Lab / experiment Grade 5 Health & Consumer Science
Covered or Open: Tracking Tap Water Chlorine at Wintercress
A grade 5 health lab in which students store tap water three ways, track its chlorine with test strips for a day, and graph the readings to set a storage rule.
Objectives
- Students will measure the chlorine in tap water stored three ways at four times over one day and record the readings in a table.
- Students will graph the readings as ordered pairs and subtract decimal readings to compare how fast each jar lost its chlorine.
- Students will use their own data to write and defend a rule for storing drinking water at home.
Vocabulary
- Parts per million (ppm)
- Parts per million tells how many tiny bits of a substance are mixed into a million bits of water. Scientists use it to measure very small amounts of chemicals in water.
Also: chlorine, germs, test strip, ordered pair, fair test
Teacher overview
Tap water leaves a treatment plant with a small amount of chlorine still in it, and that leftover amount keeps germs from growing again while the water travels through pipes and waits in a home. Under EPA rules the maximum residual disinfectant level for chlorine is 4.0 mg/L, and systems typically maintain a residual of 0.2 to 2.0 mg/L in the distribution system. In this lab, set in a fifth-grade classroom at the fictional Wintercress Elementary, students fill three identical jars with fresh tap water, store them three different ways, and track the chlorine with test strips across a school day and the following morning. Test strips report parts per million, and in water 1 ppm equals 1 mg/L, so readings can be set beside those figures, although a strip is a rough classroom measure and not an official test.
The misconception to address directly is the common belief that the chlorine in tap water is itself a pollutant, so water left out to "air out" becomes healthier to drink. The lab shows the protective residual fading in open jars, which is one reason health guidance for stored drinking water calls for clean, covered containers kept out of the sun. Students also practice a fair test, since each pair of jars they compare differs in only one way.
Test the school tap with a strip a few days before the lab. If it reads zero, the building may be on an unchlorinated well, or the local utility may use chloramine, which free-chlorine strips read low or not at all. In that case use total-chlorine strips and expect a slower, smaller drop. Window glass filters out part of the sunlight that breaks down chlorine, so the windowsill jar may lose less than students expect, and the answer key accepts whatever the group's readings show. Start the lab first thing in the morning so the hour 3 and hour 6 readings fall inside the school day, and plan the hour 24 reading and the analysis for the next morning.
Procedure
Step 1 (safety first): Put on goggles and gloves before you touch a test strip. Never drink or taste the water in this lab. Keep test strips away from your mouth and eyes. Wash your hands when you finish each reading.
Step 2 (label your jars): Use tape and a marker to label three clean jars A, B, and C. Jar A gets a lid. Jars B and C stay open the whole time.
Step 3 (fill them the same way): Run the cold tap for about half a minute. Then fill all three jars to the same line with fresh tap water. Put the lid on Jar A right away and screw it on tight.
Step 4 (place the jars): Set Jars A and B side by side on a shaded shelf. Set Jar C on a sunny windowsill. Put each jar where it cannot be bumped or knocked over.
Step 5 (take the hour 0 reading): Pour a little water from each jar into its own small paper cup. Close the lid on Jar A again right away. Dip a new strip into each cup for the time the package gives. Then hold the strip flat and wait as many seconds as the package says before you read it. Find the closest color on the chart on the bottle and write the number printed next to it. Pour the sample into the sink and throw away the cup and the strip.
Step 6 (read again at hour 3 and hour 6): Repeat Step 5 about 3 hours after you filled the jars. Repeat it again about 6 hours after you filled them. Use a new strip and a new cup every time.
Step 7 (read the next morning): Leave the jars where they are overnight. Take the last reading about 24 hours after you filled them.
Step 8 (clean up): Pour out the water, rinse the jars, and wipe up any spills so no one slips.
Data & Analysis
Write each reading in the table. Use the number printed next to the closest color on the strip bottle. The unit is ppm, which stands for parts per million.
| Hours since filling | Jar A (lid on, shade) | Jar B (open, shade) | Jar C (open, sun) |
|---|---|---|---|
| 0 | |||
| 3 | |||
| 6 | |||
| 24 |
- Make a graph on your coordinate grid. Put hours since filling on the x-axis, from 0 to 24. Put the strip reading on the y-axis, from 0 to 4. Plot the four points for each jar in its own color and connect each jar's points. Which jar has the lowest point at hour 24?
- Write the ordered pair for Jar B at hour 24. Put the hours first and the reading second.
- How much did the reading for Jar C drop from hour 0 to hour 24? Subtract the hour 24 reading from the hour 0 reading.
- How much did the reading for Jar A drop from hour 0 to hour 24? Subtract the same way.
- Suppose a class at another school set an open jar of tap water in the sun. Its readings were 1.5 ppm at hour 0, 1 ppm at hour 3, 0.5 ppm at hour 6, and 0 ppm at hour 24. How much did the reading drop from hour 0 to hour 6?
- Jars A and B sat side by side in the shade, and the only difference was the lid. Use your readings to tell what the lid did.
- Jars B and C were both open, and the only difference was where they sat. Use your readings to tell what the sunny windowsill did.
- The chlorine in tap water helps keep germs from growing while water sits. Write one rule for storing drinking water at home that your results support, and back it up with a reading from your table.
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: Jar C, the open jar in the sun, in most classes.
Accept whichever jar has the lowest hour 24 point on the student's own graph. Check that hours run along the x-axis, readings run up the y-axis, and each jar has four points in its own color. - Answer: (24, the student's hour 24 reading for Jar B).
For example, a Jar B reading of 0.5 ppm at hour 24 is written (24, 0.5). The first number tells how many hours the water sat, and the second tells the strip reading. Accept any pair whose second number matches the table. - Answer: Accept the hour 0 reading minus the hour 24 reading for Jar C.
For example, readings of 1.5 ppm and 0 ppm give a drop of 1.5 ppm. Jar C usually shows the largest drop of the three jars. Check the subtraction against the student's own table. - Answer: Accept the hour 0 reading minus the hour 24 reading for Jar A.
Jar A usually shows the smallest drop, often 0 or 0.5 ppm, because the lid kept the water closed off from the air. Check the subtraction against the student's own table. - Answer: 1 ppm.
The reading fell from 1.5 ppm at hour 0 to 0.5 ppm at hour 6, and 1.5 minus 0.5 equals 1. Checked a second way, 0.5 plus 1 equals 1.5. A student who answers 1.5 has used the hour 24 reading instead of the hour 6 reading. - Answer: The lid helped the water keep its chlorine.
Most groups find the hour 24 reading for Jar A equal to or higher than the one for Jar B. Accept any answer that compares the two jars' actual readings. If both jars read the same, accept the conclusion that the lid made no difference the strips could detect in one day, which is an honest reading of the data. - Answer: The sun made the chlorine fade faster.
Jar C usually drops sooner and farther than Jar B. Stronger answers note that the windowsill is both brighter and warmer, so this test cannot tell whether light or heat caused the difference. Accept any answer built on the group's readings. - Answer: Accept a rule such as "Keep stored drinking water in a clean jar or bottle with a lid, out of the sun."
The rule should be backed by a specific reading, for example "Jar A still read 1 ppm at hour 24, while Jar C read 0 ppm." Also accept "Pour fresh water instead of drinking from a cup that sat out all day" when a reading from an open jar supports it.
Differentiation
Support
Give these students a coordinate grid with the axes already labeled and numbered, with hours marked in steps of 3 and readings marked in steps of 0.5. In a group of three, let each student own one jar for all four readings and plot only that jar before the group combines its lines. Set up questions 3 and 4 as vertical subtraction problems with the decimal points already lined up.
Extension
Ask these students to add a Jar D that has a lid and sits in the sun. With four jars they can separate the effect of the lid from the effect of the sunny spot, and they should write a short paragraph explaining which factor mattered more in their data and how they know.
ELL supports
Pre-teach "chlorine," "germs," "lid," "shade," "reading," and "ordered pair" with a labeled picture of the three jars in their places. Provide the sentence frames "Jar ___ kept more chlorine because ___." and "My rule is ___, and my data show ___." Allow students to answer questions 6 and 7 by pointing to the two jars' lines on their graph and completing one frame.
Extensions
Have students look up their own water utility's yearly water quality report, often called a Consumer Confidence Report, and find the line that describes the disinfectant in the water. Students can compare the range the utility reports with the class's hour 0 readings and write two sentences about why the two numbers might not match exactly.