Worksheet Grade 9 Science

Chlorine Dosing at Thistle Hollow: A Grade 9 Unit-Conversion Worksheet

A grade 9 worksheet where students chain unit conversions and the pounds formula to calculate chlorine feed rates for a fictional town's water treatment plant.

Grade: Grade 9
Subject: Science
Material: Worksheet
Time: 40 min
Prep: No prep
Materials: calculator

Standards

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: Items 1 through 6 chain a flow conversion, a dosage calculation, and a residual check into one multi-step problem, requiring the consistent unit tracking this standard specifies.

All materials for HSN-Q.A.1 →

HSN-Q.A.3 Common Core Math

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

How this material aligns: Item 7 asks students to judge how many decimal places a chlorine residual reading can support given the test kit's precision, the accuracy-matching this standard requires.

All materials for HSN-Q.A.3 →

HS-ETS1-3 NGSS

Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.

How this material aligns: Items 3 and 6 evaluate whether the plant's existing 15 pound-per-day feed pump can still deliver the required dosage when summer flow rises, weighing that capacity constraint against the disinfection need item 5 explains.

All materials for HS-ETS1-3 →

Download Word file

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 convert a plant's flow rate from gallons per day to million gallons per day.
Students will apply the pounds formula to calculate a chlorine feed rate from a target dosage and flow.
Students will calculate chlorine demand from dosage and residual readings.

Vocabulary

Chlorine residual: Chlorine residual is the small amount of chlorine still left in the water after it leaves the treatment plant. It keeps protecting the water from germs all the way to your faucet.
Flow rate: Flow rate measures how much water moves past a point in a certain amount of time, like gallons per minute through a garden hose. Bigger pipes and stronger pushes give bigger flow rates.

Also: chlorine dosage, chlorine demand, pounds formula

Teacher overview

The town of Thistle Hollow runs a small surface-water treatment plant that adds chlorine to disinfect the water before it leaves for the distribution system. Operators do not add chlorine by guesswork; they calculate a precise feed rate in pounds per day using the plant's flow and a target dosage, then confirm the chemical is working by testing how much chlorine remains (the residual) after it has had time to react. This worksheet walks through that full sequence using numbers set for Thistle Hollow's plant.

A common misconception in this math is treating the plant's flow rate as if it were already in the units the pounds formula needs. The formula requires flow in million gallons per day (MGD), so a flow given in gallons per minute or gallons per day must be converted first, and skipping that conversion produces an answer that is off by several orders of magnitude. Watch for students who plug a raw gallons-per-minute number directly into the formula.

Chlorine Dosing at Thistle Hollow: A Grade 9 Unit-Conversion Worksheet — Grade 9 Science

Name: ____________________________ Date: ______________

Student Worksheet

The town of Thistle Hollow's treatment plant treats water at a flow rate of 500,000 gallons per day (gpd). All figures in this worksheet describe Thistle Hollow's fictional plant. Use these facts for all items:

Operators want a chlorine dosage of 2.5 mg/L.
One gallon of water weighs 8.34 pounds.
After chlorine reacts with the water for 30 minutes, operators test the water and measure a residual of 0.6 mg/L.
The plant's chemical feed pump can supply chlorine solution at a maximum rate of 15 pounds of chlorine per day.

Convert the plant's flow rate from 500,000 gallons per day to million gallons per day (MGD).
Using the pounds formula (pounds/day = dosage in mg/L × flow in MGD × 8.34), calculate how many pounds of chlorine the plant must feed each day to reach the 2.5 mg/L dosage.
Compare your answer to item 2 with the feed pump's maximum rate of 15 pounds per day. Can the pump supply the dosage the plant needs?
Calculate the chlorine demand using the formula: demand (mg/L) = dosage (mg/L) − residual (mg/L).
Explain in one or two sentences what "chlorine demand" represents in terms of what happens to the chlorine after it is added to the water.
If Thistle Hollow's flow rose to 800,000 gallons per day during a summer demand spike, and operators kept the same 2.5 mg/L dosage, how many pounds of chlorine per day would the plant need? Would the existing feed pump (item 3's maximum) be able to keep up?
A neighboring plant reports a residual of 0.6000 mg/L from a test kit that can only reliably measure to the nearest tenth of a milligram per liter. Rewrite that reading with an appropriate number of decimal places for the kit's precision, and explain why the extra digits are misleading.
Challenge. Chlorine demand at Thistle Hollow's plant changes with raw-water quality. On a high-demand day after heavy rain, demand reaches 2.2 mg/L. On a calm, low-demand day, demand drops to 0.9 mg/L. If operators want a 0.5 mg/L residual on both kinds of days, what dosage should they feed on the high-demand day, and what dosage on the low-demand day? (Rearrange the demand formula: dosage = demand + residual.)

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: 0.5 MGD.
500,000 gallons ÷ 1,000,000 = 0.5 MGD.
Answer: 10.425 lb.
Pounds/day = 2.5 mg/L × 0.5 MGD × 8.34 = 10.425 pounds of chlorine per day.
Answer: yes.
The plant needs 10.425 pounds per day, which is below the pump's 15 pound-per-day maximum, so the pump can supply the dosage with room to spare (about 4.575 pounds per day of extra capacity).
Answer: 1.9 mg/L.
Demand = 2.5 mg/L − 0.6 mg/L = 1.9 mg/L.
Answer: short explanation.
Chlorine demand is the portion of the chlorine dose that gets used up reacting with organic matter, minerals, and microorganisms in the water, leaving the residual as the amount left over to keep disinfecting water in the distribution system.
Answer: 16.68 lb; no.
800,000 gpd = 0.8 MGD. Pounds/day = 2.5 mg/L × 0.8 MGD × 8.34 = 16.68 pounds per day, which is above the pump's 15 pound-per-day maximum, so the existing pump could not keep up during that demand spike without a change in dosage or a pump upgrade.
Answer: 0.6 mg/L.
Because the test kit can only reliably measure to the nearest tenth of a milligram per liter, reporting "0.6000 mg/L" implies a precision the kit cannot actually deliver; the honest reading is 0.6 mg/L, with the extra zeros dropped.
Answer: 2.7 mg/L and 1.4 mg/L.
Dosage = demand + residual. On the high-demand day: 2.2 + 0.5 = 2.7 mg/L. On the low-demand day: 0.9 + 0.5 = 1.4 mg/L. The same target residual requires a higher dosage whenever demand is higher, which is why operators adjust the feed rate rather than leaving it fixed.

Differentiation

Support (below grade level)

Provide a conversion reference card listing "gallons ÷ 1,000,000 = MGD" and the pounds formula already written with labeled blanks for dosage, flow, and 8.34. Reduce item 8 to a single required calculation (the high-demand dosage only) rather than both dosages.

Extension (above grade level)

Ask students to calculate the exact flow rate, in gallons per day, at which Thistle Hollow's existing 15 pound-per-day pump reaches its maximum capacity while holding the 2.5 mg/L dosage constant, working backward through the pounds formula.

ELL supports

Pre-teach "dosage," "residual," and "demand" with a simple diagram: chlorine goes in (dosage), some reacts and disappears (demand), and some remains (residual), shown as a bar that splits into two parts. Allow item 5 to be answered with a labeled diagram plus one sentence instead of a full paragraph.