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Worksheet Grade 9 Science

Water Hardness Testing at Calderwood: A Grade 9 Titration Worksheet

A grade 9 worksheet where students turn drop-count titration readings into water hardness in mg/L as calcium carbonate, classify samples, and find percent removal.

Objectives

  1. Students will convert a drop-count titration reading into water hardness in mg/L as calcium carbonate.
  2. Students will classify water samples using a stated hardness scale.
  3. Students will calculate the percent of hardness removed by softening.

Vocabulary

Key words: hardness, titration, titrant, calcium carbonate, percent removal

Teacher overview

Hard water carries dissolved calcium and magnesium, which build scale in pipes and water heaters and make soap harder to lather. Utilities measure hardness with a simple titration: a technician adds drops of a titrant to a water sample until the color changes, then converts the volume used into hardness. This worksheet has students run those conversions for the fictional lab at Calderwood and decide how hard each sample is. A common error is forgetting that the drop-to-hardness rule only holds for a fixed sample size, so a changed sample volume changes the conversion.

Water Hardness Testing at Calderwood: A Grade 9 Titration Worksheet — Grade 9 Science
Name: ____________________________ Date: ______________

Student Worksheet

Calderwood (a fictional town) tests hardness with a drop-count titration kit. All samples are 50 milliliters (mL), and the kit's rule is:

  • Each 1.0 mL of titrant used equals 20 mg/L of hardness, reported as calcium carbonate.
  • Calderwood's lab sorts results with this scale (mg/L as calcium carbonate): soft = 0 to 60, moderately hard = 61 to 120, hard = 121 to 180, very hard = above 180.
  • The kit is read to the nearest 0.5 mL.
  1. A raw well-water sample takes 9.0 mL of titrant to change color. Calculate its hardness in mg/L as calcium carbonate.
  2. Use Calderwood's scale to classify the raw well water from item 1 as soft, moderately hard, hard, or very hard.
  3. After the water passes through the softening unit, a treated sample takes 2.5 mL of titrant. Calculate its hardness.
  4. Classify the treated water from item 3 using the same scale.
  5. Calculate the percent of hardness removed by softening, using percent removed = (raw hardness minus treated hardness) divided by raw hardness, times 100. Round to the nearest tenth.
  6. A second raw sample, from a different well, takes 6.5 mL of titrant. Calculate its hardness and classify it.
  7. Explain why the kit's rule "1.0 mL equals 20 mg/L" only works when every sample is exactly 50 mL. What would happen to the conversion if a technician used a 100 mL sample instead?
  8. A technician records a titrant reading as 9.03 mL, but the kit can only be read to the nearest 0.5 mL. Rewrite the reading at the kit's precision and explain why the extra digits are misleading.

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: 180 mg/L.
    9.0 mL x 20 mg/L per mL = 180 mg/L as calcium carbonate.

  2. Answer: Hard.
    180 mg/L falls in the 121-to-180 range, which the scale labels hard.

  3. Answer: 50 mg/L.
    2.5 mL x 20 mg/L per mL = 50 mg/L as calcium carbonate.

  4. Answer: Soft.
    50 mg/L falls in the 0-to-60 range, which the scale labels soft.

  5. Answer: 72.2 percent.
    (180 minus 50) divided by 180 = 130 divided by 180 = 0.7222, which rounds to 72.2 percent removed.

  6. Answer: 130 mg/L, hard.
    6.5 mL x 20 mg/L per mL = 130 mg/L, which falls in the 121-to-180 range, so it is classified hard.

  7. Answer: The rule is tied to the 50 mL sample size, so a larger sample changes it.
    The "20 mg/L per mL" factor already accounts for a 50 mL sample. With a 100 mL sample, the same amount of hardness would be spread through twice as much water, so each mL of titrant would stand for half as much concentration. The conversion factor would have to be cut in half to stay correct.

  8. Answer: 9.0 mL.
    The kit reads to the nearest 0.5 mL, so it cannot resolve hundredths. Reporting 9.03 mL claims a precision the kit does not have; the honest reading is 9.0 mL.

Differentiation

Support

Provide the hardness scale as a labeled number line with each category shaded, so classification is a matter of locating a value rather than recalling ranges. Work item 1 together, then let students reuse the same multiply-by-20 step for items 3 and 6.

Extension

Ask students to find the titrant volume that would place a 50 mL sample exactly at the boundary between moderately hard and hard, then explain how they worked backward from the scale to the volume.

ELL supports

Pre-teach "hardness," "titrant," and "calcium carbonate" with a simple picture of soap lathering poorly in hard water and well in soft water. Provide a sentence frame for item 7: "The rule works only at 50 mL because ___."