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Lesson plan Grade 6 Science

Coldstone Springs Well Hardness: The Limestone Underneath

A grade 6 lesson where students compare hardness data from a fictional district's limestone and granite wells and explain how water dissolving rock creates both.

Objectives

  1. Students will calculate and compare mean hardness values for wells in two rock types.
  2. Students will find the hardness of a blend made from two wells in a stated ratio.
  3. Students will explain how water dissolving limestone produces both hard water and surface features.

Vocabulary

Sinkhole
A sinkhole is a hole or depression in the ground that forms, sometimes suddenly, when underground rock dissolves or a cave roof collapses. Sinkholes are common in areas with karst topography, where rock such as limestone slowly dissolves underground.

Also: hardness, limestone, granite, dissolve, weathering

Teacher overview

Rain picks up carbon dioxide from the air and the soil, which makes it slightly acidic, and that weak acid slowly dissolves limestone. Groundwater that has traveled through limestone therefore carries dissolved calcium and magnesium, which water utilities report as hardness. This lesson uses the fictional Coldstone Springs Water District, whose five wells sit in two different kinds of bedrock, so students can see the pattern in hardness data before they explain it. A common misconception is that hard water is dirty water or that hardness means the water is unsafe; hardness is dissolved rock, and its main effects are scale inside pipes and kettles and poor lathering with soap. The fictional hardness values here are given in mg/L.

Procedure

Hook (5 min): Put a pinch of Epsom salt in one cup of water and leave the second cup plain. Add one drop of castile soap to each, stir, and have students compare the suds. Ask what could be in the first cup that changed the result, and record the guesses without correcting them.

Direct instruction (12 min): Explain that rainwater takes in carbon dioxide and becomes a weak acid, and that this weak acid dissolves limestone but has almost no effect on granite. Water that moves through cracks in limestone therefore leaves with dissolved minerals in it, which is what utilities measure as hardness. Show a simple cross section with two well casings, one reaching limestone and one reaching granite. Note that the dissolving works in both directions: the water gains minerals, and the rock loses them, which widens the cracks over thousands of years into caves, springs, and sinkholes at the surface.

Model (8 min): Work one fictional example that is not on the handout. Two wells in the same rock read 100 mg/L and 140 mg/L. Add them and divide by 2 to get a mean of 120 mg/L. Then compare that mean with a granite well reading 30 mg/L by dividing: 120 divided by 30 equals 4, so the limestone mean is 4 times the granite reading. Write both steps on the board and leave them up.

Guided and independent practice (20 min): Students complete the handout on the fictional Coldstone Springs wells. Work items 1 and 2 together, then release students to finish in pairs. Circulate during item 6 and check that students weight the blend by parts rather than simply averaging the two well values, which is the most common error here.

Closure and exit ticket (5 min): Each student writes one sentence on a card: "A new well in Coldstone Springs reads 260 mg/L. What kind of rock is it most likely drilled into, and why?" Collect the cards. If more than a quarter of the class cannot connect a high reading to limestone, review the cross section before the next class.

Coldstone Springs Well Hardness: The Limestone Underneath — Grade 6 Science
Name: ____________________________ Date: ______________

Student Handout

The Coldstone Springs Water District is fictional. It has five wells. Use the table and the district scale below for every item.

Well Bedrock Hardness (mg/L)
Bellamy limestone 240
Ridgeway limestone 180
Cutter granite 40
Fielding granite 60
Larkin limestone 300

The district sorts its water with this scale, written for this lesson:

  • Soft: below 60 mg/L
  • Moderate: 60 to 119 mg/L
  • Hard: 120 to 179 mg/L
  • Very hard: 180 mg/L and above
  1. Find the mean hardness of the three limestone wells.
  2. Find the mean hardness of the two granite wells.
  3. How many times greater is the limestone mean than the granite mean?
  4. Which category on the district scale does the Cutter well fall into?
  5. The Larkin well reads 300 mg/L. How many mg/L above the 180 mg/L cutoff for very hard is that?
  6. The district plans to blend 4 parts of Cutter water with 1 part of Larkin water. What is the hardness of the blend?
  7. Which category on the district scale does that blend fall into?
  8. Using the data in the table as evidence, explain why the limestone wells give harder water than the granite wells.
  9. The same dissolving that adds minerals to the water also removes rock underground. Explain what the land above a limestone area may look like after many thousands of years of this.

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: 240 mg/L.
    The limestone wells read 240, 180, and 300 mg/L. Their sum is 720, and 720 divided by 3 equals 240 mg/L.
  2. Answer: 50 mg/L.
    The granite wells read 40 and 60 mg/L. Their sum is 100, and 100 divided by 2 equals 50 mg/L.
  3. Answer: 4.8 times.
    240 divided by 50 equals 4.8, so the limestone mean is 4.8 times the granite mean.
  4. Answer: Soft.
    Cutter reads 40 mg/L, which is below the 60 mg/L line, so the district calls it soft.
  5. Answer: 120 mg/L.
    300 minus 180 equals 120 mg/L above the cutoff.
  6. Answer: 92 mg/L.
    The blend has 5 parts in all. Four parts at 40 mg/L contribute 160, and one part at 300 mg/L contributes 300. The total is 460, and 460 divided by 5 equals 92 mg/L.
  7. Answer: Moderate.
    92 mg/L falls between 60 and 119 mg/L, so the blend lands in the moderate category.
  8. Answer: Water that moves through limestone dissolves some of the rock and carries those minerals along, so wells drilled into limestone bring up water with more dissolved minerals in it. Granite barely dissolves, so the granite wells stay near 40 and 60 mg/L while all three limestone wells sit at 180 mg/L or above. Accept any answer that uses the table to link the rock type to the higher readings.
  9. Answer: Rock that dissolves away leaves open spaces underground, so over thousands of years the cracks widen into caves and channels, springs appear where the water comes back out, and the ground above a large opening can sag or collapse into a sinkhole. Accept any answer naming caves, springs, or sinkholes as the result of dissolved rock.

Differentiation

Support

Give students a version of the table with the limestone and granite wells already sorted into two groups and with the addition for item 1 written out. Provide a number line marked with the four category cutoffs so students can place each fictional reading by eye for items 4 and 7. Allow calculators throughout.

Extension

Ask students to find the blend ratio of Cutter water to Larkin water that would land closest to 100 mg/L, testing at least three ratios and showing the arithmetic for each. Have them explain why adding more Cutter water always lowers the result.

ELL supports

Pre-teach "hardness," "dissolve," "limestone," and "granite" with two labeled photographs and the soap demonstration from the hook. Provide the frame "Water that moves through ___ picks up more minerals, so the hardness is ___." Allow items 8 and 9 to be answered with a labeled drawing of the two wells plus one sentence.

Extensions

Have students look at their own home water utility's annual water quality report, which most systems publish online, and find whether hardness is listed. Because hardness is not a health standard, some reports leave it out, and that absence is worth discussing: students can write one paragraph on why a utility might report a number that no rule requires.