Lab / experiment Grade 3 Engineering & Design

Stonewick Settling Tank: A Grade 3 Water-Clearing Fair Test

A grade 3 engineering lab where students model a settling tank, change only the rest time in a fair test, and measure how much clear water forms on top of the mud.

Grade: Grade 3
Subject: Engineering & Design
Material: Lab / experiment
Time: 40 min
Prep: Low prep
Materials: 1 large jar or pitcher for mixing the class batch of muddy water, 2 clear jars or cups per group, Soil or dirt (about 2 spoons per group), Spoon for stirring, Water, Ruler marked in centimeters, Clock or timer, Paper towels

Standards

3-5-ETS1-3 NGSS

Plan and carry out fair tests in which variables are controlled and failure points are considered to identify aspects of a model or prototype that can be improved.

How this material aligns: Students run a fair test that keeps the mud, water amount, and jars the same and changes only the rest time, the controlled-variable testing to improve a design this standard requires.

All materials for 3-5-ETS1-3 →

3-5-ETS1-1 NGSS

Define a simple design problem reflecting a need or a want that includes specified criteria for success and constraints on materials, time, or cost.

How this material aligns: The lab defines the settling problem with a success goal of clearer water on top and a constraint of using only rest time and simple jars, matching this standard's problem definition.

All materials for 3-5-ETS1-1 →

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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 model a settling tank by letting muddy water rest in a jar.
Students will run a fair test that changes only the rest time.
Students will measure the clear water on top and compare the two jars.

Vocabulary

Key words: settle, clear, fair test, tank

Teacher overview

Real water plants often clean dirty water by letting it rest in a big tank called a settling tank. When the water sits still, the heavy dirt sinks to the bottom, and clearer water is left on top. Stonewick's plant works this way. In this lab, groups model a settling tank in two jars and run a fair test to find out whether a longer rest leaves more clear water. Groups measure the clear layer with a ruler, so the lab produces real data without any special equipment.

Stonewick Settling Tank: A Grade 3 Water-Clearing Fair Test — Grade 3 Engineering & Design

Name: ____________________________ Date: ______________

Procedure

Make the muddy water. For the whole class, stir soil into a large jar of water until it looks brown. Use about two spoons of soil for each group. Every group will use this same muddy mix so the test is fair.

Fill two jars. Pour the muddy water into two clear jars. Fill each jar to the same height, about 10 cm. Use a ruler to check that both jars start the same.

Set the rest time. Let Jar 1 rest for 5 minutes. Let Jar 2 rest for 15 minutes. The rest time is the one thing you change in this test.

Keep the test fair. Keep the mud, the water amount, and the jars the same for both. Do not stir or bump the jars while they rest.

Measure the clear water. When the time is up, look at each jar. The dirt sinks to the bottom, and clear water sits on top. Use a ruler to measure how tall the clear part is, in centimeters. Write it in the table.

Stay safe. This water is only for looking, not for drinking. Do not taste it. Wash your hands when you are done.

Data & Analysis

Jar	Rest time	Clear water on top (cm)
1	5 min
2	15 min

Which jar had more clear water on top?
Jar 2 started with 10 cm of muddy water. After it rested, 4 cm on top were clear. How many cm were still cloudy?
What did you keep the same to make the test fair?
What one thing did you change between Jar 1 and Jar 2?
Why does letting the water rest help make it clearer?
If you let a jar rest for 30 minutes, would you expect more clear water or less? Tell 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

Answer: Usually Jar 2, the jar that rested longer. Accept the jar the group actually measured with the taller clear layer.
Answer: 6 cm. The jar had 10 cm of muddy water and 4 cm turned clear, so 10 − 4 = 6 cm were still cloudy.
Answer: The mud, the amount of water, and the jars were kept the same. Accept any answer naming these as the parts held equal.
Answer: The rest time. Jar 1 rested 5 minutes and Jar 2 rested 15 minutes, and that was the only change.
Answer: When the water rests, the heavy dirt sinks to the bottom, so clearer water is left on top. Accept any answer about dirt sinking.
Answer: More clear water, because a longer rest gives the dirt more time to sink. Accept "about the same" if the student reasons that most of the dirt has already settled.

Differentiation

Support

Give these groups jars with a line already drawn at 10 cm so filling is quick and even. Pair them with a buddy who reads the ruler, and let them record the clear layer by coloring a picture of the jar instead of writing a number.

Extension

Ask these groups to add a third jar that rests for 25 minutes and to predict where its clear layer will fall compared to the other two. Have them make a simple bar graph of the three clear-water heights and describe the pattern.

ELL supports

Pre-teach "settle," "clear," and "cloudy" by showing a jar of muddy water before and after it rests. Provide the sentence frame "Jar ___ had more clear water because ___." Allow labeled drawings of the two jars in place of full sentences.

Extensions

Show a photo of a real round settling tank at a water plant and ask students to find the same idea they tested: still water and dirt sinking to the bottom. Ask why a real plant still needs a filter step after settling, even when the water already looks clearer.