Lab / experiment Grade 7 Science

Settling Jars: How Particle Size Changes Water Clarity

A grade 7 science lab where students track how muddy water clears in two jars of different sediment and connect settling speed to a treatment plant's job.

Grade: Grade 7
Subject: Science
Material: Lab / experiment
Time: 55 min
Prep: Moderate prep
Materials: 4 clear jars or cups per group (same size), Sandy soil (about half a cup), Fine soil such as potting or clay-rich soil (about half a cup), Tap water, Timer or classroom clock, 1-to-5 clarity comparison card, Spoon and labels, Paper towels for spills

Standards

MS-ESS3-3 NGSS

Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment.

How this material aligns: Students test settling as one step for cleaning cloudy water and judge, from their data, whether it alone can reach the drinking-water clarity standard, the monitor-and-minimize method this standard requires.

All materials for MS-ESS3-3 →

7.RP.A.2 Common Core Math

Recognize and represent proportional relationships between quantities, including identifying the constant of proportionality and representing proportional relationships by equations.

How this material aligns: Analysis item 4 has students treat the jar's clarity as improving at a steady rate, so the clearing rate in points per ten minutes is the constant of proportionality between clarity change and elapsed time, the proportional reasoning this standard develops.

All materials for 7.RP.A.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 measure how the clarity of muddy water changes as particles settle over time.
Students will compare the settling of coarse and fine sediment and connect it to why plants add steps beyond settling.

Vocabulary

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.
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.

Also: sediment, particle size, clarity

Teacher overview

Treatment plants slow water down in a basin so that particles can settle out before filtering, a step called sedimentation. How fast particles settle depends on their size: coarse grains drop quickly, while fine particles stay suspended much longer. In this lab, students make two jars of muddy water, one with sandy soil and one with fine soil, and watch how the clarity changes over 30 minutes. By regulation, a plant must keep finished water at or below 0.3 NTU in nearly all of its monthly samples, and this lab helps students see why settling alone cannot reach that clarity.

Settling Jars: How Particle Size Changes Water Clarity — Grade 7 Science

Name: ____________________________ Date: ______________

Procedure

Step 1 (safety, first): Use only tap water stirred with clean soil. Do not drink or taste any water in this lab. Wipe up spills right away so the floor stays dry, and wash your hands when you finish.

Step 2 (make the two samples): Label one jar A and one jar B. Fill both with the same amount of tap water. Stir a spoonful of sandy soil into jar A and a spoonful of fine soil into jar B. Stir each the same number of times so they start equally cloudy.

Step 3 (rate the start): Right after stirring, hold each jar against the 1-to-5 clarity card, where 1 is clear and 5 is very cloudy. Record the starting rating for both jars as the reading at 0 minutes.

Step 4 (let them settle): Set both jars where they will not be bumped. Do not stir them again. Rate each jar's clarity at 5 minutes, at 15 minutes, and at 30 minutes, and record every reading.

Step 5 (watch the layers): At 30 minutes, look at the bottom of each jar. Note which jar has dropped more sediment to the bottom and which water above is clearer.

Step 6 (second trial): At the same time as Step 2, set up a second pair of jars (A2 and B2) the same way, and rate them on the same schedule, recording them as Trial 2, so you can check whether your results are consistent.

Data & Analysis

Jar	0 min	5 min	15 min	30 min
A (sandy), Trial 1
A (sandy), Trial 2
B (fine), Trial 1
B (fine), Trial 2

What was jar A's clarity rating at 30 minutes? Use your own recorded data.
At 15 minutes, which jar was clearer, A or B? Use your data to decide.
Suppose jar A rated 5 at 0 minutes and 2 at 30 minutes. What is its clarity improvement in points?
Suppose jar A improved 3 points over 30 minutes at a steady rate. What is its clearing rate in points per 10 minutes?
Which sediment settled faster, sandy or fine? What does that tell you about particle size and settling speed?
A plant must reach 0.3 NTU in nearly all monthly samples. Based on your results, could settling alone reach that clarity? Explain why or why not.

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: varies by group's own data.
The value should match the group's recorded 30-minute rating for jar A, usually a low number such as 1 or 2 as the sand settles out. Accept any value from 1 to 5 that matches the data.
Answer: varies by group's own data.
Most groups find jar A (sandy) clearer at 15 minutes because coarse grains settle faster. Accept the jar the group's own numbers support.
Answer: 3 points.
5 − 2 = 3 points of clarity improvement.
Answer: 1 point per 10 minutes.
3 points over 30 minutes is 3 divided by 30 = 0.1 point per minute, which is 1 point per 10 minutes.
Answer: The sandy sediment settled faster.
Coarse, heavier grains drop quickly, while fine particles are so light they stay suspended much longer. This shows that larger particles settle faster than small ones.
Answer: No, settling alone cannot reach 0.3 NTU.
Settling removes the coarse particles you can see, but very fine particles stay in the water and keep it cloudy. Reaching 0.3 NTU takes added steps such as coagulation and filtration, which a jar cannot do. Accept any answer that explains fine particles remain.

Differentiation

Support

Build one demonstration jar as a class and rate it together before groups start, and provide a simplified data table with a single trial. Allow students to record clarity by pointing to the 1-to-5 card rather than writing a description.

Extension

Have groups add a third jar with a mix of both soils and predict, before testing, whether its 30-minute clarity will fall between jar A's and jar B's, then check the prediction against their data.

ELL supports

Provide picture cards for "coarse," "fine," "settle," and "cloudy," and pair each 1-to-5 rating with a shaded sample image. Allow analysis items 5 and 6 to be answered with a labeled drawing plus a short phrase.