Lab / experiment Grade 10 Science

Upwood Creek Chlorine Demand: A Dose and Residual Lab

Grade 10 students dose four water samples that carry increasing amounts of leaf matter, measure the free chlorine left after 15 minutes, and calculate chlorine demand.

Grade: Grade 10
Subject: Science
Material: Lab / experiment
Time: 55 min
Prep: Moderate prep
Materials: Four clear cups per group, about 250 mL each, Organic stock made by steeping dried leaves in warm water and straining, Distilled water, about 350 mL per group, Chlorine stock prepared by the teacher from unscented household bleach: about 4 mL of fresh 6 percent bleach diluted to 1 liter with distilled water gives roughly 250 mg/L, so 2 mL in a 100 mL sample is about 5 mg/L, Free chlorine test strips reading from 0 to 10 mg/L, 100 mL graduated cylinder or measuring cup, and a 5 mL syringe or dropper, Timer, safety goggles, and paper towels

Standards

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: Analysis question 4 sets the dose that meets a required residual, and question 7 has students weigh raising that dose against chemical cost, taste complaints, and disinfection byproducts.

All materials for HS-ETS1-3 →

HSN-Q.A.3 Common Core Math

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

How this material aligns: Analysis question 6 requires students to report chlorine demand to the precision their test strips actually support rather than to the digits a calculator prints.

All materials for HSN-Q.A.3 →

Download Word file

This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will measure the free chlorine remaining in four samples that differ only in organic content.
Students will calculate chlorine demand as dose minus residual and describe how demand changes with organic matter.
Students will determine the dose a treatment plant would need to hold a required residual.

Vocabulary

Key words: chlorine demand, residual, dose, free chlorine, organic matter

Teacher overview

Chlorine added to water does not simply stay there. It reacts first with whatever the water already carries, including dissolved organic matter from leaves and soil, and only what survives those reactions remains as a measurable residual. The gap between what was added and what remains is the chlorine demand, and it is the reason a plant cannot set one dose and leave it alone all year. This lab uses samples that stand in for the fictional Upwood Creek after a storm, when leaf litter and soil raise the organic load, so students can see the residual fall as the organic content climbs. Watch for the assumption that the residual a test strip reports equals the dose that was added, which turns a cup showing no chlorine into a cup that appears never to have been dosed. The classroom test uses strips rather than a laboratory colorimeter, which makes it a demonstration of the pattern rather than an official measurement.

Upwood Creek Chlorine Demand: A Dose and Residual Lab — Grade 10 Science

Name: ____________________________ Date: ______________

Procedure

Step 1 (safety first): Put on goggles before anything is poured. The chlorine stock is diluted bleach. Do not taste or drink any sample. Never mix the chlorine stock with vinegar, ammonia, or any cleaning product, because those combinations release harmful gases. Work in a well ventilated room, wipe spills at once, and wash your hands when the lab is finished.

Step 2 (record the dose): Your teacher has prepared the chlorine stock so that 2 mL of stock added to a 100 mL sample delivers a nominal dose of 5.0 mg/L. Write that dose at the top of your data table. Household bleach loses strength on the shelf, so treat this dose as approximate and use the same stock for every cup.

Step 3 (prepare the four samples): Label the cups 1 through 4. Into cup 1 measure 100 mL of distilled water with no organic stock. Into cup 2 measure 90 mL of distilled water and 10 mL of organic stock. Into cup 3 measure 80 mL of distilled water and 20 mL of organic stock. Into cup 4 measure 60 mL of distilled water and 40 mL of organic stock. Use distilled water rather than tap water, because treated tap water already carries chlorine that the strips would add to your dose. Swirl each cup gently so the contents mix.

Step 4 (record the starting appearance): Note the color of each cup before dosing, from clear to light brown. This gives you a rough picture of how much organic matter each sample carries.

Step 5 (dose every cup the same way): Add 2 mL of chlorine stock to cup 1 and start the timer. Add the same 2 mL to cups 2, 3, and 4 in turn. Swirl each cup once after dosing. Every cup receives the same dose, so any difference at the end comes from the water, not from the chlorine.

Step 6 (wait 15 minutes): Let the cups sit undisturbed for 15 minutes. This is the contact time. While you wait, write your prediction of which cup will hold the most chlorine and why.

Step 7 (measure the residual): Dip a free chlorine test strip in cup 1 for the time printed on the strip package, remove it, and compare it with the color chart. Record the reading in mg/L. Repeat with a fresh strip for each of the other three cups. Read every strip at the same time after dipping, since the color continues to change.

Step 8 (clean up): Pour the samples down a drain with plenty of running water, rinse the cups, and dispose of used strips in the trash. Wash your hands before leaving.

Data & Analysis

Cup	Organic stock added (mL)	Dose (mg/L)
1	0	5.0
2	10	5.0
3	20	5.0
4	40	5.0

Chlorine demand equals dose minus residual. Using your dose of 5.0 mg/L and your four residual readings, calculate the demand for each cup, and state which cup had the highest demand.
Using your demand values for cup 1 and cup 4, calculate the difference between them in mg/L.
Cups 2, 3, and 4 received 10, 20, and 40 mL of organic stock. Describe how demand changed as the organic stock doubled, and state whether the change was roughly proportional.
A plant must still have 0.5 mg/L of free chlorine left after contact. Using your cup 4 demand, calculate the dose that water would need.
Suppose a plant treats 2.0 MGD of water like your cup 4 sample. Using the dose you found in question 4 and the formula pounds per day equals mg/L times MGD times 8.34, calculate the pounds of chlorine the plant would feed each day.
Your test strips read in steps of about 0.5 mg/L. Explain how that limits the number of digits you should report, then write your cup 3 demand with an appropriate number of digits.
After a heavy storm, raw water carries more leaves, soil, and runoff. Using your results, predict what happens to a plant's chlorine use, and name one drawback of simply raising the dose.

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; cup 4 should show the highest demand.
Demand equals 5.0 mg/L minus the measured residual for each cup. A typical set of results runs near 0.5 to 1.0 mg/L of demand for cup 1 and 3.5 mg/L or more for cup 4. Accept any set of values calculated correctly from the group's own readings, as long as demand rises from cup 1 to cup 4.
Answer: Varies by group; commonly 2.5 to 4.0 mg/L.
Subtract the cup 1 demand from the cup 4 demand. Accept any difference that follows from the recorded residuals. A group whose cup 4 strip reads zero should note that the true demand is at least 5.0 mg/L, since the strip cannot show how much more chlorine the sample would have consumed.
Answer: Demand rose with organic stock, and roughly in proportion.
Most groups find that doubling the organic stock roughly doubles the demand at the low end, then the pattern flattens once the residual approaches zero and the strip can no longer resolve the difference. Accept either conclusion when it is supported by the group's numbers, and expect the strongest answers to mention that the flattening is a limit of the measurement rather than a limit of the chemistry.
Answer: Varies by group; dose equals the cup 4 demand plus 0.5 mg/L.
A group with a cup 4 demand of 3.6 mg/L would need a dose of 4.1 mg/L. Accept any answer that adds the required residual to the group's own demand. Students should notice that the dose is several times the residual it leaves. The 4.0 mg/L maximum residual disinfectant level for chlorine limits the chlorine left in the water, not the dose, yet a large dose into water rich in organic matter forms more disinfection byproducts, which is a reason plants remove organic matter before disinfecting rather than chasing the demand with more chemical.
Answer: Varies by group; pounds per day equals the dose times 2.0 times 8.34.
A group using a dose of 4.1 mg/L would calculate 4.1 x 2.0 x 8.34 = 68.4 pounds per day. Accept any result that follows from the group's dose with the multiplication shown. Students should carry the units through so the answer lands in pounds per day rather than mg/L.
Answer: Two significant figures at most, such as 3.5 mg/L.
The strips resolve to about half a milligram per liter, so a calculated demand of 3.47 claims precision the instrument cannot support. Reporting demand to the nearest 0.5 mg/L, or at most to one decimal place, matches the measurement. Accept a cup 3 demand written to the nearest half unit or to one decimal place, with an explanation that the strip, not the arithmetic, sets the limit.
Answer: Chlorine use rises after a storm, and higher doses bring taste and byproduct problems.
Storm runoff carries organic matter, which raises demand exactly as the added organic stock did in this lab, so the plant must feed more chlorine to hold the same residual. Drawbacks students should raise include the cost of the extra chemical, complaints about chlorine taste and odor, and the formation of disinfection byproducts when chlorine reacts with organic matter. Strong answers note that removing the organic matter first, through coagulation and filtration, is usually better than raising the dose alone.

Differentiation

Support

Run cups 1 and 4 as a class demonstration before groups work, so students see the largest contrast first and know what a low residual strip looks like. Provide a data table with the dose column already filled in and the subtraction written as a reminder at the top, and let students use a calculator with the formula demand equals dose minus residual displayed beside them.

Extension

Have groups run a second round on cup 4 water that they first filter through a coffee filter packed with a spoonful of activated carbon from an aquarium supply, then compare the demand before and after. Ask them to state, using both sets of numbers, how much of the fictional Upwood Creek dose the plant could avoid by treating the organic matter before disinfection.

ELL supports

Post a three-word visual chart showing dose, residual, and demand as a bar that shrinks, so the subtraction is visible before it is calculated. Pre-teach "organic matter" with samples of the leaf stock beside clear tap water, and provide the sentence frame "Cup ___ had the highest demand because ___" for question 1.

Extensions

Ask students to write the procedure they would use to find the lowest dose that still leaves 0.5 mg/L of residual in the fictional Upwood Creek sample, using the same materials. The procedure should state how many cups they would prepare, what dose each would receive, how long the contact time would run, and how they would decide which cup gives the answer.