle faster. This lab lets students test that idea for the Mabry plant, a fictional water plant. Students compare two jars of muddy water, one plain and one with a small amount of alum, and rate how clear each becomes over 15 minutes.

Does a Coagulant Help Sediment Settle? A Mabry Plant Test — Grade 6 Science

Name: ____________________________ Date: ______________

Procedure

Each group tests two jars filled from the same muddy water, so the only difference is the coagulant. Rate clarity with the 1-5 card, where 1 is clear and 5 is very cloudy.

Step 1. Make muddy water by stirring soil into water in a large container until it looks about the same cloudiness throughout. Share it among the groups.

Step 2. Fill two jars from the muddy water so both start at the same clarity. Label them Jar A and Jar B.

Step 3. Safety note. Wear safety goggles while handling the alum. Do not taste or drink any water in this lab. Do not eat the alum. Wash your hands after handling the alum and the muddy water.

Step 4. Add about 1/4 teaspoon of alum to Jar B only. Add nothing to Jar A. Stir each jar gently for ten seconds with a clean stick.

Step 5. Right after stirring, rate the clarity of each jar with the 1-5 card. This is the reading at 0 minutes. Record both.

Step 6. Let the jars sit still. Rate each jar again at 5 minutes, at 10 minutes, and at 15 minutes, and record each reading. Do not bump the table.

Step 7. Pour the water off into a sink, put the settled mud in the trash or on a garden bed, never down a storm drain, and clean up as directed.

Data & Analysis

Fill in your own readings, or use the sample data below from a Mabry plant test if your jars did not settle in time.

Time (min)	Jar A clarity, no coagulant (1-5)	Jar B clarity, with coagulant (1-5)
0	5	5
5	5	4
10	4	2
15	4	1

In Jar B, how many points did the cloudiness rating drop from 0 minutes to 15 minutes?
In Jar A, how many points did the cloudiness rating drop from 0 minutes to 15 minutes?
Which jar cleared more, and by how many more points did its rating drop than the other jar's?
At 15 minutes, Jar B's rating is what fraction of its starting rating, written as a percent?
A real settling basin gives water 2 to 4 hours to settle, but this test ran only 15 minutes. Explain why a real plant gives the water so much more time.
Based on your data, which design would you recommend to the Mabry plant, plain settling or settling with a coagulant, and what evidence supports your choice?

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: 4 points.
Jar B went from 5 at 0 minutes to 1 at 15 minutes, a drop of 5 minus 1 = 4 points.
Answer: 1 point.
Jar A went from 5 to 4, a drop of 5 minus 4 = 1 point.
Answer: Jar B, by 3 more points.
Jar B dropped 4 points and Jar A dropped 1 point, so Jar B cleared 4 minus 1 = 3 more points than Jar A.
Answer: 20 percent.
Jar B's ending rating 1 divided by its starting rating 5 = 0.2, which is 20 percent.
Answer: Accept answers explaining that settling takes time and the finest bits sink very slowly. A real plant treats far more water and needs the tiny particles to settle out too, so it gives the water 2 to 4 hours in a large basin, much longer than a 15-minute jar test.
Answer: Most students should recommend settling with a coagulant. The evidence is that Jar B cleared far more than Jar A, dropping 4 points versus 1 in the same 15 minutes. Accept any recommendation supported by the group's own clarity data.

Differentiation

Support

Pre-make one demonstration pair of jars as a class so students see the steps before running their own. Give these groups a data table with only the 0-minute and 15-minute rows, so they compare a clear start and end rather than tracking four readings.

Extension

Have students run a third jar with twice the alum and compare its 15-minute clarity with Jar B's, then write whether more coagulant kept clearing the water or reached a point where extra made little difference.

ELL supports

Provide picture cards for sediment, coagulant, settle, and clarity, and post the 1-5 cloudiness card where all students can see it. Allow students to record clarity by circling a number on the card rather than writing a description, and give the frame "Jar ___ cleared more because ___."