Lab / experiment Grade 9 Health & Consumer Science

Ivelstone Storm Kit: How Stored Tap Water Loses Its Chlorine

Students store tap water four ways, track free and total chlorine with test strips for two days, and turn the results into storage rules for a storm kit.

Grade: Grade 9
Subject: Health & Consumer Science
Material: Lab / experiment
Time: 3 sessions, ~85 min total
Prep: Moderate prep
Materials: Three clear 500 mL plastic water bottles with caps per group (containers A, B, and D), One clear wide-mouth cup or jar that holds at least 500 mL, with no lid, per group (container C), Container D prepared by the teacher the day before: a bottle swirled with about a tablespoon of milk, emptied, and rinsed once with tap water, Free and total chlorine test strips made for drinking water or for pools and spas, about 15 per group, Measuring cup or graduated cylinder marked in milliliters, Four small clean test cups per group, labeled A to D, Masking tape and a permanent marker for labels, A dark cabinet and a sunny windowsill or sheltered outdoor ledge, Timer or phone stopwatch, Paper towels

Standards

RST.9-10.3 Common Core ELA

Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks, attending to special cases or exceptions defined in the text.

How this material aligns: Students follow the multistep strip-testing procedure in Steps 4 through 6 exactly, including the timed read window and the two special cases the text defines: a reading that falls between color blocks, and water treated with chloramine instead of chlorine.

All materials for RST.9-10.3 →

HSN-Q.A.3 Common Core Math

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

How this material aligns: The recording rule in Step 5 and analysis question 5 require students to report chlorine readings only to the resolution of the strip's color blocks and to reject a computed loss that claims more precision than the strip can give.

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 2026-09-24.

Objectives

Students will measure free and total chlorine in tap water stored four different ways at the start, after one day, and after two days.
Students will compare pairs of containers that differ in one condition to explain how open air, sunlight, and leftover food residue each reduce a chlorine residual.
Students will report strip readings at the precision the strip allows and explain why a false level of precision is misleading.
Students will write evidence-based storage rules for a household emergency water supply.

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.

Also: free chlorine, total chlorine, chloramine, chlorine demand, contamination

Teacher overview

When winter storms knock out power in the fictional town of Ivelstone, the water utility's booster pumps can stop, and the town asks every household to keep a supply of drinking water at home. This lab asks a practical health question about that advice: how do the container and the storage spot change how long tap water keeps the chlorine residual that protects it from new germs? Students store school tap water in four containers, test each one with chlorine strips at the start, after one day, and after two days, and then write storage rules for an Ivelstone storm kit. Public water systems typically keep a residual of about 0.2 to 2.0 mg/L in their pipes, well below the regulatory maximum of 4.0 mg/L, so most school taps start inside the range that common strips can read.

Teaching note: many students assume that water which was safe at the tap stays safe in any container, because it "was already treated" at the plant. Treatment is a one-time event, while the residual is a small supply of disinfectant that keeps working after the water leaves the plant and gets used up as it works. Make a second point just as plainly: water that has lost its measurable chlorine is not automatically unsafe. It has lost its protection against germs that arrive later, from a dipped cup, a dirty container, or unwashed hands, which is why storage habits matter.

Preparation: check your utility's annual water quality report to learn whether it disinfects with chlorine or with chloramine. With chloramine, free chlorine reads near zero from the start and total chlorine falls more slowly, so have students track the total chlorine columns and, if the schedule allows, extend the storage to four days. If your school is on an untreated well, fill the containers from a chlorinated public tap instead. Window glass blocks much of the ultraviolet light that breaks down chlorine, so a sheltered outdoor ledge gives a clearer sunlight result than an indoor sill when your school allows it. Prepare container D the day before for each group and remind students that it is thrown away at the end of the lab. Ask about milk allergies first, and handle container D yourself for any group that includes an allergic student.

Ivelstone Storm Kit: How Stored Tap Water Loses Its Chlorine — Grade 9 Health & Consumer Science

Name: ____________________________ Date: ______________

Procedure

You will store tap water in four containers for two days and test the chlorine in each one three times. Read every step before you begin, because the order matters.

Step 1. Label your four containers with your group name, today's date, and a letter.
- Container A is a clean bottle with its cap on, kept in the dark cabinet.
- Container B is a clean bottle with its cap on, kept in the sunniest spot your teacher names.
- Container C is the clean wide-mouth cup with no cover, kept in the dark cabinet.
- Container D is the bottle that once held milk and was rinsed one time. Keep its cap on and keep it in the dark cabinet.
Step 2. Run the cold tap until the water feels cold. Measure 500 mL of tap water into each container. Put the caps on A, B, and D. Leave C open.
Step 3. Pour a small amount from each container into the test cup with the same letter. Use just enough to cover the pads on a strip.
Step 4. Test each cup this way. Dip one strip into the cup for the time printed on the strip package, then take it out. Hold it flat and do not shake it. Wait the exact number of seconds the package gives, then compare both pads to the color chart in good light. The package also gives a time limit for reading the colors, and a reading taken after that limit does not count.
Step 5. Record the value of the color block that matches best, in mg/L, for free chlorine and for total chlorine. If a color falls between two blocks, write both values, such as "0.5 to 1." Never write a number that is not printed on the chart.
Step 6. Watch for two special cases. If free chlorine reads 0 but total chlorine reads above 0 on the first day, your water is probably treated with chloramine, so tell your teacher and use the total chlorine columns for every question. If both pads read 0 on the first day, stop and tell your teacher, because this test needs chlorinated water.
Step 7. Pour the used test water into the sink. Never pour test water back into a container. Put each container in its storage spot.
Step 8. Repeat Steps 3 through 7 after one day and again after two days. Use a new strip for every reading.
Safety. Do not drink or taste any sample, including the fresh tap water on the first day. Hold strips by the end without pads, keep the strip bottle closed so moisture does not ruin the strips, and wash your hands when you finish. At the end of the lab, empty container D into the sink and throw the bottle away.

Data & Analysis

Chlorine strips report milligrams per liter (mg/L), which in water is the same as parts per million. Fill in the table as you test. If your water uses chloramine, answer every question with your total chlorine readings.

Container	Day 0 free (mg/L)	Day 0 total (mg/L)	Day 1 free (mg/L)	Day 1 total (mg/L)	Day 2 free (mg/L)	Day 2 total (mg/L)
A: capped, dark
B: capped, sunny
C: open, dark
D: milk bottle, capped, dark

Which container had the highest free chlorine reading on day 2?
Containers A and C are different in only one way. Compare their readings from day 0 to day 2 and explain what being open to the air did to the chlorine.
Containers A and B are different in only one way. Compare their readings from day 0 to day 2 and explain what the sunny spot did to the chlorine.
Container D is different from A only because it once held milk. Describe what happened to the chlorine in D and explain why leftover milk would cause it.
Suppose container A read 1 mg/L on day 0 and "0.5 to 1" on day 2. A student writes that A lost exactly 0.25 mg/L. What should the student report instead?
Stored water with no chlorine left is not automatically unsafe to drink. Explain what losing the residual does change for a family that is using stored water.
A family in the fictional town of Ivelstone dips their own cups into a jug of stored water to get a drink. Explain why this habit is a bigger risk once the chlorine is gone.
Write the one storage rule you would put first in the Ivelstone storm kit, and name the comparison from your data that supports it.

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: Container A, capped and kept in the dark, in most groups.
Accept the student's own highest day 2 reading. A clean, sealed container away from light loses chlorine the most slowly because nothing in it or around it is using the chlorine up. If two containers tie, accept either with a note that the strip could not tell them apart.
Answer: The open cup (C) lost more chlorine than the capped bottle (A).
Chlorine slowly escapes from an open water surface into the air, and a wide mouth gives it more surface to leave from. Accept a conclusion that matches the student's readings. If A and C ended on the same color block, accept an answer that says any difference was smaller than one block on the strip.
Answer: The bottle in the sunny spot (B) lost more chlorine than the bottle in the dark (A).
Ultraviolet light in sunlight breaks free chlorine apart, and a sunny spot also warms the water, which speeds up the loss. Window glass blocks much of the ultraviolet light, so accept a small or zero difference for bottles kept indoors when the student explains it that way.
Answer: Container D lost its chlorine fastest, often dropping to 0 by day 1 or even on day 0.
Chlorine reacts with the proteins, sugars, and fats in the thin film of milk that one rinse leaves behind, and each reaction uses up some chlorine. The amount of chlorine that material uses up is called chlorine demand. Any leftover food or drink in a container does the same thing, which is why used milk and juice jugs make poor storage containers. Accept the student's own readings whenever D ends at or below A, with this explanation.
Answer: A loss of less than 0.5 mg/L, with no exact value.
The day 2 reading fell somewhere between the 0.5 and 1 blocks, so the most the student can say is that A lost some chlorine, and less than 0.5 mg/L. Writing 0.25 claims a precision the strip does not have, because the chart has no block between 0.5 and 1.
Answer: It removes the protection against new germs.
The water was safe when it was stored, and it does not become harmful just because the chlorine is gone. What changes is that any germs that get in later, from a cup, hands, dust, or a dirty container, are no longer killed and can survive or grow. That makes clean containers, tight caps, and careful pouring more important the longer the water is stored.
Answer: Germs from the cups and hands get into the water, and with no chlorine left nothing kills them.
Each dip can add germs from mouths and fingers, and the whole jug is then shared by the family. Accept answers that add a safer habit, such as pouring from the jug into a clean cup or using a container with a spout.
Answer: Accept any clear rule that is supported by a named comparison from the student's data.
Strong answers include keeping containers capped (A compared with C), storing them in a cool, dark place (A compared with B), and using clean containers that never held milk or juice (A compared with D). The rule should be written as an action a family can take, not as a general warning.

Differentiation

Support

Give these groups only containers A and C so that every reading answers one comparison, and pre-label the table rows. Enlarge a copy of the strip color chart and model one complete strip reading with the class, saying each step of Step 4 aloud and pointing to the timer. Provide a frame for questions 2 through 4: "Container ___ lost more chlorine than container ___, because ___."

Extension

Add a container E, a clean capped bottle stored in a refrigerator, and ask students to predict before testing whether cold storage keeps chlorine better than the dark cabinet. Students then design a follow-up test of their own, such as a milk bottle washed with hot soapy water compared with one rinsed once, and state the one variable they changed and what they kept the same.

ELL supports

Pre-teach "residual," "container," "capped," "sunlight," and "germs" with a labeled photo of the four containers in their storage spots. Post the Step 4 strip routine as four numbered pictures: dip, hold flat, wait, compare. Accept answers to questions 6 and 7 as labeled drawings with one sentence each, and allow a bilingual glossary during the analysis.

Extensions

Groups can turn their answers into a one-page Ivelstone storm kit card that a family could tape inside a cabinet door, with each rule followed by the class data that supports it. A water quality technician from a local utility can visit to explain how the utility checks the residual at the far ends of its pipe network, where water has traveled longest, and why that reading guides the amount of chlorine added at the plant.