Lab / experiment Grade 7 Health & Consumer Science
Kirkstall Park Splash Pad: A Water Turnover Dilution Test
Groups model a splash pad's water turnovers with food coloring, rate the fading color, and calculate why fresh water alone cannot clean up a contamination event.
Objectives
- Students will model repeated water turnovers and record how the color of a marked sample fades.
- Students will calculate the fraction of water replaced per turnover and the fraction of color remaining after two turnovers.
- Students will explain why an operator closes and disinfects a splash pad rather than waiting for dilution.
Vocabulary
- Disinfection
- Disinfection is the step that kills or disables germs in water so it is safe to drink. Water plants usually disinfect with chlorine, ultraviolet light, or ozone.
Also: turnover, dilution, recirculate, fraction
Teacher overview
A splash pad sends the same water past the same children all afternoon, so anything that enters the water reaches everyone who plays in it until the operator deals with it. This lab models the fictional Kirkstall Park splash pad in the town of Bramden as a pitcher of colored water, and students replace a set volume over and over to see how slowly the color fades. Teaching note: this lab targets the belief that moving water cleans itself, or that adding fresh water fixes contamination. Replacing a tenth of the water leaves nine tenths of the problem behind, and disinfection is what protects swimmers.
Procedure
- Fill your pitcher with 2,000 mL of water. This models the splash pad tank.
- Add 10 drops of blue food coloring and stir well. The color models material that entered the water in one event.
- Pour a 20 mL sample into the cup labeled 0 and set it on white paper. This is your starting color.
- Measure 200 mL out of the pitcher and pour it into the waste bucket. Add 200 mL of clean water and stir. That is one turnover.
- Pour a 20 mL sample into the cup labeled 1 and set it beside cup 0.
- Repeat the turnover four more times. After each one, take a 20 mL sample into the next numbered cup.
- Line the six cups up on white paper in order. Rate each cup from 5 for the darkest color down to 0 for no color you can see.
- Record every rating in your data table before you clean up.
- Safety: do not drink or taste any sample. Food coloring stains skin and clothing, so wear an apron and wipe spills right away.
Data & Analysis
Rate the color of each cup against the cup labeled 0, which is your darkest.
| Turnover | Color rating (0-5) | Fraction of the original color left (calculated) |
|---|---|---|
| 0 | 5 | 1.00 |
| 1 | ||
| 2 | ||
| 3 | ||
| 4 | ||
| 5 |
At which turnover did your color rating first drop to 2 or lower?
Each turnover removed 200 mL from a 2,000 mL pitcher and replaced it with clean water. What fraction of the water was replaced in one turnover?
Suppose one turnover leaves 0.9 of the color in the water. What fraction of the color is left after two turnovers?
The pitcher started with 10 drops of color. Using the fraction from question 3, about how many drops' worth of color remain after two turnovers?
Compare your color ratings with the calculated fractions. Did the color seem to fade faster or slower than the numbers say it should? Give one reason your eyes and the numbers might disagree.
A splash pad operator cannot wait for dilution after something contaminates the water. Use your data to explain why the operator closes the feature and disinfects instead.
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: Accept whichever turnover the student recorded, most often turnover 4 or 5. The rating should fall gradually rather than dropping to 0 in one step.
Answer: 0.1 of the water. 200 mL divided by 2,000 mL equals 0.1, which is one tenth, or 10 percent of the pitcher.
Answer: 0.81 of the color. After one turnover 0.9 is left, and 0.9 times 0.9 equals 0.81 after the second turnover.
Answer: 8.1 drops. 10 drops times 0.81 equals 8.1 drops, so about 8 drops' worth of color is still in the water.
Answer: Accept either answer with a reason. Most groups say the color seemed to fade faster than the numbers, because the eye compares each cup to the one next to it rather than to cup 0, and thin layers of liquid look lighter than deep ones.
Answer: Dilution removes only a small share each turnover, so after two turnovers about four fifths of the original material is still in the water and every child in the spray is still in contact with it. Closing the feature stops new exposure, and disinfection acts on the germs themselves instead of waiting for them to be carried out. Some germs also resist normal disinfectant levels, which is why a serious event can require draining rather than treating.
Differentiation
Support
Run three turnovers instead of five with these groups, and give them the fraction column already filled in so their work is the color rating and the comparison. A printed color strip showing five shades of blue makes the rating step concrete.
Extension
Ask students to predict how many turnovers would be needed to bring the color below one tenth of the original, then test the prediction by continuing the procedure and recording the turnover where it happens. Have them explain why the prediction is hard to make with ratings alone.
ELL supports
Pre-teach "turnover," "dilute," and "sample" by demonstrating one full turnover while narrating each step in short sentences. Provide the data table with the first row completed, and give a sentence frame for question 6: "The operator closes the splash pad because ___, and disinfects because ___."
Extensions
Groups can repeat the test using 400 mL per turnover instead of 200 mL and compare how many turnovers each version needs to reach the same color rating. A parks aquatics supervisor can be invited to describe what their facility actually does after a contamination event, including how long the feature stays closed and who decides when it reopens.