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Worksheet Grade 8 Science

Slagford Creek Mine Drainage: pH, Iron, and a Limestone Bed

A grade 8 science worksheet using powers of ten to compare acid strength above and below a fictional limestone treatment bed on a mine drainage stream.

Objectives

  1. Students will express hydrogen-ion amounts as a single digit times an integer power of ten.
  2. Students will state how many times as much hydrogen ion one water sample holds compared with another.
  3. Students will calculate iron removal and the daily limestone use of a passive treatment bed.

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: mine drainage, hydrogen ion, limestone bed, removal efficiency, power of ten

Teacher overview

Water that drains out of an abandoned coal mine is often acidic and stained orange, because sulfur minerals exposed to air and water produce acid, and that acid dissolves iron out of the rock. One low-cost fix is a bed of crushed limestone that the water flows through slowly, since limestone neutralizes acid and lets the dissolved iron settle out as a solid. This worksheet uses a fictional monitoring record from one such bed, and it treats pH as what it is, a count of powers of ten. Students often read pH 3 and pH 6 as "twice as acidic," when the water at pH 3 actually holds a thousand times as much hydrogen ion.

Slagford Creek Mine Drainage: pH, Iron, and a Limestone Bed — Grade 8 Science
Name: ____________________________ Date: ______________

Student Worksheet

The fictional Blackdamp Mine closed sixty years ago above the town of Colliery Bend. Water still runs out of the mine opening into Slagford Creek. The fictional Slagford Valley Conservancy built a limestone bed to treat it.

Use this rule for pH. Water at pH 7 holds 1 x 10^-7 moles of hydrogen ion in each liter. Each drop of one pH unit means ten times as much hydrogen ion. So pH 6 water holds 1 x 10^-6 moles per liter, and pH 5 water holds 1 x 10^-5 moles per liter.

Sampling site pH Iron (mg/L)
Mine opening 3 42
Just above the limestone bed 4 30
Just below the limestone bed 6 4
Slagford Creek upstream of the mine 7 0.2

The mine opening discharges 90 gallons per minute. The limestone bed uses 0.5 pounds of crushed limestone for every 1,000 gallons it treats.

  1. Water at the mine opening has a pH of 3. Write its hydrogen-ion amount as a single digit times a power of ten, in moles per liter.
  2. Creek water upstream of the mine has a pH of 7. How many times as much hydrogen ion does the mine water hold as the upstream creek water?
  3. Water just below the limestone bed has a pH of 6. How many times as much hydrogen ion does the mine water hold as the water below the bed?
  4. How many milligrams per liter of iron were removed between the mine opening and the point just below the bed?
  5. Removal efficiency is the amount removed divided by the amount that came in, times 100. Find the iron removal efficiency between the mine opening and the point below the bed. Round to the nearest tenth of a percent.
  6. The mine opening discharges 90 gallons per minute. How many gallons flow out in one full day?
  7. The bed uses 0.5 pounds of limestone for every 1,000 gallons treated. How many pounds does it use in one day?
  8. Write your answer to item 6 as a single digit times a power of ten, rounded to one digit.
  9. Explain what the limestone bed does to the water, and why the iron amount drops after the water passes through it.
  10. The Conservancy samples both above and below the bed every month. Explain why sampling in both places tells more than sampling in one, and name one result that would tell the crew the bed needs new limestone.

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: 1 x 10^-3 mol/L.
    Water at pH 7 holds 1 x 10^-7 moles per liter. Going from pH 7 down to pH 3 is four steps, and each step multiplies by ten, so the exponent rises from negative 7 to negative 3.
  2. Answer: 10,000 times.
    Dividing 1 x 10^-3 by 1 x 10^-7 subtracts the exponents, giving 10^4, which is 10,000. The four pH units between the samples are four factors of ten.
  3. Answer: 1,000 times.
    Dividing 1 x 10^-3 by 1 x 10^-6 gives 10^3, which is 1,000. This is the item that corrects the idea that pH 3 is about twice as acidic as pH 6.
  4. Answer: 38 mg/L.
    Subtracting the iron below the bed from the iron at the mine opening gives 42 minus 4, which equals 38 milligrams per liter.
  5. Answer: 90.5 percent.
    Dividing the amount removed by the amount that came in gives 38 divided by 42, which is 0.90476. Multiplying by 100 gives 90.476, which rounds to 90.5 percent.
  6. Answer: 129,600 gallons per day.
    In one hour the mine discharges 90 times 60, or 5,400 gallons. In 24 hours that is 5,400 times 24, which equals 129,600 gallons.
  7. Answer: 64.8 pounds per day.
    Dividing the daily flow by 1,000 gives 129,600 divided by 1,000, or 129.6 thousand-gallon units. Multiplying by 0.5 pounds gives 64.8 pounds of limestone per day.
  8. Answer: 1 x 10^5 gallons.
    The daily discharge of 129,600 gallons lies between 100,000 and 1,000,000. Rounded to a single digit times a power of ten it is 1 x 10^5 gallons, which is the estimate an operator would use for planning.
  9. Answer: The limestone neutralizes the acid, which raises the pH and lets the dissolved iron settle out.
    Crushed limestone reacts with the acid in the water, so the hydrogen-ion amount falls and the pH climbs from 3 toward 6. Iron stays dissolved in strongly acidic water. Once the water is closer to neutral, the iron combines with oxygen and forms an orange solid that settles in the pond below the bed, so the amount measured in the water drops.
  10. Answer: Sampling above and below shows how much of the change the bed itself caused.
    A single sample below the bed cannot separate the bed's effect from a change in the mine water. Two samples taken the same day give a before and after pair, so the crew can calculate removal for that day. A result showing the pH below the bed falling toward the pH above it, or the iron removal across the bed (now about 87 percent, from 30 mg/L above the bed to 4 mg/L below it) dropping well below that level, would mean the limestone has been coated or used up and needs replacing.

Differentiation

Support

Give these students a completed first row for items 1 through 3, showing the mine opening concentration already written as 1 x 10^-3, and a number line of powers of ten from 10^-7 to 10^-1 to count along. For items 6 and 7, provide the two conversion steps as labeled blanks: gallons per hour, then gallons per day.

Extension

Suppose the Conservancy doubles the size of the bed and the water leaves at pH 7 instead of pH 6. Ask students to state how many times as much hydrogen ion the current treated water holds compared with that improved result, using the wording of items 2 and 3, and to explain why doubling the bed does not double the pH number.

ELL supports

Pre-teach "drainage," "neutralize," "dissolved," and "efficiency" using a labeled side view of the mine opening, the limestone bed, and the settling pond. Provide the sentence frame "The water at pH ___ holds ___ times as much hydrogen ion as the water at pH ___" for items 2 and 3, and a two-column chart headed "before the bed" and "after the bed" for item 9.

Extensions

Students design a simple test of the same idea using a fictional stream and common materials, describing how they would compare the pH of vinegar-treated water before and after it passes through crushed antacid tablets. The write-up should name the variable being tested, what stays the same, and how many measurements they would take.