Water for Schools
Menu

Worksheet Grade 7 Science

Marren Falls Well Water: Hardness Testing and Percent Softening

A grade 7 science worksheet where students classify the hardness of two Marren Falls wells and calculate the percent of hardness a softener removes.

Objectives

  1. Students will classify water hardness values using a given hardness scale.
  2. Students will calculate the percent of hardness a softener removes from a water sample.
  3. Students will apply a known percent reduction to predict a second sample's softened hardness.

Vocabulary

Groundwater
Groundwater is water that has soaked into the ground and fills the spaces between soil particles and cracks in rock underground. It is a large underground source of fresh water that wells and springs draw from.

Also: hardness, softener, calcium, milligrams per liter

Teacher overview

Hard water carries dissolved minerals, mostly calcium and magnesium, that groundwater picks up as it moves through rock. Water utilities and homeowners measure hardness to decide whether to add a softener, because very hard water leaves scale inside pipes and water heaters. This worksheet uses two wells in one town so students classify hardness values and then work with percent removal, a common grade 7 percent task set in a real water context.

Marren Falls Well Water: Hardness Testing and Percent Softening — Grade 7 Science
Name: ____________________________ Date: ______________

Student Worksheet

The town of Marren Falls draws its water from two wells. Water hardness is reported in milligrams per liter (mg/L) of calcium carbonate. The town lab uses this hardness scale:

  • Soft: below 60 mg/L
  • Moderately hard: 60 to 120 mg/L
  • Hard: 120 to 180 mg/L
  • Very hard: above 180 mg/L

Test results:

  • Well A raw hardness: 250 mg/L
  • Well B raw hardness: 90 mg/L
  • A softener on Well A lowers its hardness from 250 mg/L to 40 mg/L.
  1. Use the scale to classify Well A's raw hardness of 250 mg/L. Write the category.
  2. Use the scale to classify Well B's raw hardness of 90 mg/L. Write the category.
  3. Well A's hardness of 250 mg/L is made of 160 mg/L from calcium and the rest from magnesium. How many mg/L of Well A's hardness comes from magnesium?
  4. The softener lowers Well A from 250 mg/L to 40 mg/L. What percent of Well A's hardness does the softener remove? Round to the nearest whole percent.
  5. How many times harder is Well A (250 mg/L) than Well B (90 mg/L)? Round to the nearest tenth.
  6. Suppose the same softener removes the same percent you found in item 4 from Well B. Starting at 90 mg/L, what would Well B's softened hardness be? Round to the nearest tenth of a mg/L.
  7. The town blends equal amounts of raw Well A water (250 mg/L) and raw Well B water (90 mg/L). Find the hardness of the blend by averaging the two values, then classify the blend using the scale.
  8. Marren Falls sits on a bed of limestone, a rock rich in calcium. Explain why groundwater from these wells is hard, and why a well drilled into a different kind of rock nearby might be softer.
  9. Explain one reason a homeowner might install a softener even though softening adds cost.

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: Very hard. 250 mg/L is above 180 mg/L, so it falls in the "very hard" category.
  2. Answer: Moderately hard. 90 mg/L is between 60 and 120 mg/L.
  3. Answer: 90 mg/L. Total hardness minus calcium hardness is 250 - 160 = 90 mg/L from magnesium.
  4. Answer: 84 percent. The softener removes 250 - 40 = 210 mg/L. Then 210 / 250 = 0.84, which is 84%.
  5. Answer: about 2.8 times. 250 / 90 = 2.777..., which rounds to 2.8 times as hard.
  6. Answer: 14.4 mg/L. Removing 84% leaves 16%, so 90 x 0.16 = 14.4 mg/L. (Keeping 90 - 0.84 x 90 gives the same result.)
  7. Answer: 170 mg/L, hard. The average is (250 + 90) / 2 = 170 mg/L, which is between 120 and 180 mg/L, so the blend is hard.
  8. Answer: Groundwater flowing through limestone dissolves calcium from the rock, which raises hardness. A well in sandstone or granite, which holds little calcium, would pick up less mineral and give softer water.
  9. Answer: Accept any reasonable answer, such as reducing scale buildup in pipes and water heaters, making soap lather better, or extending the life of appliances.

Differentiation

Support

Give students the hardness scale as a printed number line with the four categories shaded, so classifying items 1, 2, and 7 becomes a matter of locating a value. Work item 4 together as a model before students try items 5 and 6, and allow a calculator for the division steps.

Extension

Ask students to find the blend ratio that would bring the town of Marren Falls' mixed water to exactly 150 mg/L, using more Well B water than Well A water, and to show the calculation that supports their mix.

ELL supports

Pre-teach "hardness," "softener," "calcium," and "magnesium" with a simple picture of scale building up inside a pipe. Provide the sentence frame "Well ___ is ___ because its hardness is ___ mg/L" for the classifying items, and allow category names as short answers.