Lab / experiment Grade 10 Science

Clearing Cloudy Water: A Coagulation and Flocculation Jar Test

A grade 10 lab where students add different alum doses to muddy water, run a jar test, and rate how coagulation and settling change water clarity.

Grade: Grade 10
Subject: Science
Material: Lab / experiment
Time: 55 min
Prep: Moderate prep
Materials: Four clear cups or jars of the same size per group (about 250 mL each), Muddy water made by stirring 2 tablespoons of soil into 1 liter of water, Alum (aluminum sulfate) solution, about 1 teaspoon of alum dissolved in 100 mL of water, with a dropper, Stirring stick per group, Timer or clock, 1-to-5 cloudiness comparison card (1 = clear, 5 = very cloudy), Safety goggles for each student

Standards

HS-PS1-3 NGSS

Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.

How this material aligns: In analysis question 4 students use their settling results to infer how the coagulant changes the electrical forces holding like-charged clay particles apart, the bulk-to-particle reasoning this standard targets.

All materials for HS-PS1-3 →

HSN-Q.A.1 Common Core Math

Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays.

How this material aligns: Students record clarity ratings and calculate the change and the percentage of jars that improved in analysis questions 2 and 5, keeping their units consistent.

All materials for HSN-Q.A.1 →

Download Word file

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 jar test to compare how different coagulant doses affect water clarity.
Students will rate clarity before and after settling and explain the result in terms of particle charge.

Vocabulary

Coagulation: Coagulation is the first cleaning step at a drinking water plant. Operators quickly mix in a chemical that makes tiny bits of dirt stop pushing each other away so they can start sticking together.
Flocculation: Flocculation is the slow, gentle stirring step at a water plant that lets tiny sticky particles bump into each other and clump into bigger pieces called floc, heavy enough to settle out of the water.
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.
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: colloid

Teacher overview

Fine clay particles make water cloudy, and they refuse to settle on their own because each particle carries the same electric charge and the particles push one another apart. Treatment plants fix this by adding a coagulant, which cancels those charges so the particles clump into larger clusters called floc that sink. This lab lets students run a small version of that process, called a jar test, using a stand-in for a plant's raw water from the fictional Pineglen source. The classroom test uses a simple cloudiness scale rather than a lab turbidity meter, so it shows the principle rather than an official measurement.

Clearing Cloudy Water: A Coagulation and Flocculation Jar Test — Grade 10 Science

Name: ____________________________ Date: ______________

Procedure

Step 1 (safety first): Put on goggles. Do not drink or taste any water in this lab. Alum can irritate skin and eyes, so wipe up spills and wash your hands when you finish.

Step 2 (make the cloudy water): Stir the soil into the liter of water until it is evenly muddy. Pour about 250 mL of this muddy water into each of your four cups so all four start equally cloudy.

Step 3 (rate the starting clarity): Hold cup 1 against the 1-to-5 cloudiness card and record its rating in the "before" column. Because all four cups came from the same mix, use this rating as the starting clarity for every cup.

Step 4 (add the coagulant): Leave cup 1 with no alum as your control. Add 5 drops of alum solution to cup 2, 10 drops to cup 3, and 20 drops to cup 4.

Step 5 (rapid mix): Stir each cup quickly for about 30 seconds right after adding the alum. This spreads the coagulant through the water so it can reach every particle.

Step 6 (slow mix for flocculation): Stir each cup slowly for about 3 minutes. Gentle mixing lets the tiny clumps bump into one another and grow into floc large enough to settle.

Step 7 (let it settle): Stop stirring and let all four cups sit undisturbed for 15 minutes. Watch for floc forming and sinking.

Step 8 (rate the final clarity): After settling, rate the clarity of each cup against the 1-to-5 card and record it in the "after" column, and note whether you saw floc in each cup.

Data & Analysis

Cup	Alum added (drops)	Clarity before (1-5)	Clarity after settling (1-5)	Floc seen?
1 (control)	0
2	5
3	10
4	20

Which cup was the clearest after settling? Give its cup number and its final rating.
For your clearest cup, calculate the change in clarity rating by subtracting the "after" rating from the "before" rating.
Did the highest dose (cup 4, 20 drops) always give the clearest water, or did a middle dose do as well or better? Use your ratings to support your answer.
The clay particles stayed suspended because they carry the same electric charge and repel one another. Using your results, explain what the alum did to those forces so the particles could clump and settle.
Of the three cups that received alum, what percentage ended up clearer than the no-alum control? Show your calculation.
A real plant must keep its finished water at or below 0.3 NTU in almost all of its samples. Based on your results, could coagulation and settling alone reliably reach that level, or would filtration still be needed? Explain.

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. Most groups find cup 3 (10 drops) or cup 4 (20 drops) is clearest; accept whichever cup the student's own ratings identify, with its recorded final rating.
Answer: Varies by group. The change equals the before rating minus the after rating; a larger positive number means more cloudiness was removed. Accept any value that matches the student's recorded ratings.
Answer: Often a middle dose does as well as or better than the highest dose, because too much coagulant can recharge the particles and keep them suspended. Accept any answer supported by the group's ratings that recognizes there can be a best dose rather than "more is always better."
Answer: The alum added positively charged ions that neutralized the negative charge on the clay particles. With the repelling charge canceled, the particles could collide and stick together into floc heavy enough to settle, which is why the dosed cups cleared while the control stayed cloudy.
Answer: Varies by group. Divide the number of dosed cups that ended clearer than the no-alum control by the three dosed cups, then multiply by 100. A group whose three dosed cups all improved reports 100 percent; two of three is about 67 percent, and one of three is about 33 percent. Accept whatever percentage matches the group's own recorded ratings, as long as the calculation is shown.
Answer: No, not by coagulation and settling alone. A jar test drops out the larger floc, but the fine particles that remain keep the water above 0.3 NTU. A real plant follows coagulation and settling with filtration to reach that low a turbidity, so filtration is still needed.

Differentiation

Support

Run cups 1 and 3 as a class demonstration before groups work independently, and provide a simplified data table with only the control and one alum dose so students compare two cups instead of four.

Extension

Have students design a follow-up test that finds the best alum dose more precisely by adding cups at 12, 15, and 18 drops, and predict where the clearest result will fall based on their first round.

ELL supports

Pair "coagulation," "floc," and "colloid" with pictures showing scattered charged particles, then clumps forming, then clumps settling. Allow students to record ratings using the numbered card alone and to answer question 4 with a labeled sketch of particles before and after the alum is added.