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Lesson plan Grade 6 Science

Road Salt in Merribeck Creek: A Winter Chloride Record

A grade 6 lesson where students summarize a fictional town's winter chloride readings, compare them with a creek guideline, and plan how to monitor and cut salt use.

Objectives

  1. Students will summarize a set of weekly chloride measurements using mean, median, and range.
  2. Students will compare measured chloride values with a stated creek guideline.
  3. Students will propose one monitoring method and one salt-reduction step for a town.

Vocabulary

Key words: chloride, dissolve, runoff, median, range, guideline

Teacher overview

Salt spread on winter roads does not stay on the roads. It dissolves in melting snow, runs into storm drains, and arrives in streams as chloride, which stays dissolved rather than settling out or breaking down. This lesson uses eight weeks of fictional chloride readings from Merribeck Creek in the fictional town of Talbridge so students can summarize a realistic seasonal data set and then decide what the town should do about it. The misconception worth naming here is that a pollutant you cannot see must have washed away; students often assume that clear water is clean water, while dissolved chloride is invisible and persistent, which is exactly why it has to be measured rather than judged by eye. All concentrations in this lesson are in mg/L.

Procedure

Hook (5 min): Stir a spoonful of table salt into a jar of water beside a jar of plain water and pass both around. Ask students which jar has salt in it and how they could tell without tasting. Steer the discussion toward measurement, and write "chloride" on the board.

Direct instruction (12 min): Explain that road salt dissolves into separate particles in water, and that the chloride part stays dissolved as the water moves. Settling tanks do not remove it, plants along the bank do not take it up in any large amount, and rain only dilutes it. Explain that towns therefore track chloride at a fixed spot in a creek across the winter, and that they manage it at the source by spreading less salt. Name two real practices: wetting the salt with brine before spreading so less bounces off the road, and calibrating spreaders so each truck releases a known amount per lane mile.

Model (8 min): Work one fictional example that is not on the handout. Four readings from a different creek are 40, 60, 100, and 200 mg/L. Add them to get 400, then divide by 4 to get a mean of 100 mg/L. Order them and take the middle pair, 60 and 100, whose average is a median of 80 mg/L. Subtract the smallest from the largest for a range of 160 mg/L. Leave all three worked steps on the board.

Guided and independent practice (20 min): Students complete the handout on the fictional Merribeck Creek data. Work item 1 together, then release students to finish in pairs. Circulate during item 2 and check that students order the readings before finding the median, which is the most common error in this set because the readings arrive in calendar order.

Closure and exit ticket (5 min): Each student answers one question on a card: "Talbridge spreads no salt at all during one cold, dry week in February when no snow falls and nothing melts. Would you expect that week's chloride reading to be higher or lower than the week before, and why?" Collect the cards and scan for answers that connect less salt on the road to less chloride in the creek. If more than a quarter of the class cannot make that link, reteach the runoff path before moving on.

Road Salt in Merribeck Creek: A Winter Chloride Record — Grade 6 Science
Name: ____________________________ Date: ______________

Student Handout

Merribeck Creek runs through Talbridge, a fictional town. The town measured chloride at the same spot in the creek once a week for eight weeks. All of these numbers are fictional.

Week Chloride (mg/L)
1 20
2 40
3 150
4 250
5 300
6 180
7 60
8 40

Talbridge uses its own creek guideline, written for this lesson: chloride should stay at or below 230 mg/L to protect the animals that live in the creek year round.

  1. Find the mean of the eight weekly readings.
  2. Find the median of the eight weekly readings.
  3. Find the range of the eight weekly readings.
  4. How many of the eight weeks were above the town guideline of 230 mg/L?
  5. The week 5 reading is how many mg/L above the guideline?
  6. Suppose Talbridge changes how it spreads salt and every reading drops by 20 percent. What would the new week 5 reading be?
  7. Would that new week 5 reading still be above the guideline?
  8. Explain why chloride from road salt does not settle to the bottom of the creek the way sand does after a storm.
  9. Name one way Talbridge could monitor chloride in Merribeck Creek through the winter.
  10. Name one change Talbridge could make so that less chloride reaches the creek in the first place.

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: 130 mg/L.
    The eight fictional readings add to 1,040 mg/L, and 1,040 divided by 8 equals 130 mg/L.
  2. Answer: 105 mg/L.
    In order the readings are 20, 40, 40, 60, 150, 180, 250, 300. The middle two are 60 and 150, and their average is 105 mg/L.
  3. Answer: 280 mg/L.
    The largest reading is 300 and the smallest is 20, so 300 minus 20 equals 280 mg/L.
  4. Answer: 2 weeks.
    Week 4 at 250 mg/L and week 5 at 300 mg/L are above 230 mg/L. No other week reaches it.
  5. Answer: 70 mg/L.
    300 minus 230 equals 70 mg/L above the guideline.
  6. Answer: 240 mg/L.
    A drop of 20 percent leaves 80 percent, and 0.80 times 300 equals 240 mg/L.
  7. Answer: Yes.
    240 mg/L is still above the 230 mg/L guideline, though by only 10 mg/L rather than 70 mg/L.
  8. Answer: Salt dissolves in water, so the chloride is spread evenly through the water instead of sitting in it as separate grains. Sand does not dissolve, so gravity pulls the grains down once the water slows. Accept any answer that contrasts a dissolved substance with a settling solid.
  9. Answer: Take a water sample at the same spot in the creek each week through the winter and have it tested for chloride. Accept a continuous sensor left in the creek, or sampling before and after each storm, as equally correct.
  10. Answer: Wet the salt with brine before spreading it so less bounces off the road. Accept calibrating the spreaders, plowing more before salting, or salting only the hills and intersections as equally correct.

Differentiation

Support

Give students the eight fictional readings already sorted from smallest to largest on a separate strip of paper, so ordering is not an obstacle to finding the median. Provide the mean as a labeled two-step recipe, add then divide by how many, and allow calculators throughout. Accept one-word answers of higher or lower on the exit ticket.

Extension

Ask students to graph the eight fictional readings as a line graph with weeks on the horizontal axis, draw the guideline as a horizontal line across the graph, and write two sentences about what the shape of the line suggests about when Talbridge spread the most salt.

ELL supports

Pre-teach "chloride," "dissolve," "runoff," and "guideline" using the two jars from the hook plus a simple drawing of a road, a storm drain, and a creek connected by arrows. Provide the frame "Week ___ was above the guideline because the reading was ___." Allow items 8 through 10 to be answered in one short sentence each or with a labeled drawing.

Extensions

Have students find out what their own town or county uses on winter roads by reading the public works page of its website, then write a short paragraph comparing that practice with the fictional changes Talbridge is considering. Classes in warm regions can substitute the salt used to soften water in home water softeners, which reaches wastewater plants and creeks by a different path.