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Lesson plan Grade 9 Health & Consumer Science

The Marwick Household Water Audit: A Grade 9 Home-Use Model Lesson

Students build and graph an equation for a fictional family's daily water use, then test which single change saves the most water at home.

Objectives

  1. Students will write a two-variable equation that relates shower gallons to shower length and graph it with labeled, scaled axes.
  2. Students will compare the daily water savings of three household changes using their own calculations.

Vocabulary

Water audit
A careful check of where water goes, either in a home or across a whole water system. Audits find leaks and waste so they can be fixed.
Flow rate
Flow rate measures how much water moves past a point in a certain amount of time, like gallons per minute through a garden hose. Bigger pipes and stronger pushes give bigger flow rates.

Also: gallons per minute, conservation

Teacher overview

A home water audit turns a monthly bill into something a student can model with algebra. This lesson follows the fictional Marwick family as they measure a few fixtures and estimate their daily use, then decide which change would save the most water. A common misconception is that shortening a shower always saves more than any other fix; the numbers in this lesson show that a running toilet can waste more per day than a family saves by switching every shower to a low-flow head, so the comparison has to be made with data rather than intuition. The average US household uses about 300 gallons per day, which gives students a benchmark to check their estimate against.

Procedure

Hook (5 min): Ask students to estimate, in writing, how many gallons their own morning shower uses. Collect a few guesses on the board without judging them. Tell them that by the end of class they will be able to compute it from two numbers: the flow rate and the time.

Direct instruction (12 min): Define flow rate as gallons per minute (gpm) and show the fictional Marwick showerhead measured at 2.5 gpm. Model writing the equation for shower gallons: g equals 2.5 times t, where t is minutes. Graph it on labeled, scaled axes (minutes on the x-axis, gallons on the y-axis) and read a value off the line, for example a 9-minute shower at 22.5 gallons. State plainly that the slope is the flow rate.

Guided practice (10 min): Work item 4 of the Student Handout together as a class, switching the Marwicks to a WaterSense head at 2.0 gpm and rewriting the equation as g equals 2.0 times t. Have students graph the second line on the same axes and describe, in one sentence, what the smaller slope means for the family.

Independent practice (15 min): Students complete the remaining Student Handout items on their own or in pairs, comparing shower savings against a fixed toilet-leak figure and against their household benchmark. Circulate and check that units stay consistent from step to step.

Formative check and closure (8 min): As an exit ticket, students answer one question on a slip: "Which single change saves the Marwicks the most water per day, and what is your evidence?" Collect the slips and scan for whether students compared the leak figure against the shower savings rather than guessing. If more than a third choose shorter showers without citing the leak number, reteach item 10 the next day.

The Marwick Household Water Audit: A Grade 9 Home-Use Model Lesson — Grade 9 Health & Consumer Science
Name: ____________________________ Date: ______________

Student Handout

Suppose the Marwick family of four is auditing its water use. Their current showerhead flows at 2.5 gallons per minute, and each person takes one shower a day. Use these fictional figures for every item, and show the units in each step.

  1. Write an equation for the gallons of water, g, used in a shower that lasts t minutes with the current 2.5 gpm head.
  2. Using your equation, how many gallons does one 9-minute shower use?
  3. All four family members each take one 9-minute shower per day. How many shower gallons does the household use in one day?
  4. The Marwicks switch to a WaterSense showerhead that flows at 2.0 gpm. How many gallons does one 9-minute shower use now?
  5. How many gallons per day does the whole family save on showers by switching to the 2.0 gpm head, if everyone still showers 9 minutes?
  6. Over a 30-day month, how many gallons does the family save on showers with the new head?
  7. A silently running toilet in the Marwick home wastes about 9,400 gallons in a year. About how many gallons per day is that? Round to the nearest gallon.
  8. The Marwicks estimate their total household use at 380 gallons per day. The average US household uses about 300 gallons per day. How many gallons above the average is the Marwick estimate?
  9. What percent of the average household's daily use is the Marwicks' 380 gallons per day? Round to the nearest whole percent.
  10. Explain, using your numbers from items 5 and 7, which single change saves more water per day: switching every shower to the 2.0 gpm head, or fixing the running toilet.

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: g = 2.5t gallons. The flow rate of 2.5 gallons per minute is the constant multiplied by the number of minutes, t.
  2. Answer: 22.5 gallons. g = 2.5 times 9 = 22.5 gallons for a 9-minute shower.
  3. Answer: 90 gallons. Each 9-minute shower is 22.5 gallons, and 4 people times 22.5 = 90 gallons per day.
  4. Answer: 18 gallons. With the 2.0 gpm head, g = 2.0 times 9 = 18 gallons for one 9-minute shower.
  5. Answer: 18 gallons per day. Each shower now saves 22.5 minus 18 = 4.5 gallons, and 4 showers times 4.5 = 18 gallons per day.
  6. Answer: 540 gallons. 18 gallons saved per day times 30 days = 540 gallons in the month.
  7. Answer: 26 gallons per day. 9,400 gallons per year divided by 365 days is about 25.75, which rounds to 26 gallons per day.
  8. Answer: 80 gallons. 380 minus 300 = 80 gallons per day above the average household.
  9. Answer: 127 percent. 380 divided by 300 = 1.2667, which is about 127 percent of the average.
  10. Answer: Fixing the running toilet. It wastes about 26 gallons per day, which is more than the 18 gallons per day the family saves by putting the low-flow head on all four showers, so the leak repair is the bigger daily saving.

Differentiation

Support

Give students a partly worked handout with item 1 already solved and the axis labels pre-printed on the graph. Allow calculators for every step, and provide a units checklist (gpm times minutes equals gallons) taped to the desk so students can confirm each line.

Extension

Ask students to write a single two-variable equation for total daily household water that combines showers and the toilet leak, then use it to find how many shower minutes per person would push the family back down to the 300-gallon average.

ELL supports

Pre-teach "flow rate," "gallons per minute," and "audit" with a short demonstration: run a measuring jug under a tap for ten seconds and compute the rate together. Provide a sentence frame for item 10: "The bigger daily saving is ___ because it saves about ___ gallons per day, which is more than ___."