Lab / experiment Grade 8 Health & Consumer Science
Faircombe Main Break: Testing How Much Rinse Water Hands Need
During a fictional water main outage, students test how rinse volume and soap change the residue left on hands and pick a rinse amount for school jug stations.
Objectives
- Students will measure how the amount of rinse water and the use of soap change the residue left on hands after a timed wash.
- Students will graph class data and describe where added rinse water stops improving the result.
- Students will recommend a rinse amount that balances clean hands against a limited emergency water supply.
Vocabulary
- Water main
- A big underground pipe that carries drinking water beneath the streets to a neighborhood. Smaller service lines branch off it to reach each building.
Also: simulant, residue, control, nonlinear, trade-off
Teacher overview
In our scenario, an old water main under Wharton Street in the fictional town of Faircombe breaks on a Monday morning, and Faircombe Middle School has no running water until crews finish the repair. The school keeps classes going with handwashing stations made from large jugs with spigots, filled from a water truck, and the school nurse needs to know how much rinse water each handwash should use so the supply lasts the day. Students coat their hands with a cinnamon-and-oil simulant, wash with a fixed amount of soap and scrubbing, rinse with different measured volumes, and score the residue left behind. Teaching note: two misconceptions come up in most classes. Many students believe hot water is what kills germs during handwashing, when water comfortable enough for hands is far too cool to kill germs and it is the soap and scrubbing that loosen and lift them. Many also assume that more rinse water always means cleaner hands, and the class graph usually shows the benefit leveling off, which is the pattern this lab is built to reveal.
Procedure
Work in groups of four or five. Each student runs one wash, and the group shares its data.
- Step 1. Get your assignment. Your teacher will give each student a rinse amount: 50, 100, 200, or 400 milliliters with soap, or 400 milliliters with no soap. The no-soap wash is the control.
- Step 2. Check your skin. If cinnamon or oil bothers your skin, or you have cuts on your hands, tell your teacher. You can be the pourer and scorer instead.
- Step 3. Coat your hands. Put half a teaspoon of the cinnamon-oil mix in one palm. Rub it over both hands for 15 seconds, including the backs, thumbs, fingertips, and the spaces between the fingers. Wait 1 minute. Do not touch your face or eyes.
- Step 4. Wet and lather. Hold your hands over the basin while your partner pours 50 milliliters of room-temperature water over them. Add one pump of soap, unless you are doing the no-soap wash. Scrub all parts of both hands for 20 seconds while your partner times you.
- Step 5. Rinse. Your partner measures your assigned rinse amount and pours it slowly over both hands into the basin. Turn your hands so the water reaches the fronts and backs.
- Step 6. Blot. Press both hands gently on a white paper towel. Do not rub.
- Step 7. Score. Check five zones on both hands: palms, backs, thumbs, fingertips and nails, and the spaces between the fingers. Look at the paper towel too. Give 1 point for each zone that still shows cinnamon or feels oily. Your score will be from 0 (clean) to 5 (residue in every zone).
- Step 8. Clean up. Wash your hands fully at the sink with soap. Wipe up spills right away, since oil on the floor is slippery. Empty the basin into the sink your teacher chooses.
Data & Analysis
Group results:
| Student | Soap (yes or no) | Rinse amount (mL) | Residue score (0 to 5) | Zones that still had residue |
|---|---|---|---|---|
Class results (copy from the board):
| Wash | Scores recorded | Mean score |
|---|---|---|
| 50 mL rinse, soap | ||
| 100 mL rinse, soap | ||
| 200 mL rinse, soap | ||
| 400 mL rinse, soap | ||
| 400 mL rinse, no soap |
Make a scatter plot of the class results for the washes with soap. Put rinse amount on the x-axis and residue score on the y-axis. Describe the association. Do the points follow a straight line, or do they level off?
Describe in words how the residue score changes as the rinse amount goes from 50 to 400 milliliters. Where does adding more water make the biggest difference, and where does it make little difference?
Compare the mean score for the 400-milliliter wash with soap to the mean score for the 400-milliliter wash without soap. Which washed better, and by how many points?
Which zones of the hand held residue most often in your class? Give one reason those zones are harder to clean.
Suppose the nurse's jugs hold 240 liters of water for one school day. Each handwash uses 50 milliliters to wet the hands and 200 milliliters to rinse. How many full handwashes can the supply cover? (1 liter = 1,000 milliliters)
Suppose each handwash instead uses 50 milliliters to wet the hands and 400 milliliters to rinse. How many full handwashes can the same 240 liters cover?
In our scenario, the school has 450 students and staff, and the nurse wants everyone to be able to wash at least twice during the day. Using your graph and your answers to questions 5 and 6, which rinse amount would you recommend? Explain the trade-off.
A classmate says the jugs should be filled with hot water because hot water kills germs. Use your soap and no-soap results to explain what really does most of the cleaning.
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: A negative association that levels off, so the points do not follow a straight line in most classes.
Scores usually fall as the rinse amount rises, then change little. Accept any description that matches the student's own plot and names the direction of the association.Answer: In most classes the score drops most between 50 and 200 milliliters and changes little after that.
Accept any description that matches the class graph, as long as it names where the graph falls steeply and where it flattens. The flat part shows where extra water adds little cleaning.Answer: The wash with soap, by the difference between the two class means.
In most classes the no-soap mean is clearly higher, even though both groups used the largest rinse. Accept the student's own difference.Answer: Usually the fingertips and nails, the thumbs, and the spaces between the fingers.
These places are curved or tucked away, so scrubbing and rinse water reach them less. Accept any zones supported by the class data with a reasonable reason.Answer: 960 handwashes.
240 liters × 1,000 = 240,000 milliliters. Each wash uses 50 + 200 = 250 milliliters. 240,000 ÷ 250 = 960 full handwashes.Answer: 533 handwashes.
Each wash uses 50 + 400 = 450 milliliters. 240,000 ÷ 450 = 533.3, so only 533 full handwashes fit.Answer: About 200 milliliters in most classes.
Two washes each for 450 people is 900 washes. The 200-milliliter rinse covers 960 washes, which is enough, while the 400-milliliter rinse covers only 533. If the class graph shows little extra cleaning above 200 milliliters, the larger rinse uses water the school cannot spare. Accept 100 milliliters if the class data show scores at 100 close to those at 200, since that rinse would cover even more washes: 240,000 ÷ 150 = 1,600.Answer: Soap and scrubbing do most of the cleaning.
The no-soap group used the largest rinse and still scored worse than the soap groups, so the water alone did not remove the residue. Water that feels comfortable on the hands is not hot enough to kill germs, and water hot enough to kill them would burn the skin. Soap loosens oils and the germs stuck in them, and the rinse carries them away.
Differentiation
Support
Give these groups a class table with the means already calculated and a graph with labeled axes, so they begin by plotting points. For questions 5 and 6, provide a two-row table with columns for milliliters per wash and total milliliters, and remind students to change liters to milliliters first.
Extension
Have students add a sixth condition, such as a 10-second scrub with a 200-milliliter rinse, and decide whether scrub time or rinse volume matters more to the score. Ask them to rewrite the nurse's handwashing sign for the jug stations so it tells students exactly how long to scrub and how much to rinse.
ELL supports
Pre-teach "residue," "rinse," "lather," "control," and "trade-off" with photos of each step at a jug station. Provide sentence frames for questions 7 and 8: "I recommend ___ milliliters because ___." and "The soap matters more than hot water because ___."
Extensions
Students can find out how their own school would provide handwashing water during an outage and whether a plan already exists. A school nurse or a local water utility's emergency coordinator can be invited to describe how a boil-water notice or a main break changes what staff tell students about drinking fountains and sinks.