Worksheet Grade 11 Science

Thornbury's Jar Test: Coagulation Chemistry and Turbidity Removal

Grade 11 students calculate turbidity-removal percentages from a fictional plant's alum jar test and explain coagulation using particle surface charge.

Grade: Grade 11
Subject: Science
Material: Worksheet
Time: 40 min
Prep: No prep
Materials: calculator (optional)

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: Items 1, 2, and 9 require students to explain Thornbury's cloudy raw water and coagulant overdosing in terms of the strength of electrical forces between colloidal particles, exactly this standard's bulk-scale evidence.

All materials for HS-PS1-3 →

HSN-Q.A.3 Common Core Math

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

How this material aligns: Items 3 through 7 ask students to round turbidity-removal percentages to one decimal place, a level of precision appropriate to a jar-test reading, the accuracy judgment 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 2026-07-16.

Objectives

Students will explain coagulation in terms of colloidal particle surface charge and charge neutralization.
Students will calculate turbidity-removal efficiency from jar-test data using the removal-efficiency formula.
Students will connect jar-test results to the regulatory turbidity limit finished water must meet.

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.
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, zeta charge

Teacher overview

Thornbury (a fictional town created for this worksheet) draws its raw water from a reservoir that runs cloudy after storms. Operators at Thornbury's plant use a jar test, a small-scale bench experiment, to find the coagulant dose that clears the water most effectively before it reaches full-scale sedimentation and filtration basins. This worksheet uses one afternoon's jar-test results to connect the chemistry of colloidal particles to the arithmetic operators use every day. A common misconception worth addressing directly: students often assume that adding more coagulant always removes more turbidity, when in fact overdosing can make water cloudier again by reversing the particles' surface charge.

Thornbury's Jar Test: Coagulation Chemistry and Turbidity Removal — Grade 11 Science

Name: ____________________________ Date: ______________

Student Worksheet

All data below come from a jar test run at Thornbury's plant (a fictional facility used for this exercise). The raw water entering the test measured 42.0 NTU of turbidity. Operators dosed five jars with different amounts of alum (aluminum sulfate) and measured the turbidity remaining in each after settling.

Jar	Alum dose (mg/L)	Final turbidity (NTU)
1	10	18.0
2	20	9.5
3	30	3.0
4	40	1.2
5	50	2.5

Explain why Thornbury's raw water is cloudy before treatment. Refer to the surface charge that colloidal particles carry and what that charge does to how the particles behave in water.
Explain, in chemical terms, how adding alum begins to clear the water. Describe what the aluminum ions do to the colloidal particles' surface charge and what that change allows to happen next.
Using the removal-efficiency formula (Removal % = (influent − effluent) ÷ influent × 100), calculate the percent turbidity removal for Jar 1. Round each percentage in items 3 through 7 to the nearest tenth of a percent.
Calculate the percent turbidity removal for Jar 2.
Calculate the percent turbidity removal for Jar 3.
Calculate the percent turbidity removal for Jar 4.
Calculate the percent turbidity removal for Jar 5.
Which jar produced the best (highest) removal efficiency? State the jar number and its alum dose.
Jar 5 used a higher alum dose than Jar 4, yet its turbidity is worse. Using the concept of particle surface charge, explain why overdosing coagulant can make turbidity rise again instead of continuing to fall.
State the regulatory turbidity limit finished water leaving a plant must meet, and explain what additional treatment steps after coagulation must accomplish, given that even Jar 4's turbidity (1.2 NTU) does not meet that limit on its own.
Suppose Thornbury's plant treats an average flow of 2.5 MGD (a flow set for this exercise). Using the optimal alum dose you identified in item 8 and the pounds formula (Loading in lbs/day = dose in mg/L × flow in MGD × 8.34), calculate how many pounds of alum the plant must feed each day.

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: Colloidal particles carry a negative surface charge, which makes them repel one another instead of colliding and clumping together, so they stay suspended and keep the water cloudy.
Answer: Alum releases positively charged aluminum ions into the water. Those ions neutralize the negative surface charge on the colloidal particles, which lets the particles collide, stick together, and form larger clumps called floc that can settle out.
Answer: 57.1 percent. (42.0 − 18.0) ÷ 42.0 × 100 = 24.0 ÷ 42.0 × 100 = 57.14%, which rounds to 57.1%.
Answer: 77.4 percent. (42.0 − 9.5) ÷ 42.0 × 100 = 32.5 ÷ 42.0 × 100 = 77.38%, which rounds to 77.4%.
Answer: 92.9 percent. (42.0 − 3.0) ÷ 42.0 × 100 = 39.0 ÷ 42.0 × 100 = 92.86%, which rounds to 92.9%.
Answer: 97.1 percent. (42.0 − 1.2) ÷ 42.0 × 100 = 40.8 ÷ 42.0 × 100 = 97.14%, which rounds to 97.1%.
Answer: 94.0 percent. (42.0 − 2.5) ÷ 42.0 × 100 = 39.5 ÷ 42.0 × 100 = 94.05%, which rounds to 94.0%.
Answer: Jar 4, a 40 mg/L alum dose. It produced the highest removal efficiency (97.1 percent) and the lowest turbidity (1.2 NTU) of the five jars tested.
Answer: At high enough doses, excess positively charged aluminum ions can coat the particle surfaces so completely that the particles themselves become positively charged. Positively charged particles repel each other just as negatively charged ones did, so they stop clumping and turbidity rises again.
Answer: 0.3 NTU in at least 95 percent of monthly samples, never exceeding 1 NTU. Jar 4's 1.2 NTU is already a settled-water result, so the full-scale plant needs flocculation and sedimentation to match the jar and then filtration to strain out the remaining fine floc and bring turbidity to 0.3 NTU or below before the water leaves the plant.
Answer: 834 lbs/day. 40 mg/L × 2.5 MGD × 8.34 = 100 × 8.34 = 834 pounds of alum per day.

Differentiation

Support (below grade level)

Provide the removal-efficiency formula already set up with the raw-water value filled in ((42.0 − ___) ÷ 42.0 × 100) so students only need to substitute each jar's final turbidity. Work through Jar 1 together as a class before releasing students to Jars 2 through 5.

Extension (above grade level)

Ask students to sketch a rough graph of alum dose (x-axis) versus final turbidity (y-axis) using the five jars' data, then use the shape of that graph to explain why jar testing, rather than simply using the highest available dose, is standard practice at every treatment plant.

ELL supports

Pre-teach "coagulation," "colloid," "floc," and "turbidity" with a simple diagram showing scattered charged particles before dosing and clumped floc after dosing. Allow items 1, 2, and 9 to be answered using labeled diagrams supplemented with short phrases rather than full paragraphs.