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Worksheet Grade 6 Health & Consumer Science

The Alderpine Community Pool Saturday Chemical Log

A grade 6 worksheet on a fictional pool's four daily chlorine and pH readings, covering mean, range, percent change, ratios, and why a chlorine smell misleads.

Objectives

  1. Students will compare logged readings against a posted operating range.
  2. Students will find the mean, range, and percent change of a set of four readings.
  3. Students will explain why a strong pool smell does not mean a high chlorine reading.

Vocabulary

pH
pH is a number that tells whether water is acidic, like lemon juice, or basic, like baking soda mixed in water. Pure water sits in the middle of the scale at a pH of 7.

Also: free chlorine, combined chlorine, operating range

Teacher overview

Public pools post a chemical log because chlorine gets used up as swimmers enter the water, and a reading that drifts below the posted band is the first sign that the pool needs attention. This worksheet gives students one fictional Saturday of readings from the Alderpine Community Pool and asks them to summarize it the way an operator would, with a mean, a range, and a percent change. The last two items target a common misconception, that a strong chlorine smell means a pool has too much chlorine. The smell comes from combined chlorine, which forms as free chlorine is consumed, so the strongest smell often sits alongside the lowest reading of the day.

The Alderpine Community Pool Saturday Chemical Log — Grade 6 Health & Consumer Science
Name: ____________________________ Date: ______________

Student Worksheet

The Alderpine Community Pool is a fictional town pool. A sign by the gate posts the operating ranges the staff must hold: free chlorine between 1.0 mg/L and 3.0 mg/L, and pH between 7.2 and 7.8. A lifeguard tests and logs the water four times a day.

When free chlorine does its job on sweat, sunscreen, and body oils, it turns into combined chlorine. Combined chlorine is what people smell. A pool that smells strongly usually has a lot of combined chlorine and not much free chlorine left.

Saturday log

Time Free chlorine (mg/L) pH Swimmers in the pool
8:00 a.m. 2.8 7.4 6
11:00 a.m. 2.2 7.5 34
2:00 p.m. 1.4 7.7 60
5:00 p.m. 0.8 7.9 48

Show your work for every item.

  1. How many of the four free chlorine readings fall inside the posted range of 1.0 mg/L to 3.0 mg/L?
  2. The 5:00 p.m. free chlorine reading is 0.8 mg/L. How far below the bottom of the posted range of 1.0 mg/L is it?
  3. Find the mean of the four free chlorine readings: 2.8, 2.2, 1.4, and 0.8 mg/L.
  4. Find the range of those same four readings by subtracting the smallest from the largest.
  5. Free chlorine fell from 2.8 mg/L at 8:00 a.m. to 0.8 mg/L at 5:00 p.m. What percent of the morning reading was lost during the day?
  6. There were 6 swimmers at 8:00 a.m. and 60 swimmers at 2:00 p.m. How many times as many swimmers were in the pool at 2:00 p.m.?
  7. Write the ratio of swimmers at 11:00 a.m. to swimmers at 5:00 p.m. in simplest form. Use the counts 34 and 48.
  8. The posted pH range is 7.2 to 7.8. Which one logged pH reading falls outside that range?
  9. By how much does that pH reading sit above the top of the posted range of 7.8?
  10. At 5:00 p.m. the operator adds chlorine and the free chlorine reading rises from 0.8 mg/L to 2.0 mg/L. How much did the reading increase?
  11. Look at the swimmer counts beside the free chlorine readings across the four logged times. Describe the pattern you see and explain why more swimmers would change the chlorine reading.
  12. A swimmer arrives at 5:00 p.m., smells chlorine strongly, and says the staff must have added too much. Use the log and the note above the table to explain why that smell is not evidence of a high free chlorine reading.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 3 readings.
    The readings of 2.8, 2.2, and 1.4 mg/L all sit between 1.0 and 3.0 mg/L. The 5:00 p.m. reading of 0.8 mg/L falls below the bottom of the posted range.
  2. Answer: 0.2 mg/L.
    Subtract 0.8 mg/L from the 1.0 mg/L bottom of the range: 1.0 minus 0.8 equals 0.2 mg/L below the range.
  3. Answer: 1.8 mg/L.
    Add the four readings: 2.8 plus 2.2 plus 1.4 plus 0.8 equals 7.2. Divide by the 4 observations: 7.2 divided by 4 equals 1.8 mg/L.
  4. Answer: 2.0 mg/L.
    The largest reading is 2.8 mg/L and the smallest is 0.8 mg/L, so the range is 2.8 minus 0.8, which equals 2.0 mg/L.
  5. Answer: about 71 percent.
    The drop is 2.8 minus 0.8, which is 2.0 mg/L. Divide the drop by the morning reading: 2.0 divided by 2.8 equals about 0.714, or about 71 percent of the morning reading.
  6. Answer: 10 times.
    Divide the 2:00 p.m. count by the 8:00 a.m. count: 60 divided by 6 equals 10, so ten times as many swimmers were in the pool.
  7. Answer: 17 to 24.
    The counts are 34 and 48. Both divide by 2, giving 17 to 24. Nothing divides both 17 and 24, so 17 to 24 is simplest form.
  8. Answer: 7.9.
    The 5:00 p.m. pH reading of 7.9 is the only one outside the posted band. The readings of 7.4, 7.5, and 7.7 all sit between 7.2 and 7.8.
  9. Answer: 0.1 pH unit.
    Subtract the top of the posted range from the reading: 7.9 minus 7.8 equals 0.1.
  10. Answer: 1.2 mg/L.
    Subtract the reading before the addition from the reading after it: 2.0 minus 0.8 equals 1.2 mg/L.
  11. Answer: Free chlorine fell as the swimmer count rose.
    Accept any answer describing the inverse pattern in the log, that the chlorine readings dropped from 2.8 to 0.8 mg/L while swimmers rose from 6 to 60 and then eased to 48. The explanation should say that free chlorine is used up as it reacts with what swimmers bring into the water, so a busier pool consumes it faster.
  12. Answer: The smell comes from combined chlorine, which forms as free chlorine is used up.
    Accept any answer that connects the note above the table to the 5:00 p.m. row. The log shows the lowest free chlorine reading of the day at that hour, 0.8 mg/L, so the strong smell points to a pool that needs more chlorine rather than one that has too much.

Differentiation

Support

Give these students a version of the fictional log with the posted range drawn as a shaded band across the chlorine column, so items 1 and 2 become a matter of reading which values fall outside the shading. Provide the mean and range calculations with the addition already written out.

Extension

Ask students to write the 8:00 p.m. row they would expect if 20 more swimmers entered after 5:00 p.m. and no chlorine were added, then justify each number they chose using the pattern in the fictional log.

ELL supports

Pre-teach "free chlorine," "combined chlorine," "range," and "log" with a labeled number line showing the posted band shaded and each Saturday reading marked on it. Provide the sentence frame: "The reading at ___ was ___, which is ___ the posted range."

Extensions

Have students find the pool rules posted at a local recreation center and compare which readings that facility records with the four columns in this log, then write two sentences on what a swimmer could tell from the posted sheet.