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Lesson plan Grade 8 Math

Sculpin Creek Oxygen and Temperature: Scatter Plots and Trend Lines

A 55-minute grade 8 lesson where students scatter-plot a fictional creek's dissolved oxygen against temperature, describe the association, and fit a trend line by eye.

Objectives

  1. Students will construct a scatter plot and describe its pattern of association.
  2. Students will informally fit a trend line and interpret its slope in context.

Vocabulary

Key words: scatter plot, association, line of best fit, slope

Teacher overview

Warm water holds less dissolved oxygen than cold water, so a creek's oxygen readings usually fall as temperature rises, which shows up as a downward scatter plot. This lesson has students plot a fictional creek's data, describe the negative association, and fit a trend line by eye to estimate where oxygen would drop below the level fish need. A common misconception this lesson addresses is drawing the trend line only through the first and last points; a good line of best fit balances all the points, with about as many above as below.

Procedure

Hook (5 min): Ask students what happens to a warm, flat soda compared with a cold, fizzy one, and connect it to the idea that warm water holds less gas. Tell them they will test whether a creek's oxygen follows the same pattern.

Direct instruction (12 min): Define a scatter plot and the kinds of association: positive, negative, linear, nonlinear, and note clustering and outliers. Using the fictional Sculpin Creek data, model plotting the first three points, with temperature on the x-axis and dissolved oxygen on the y-axis, and describe the early downward pattern.

Guided plotting (13 min): Students plot the remaining points from the handout table. Circulate and check axis labels and scale. Ask the class to describe the association they see so far, and confirm it looks negative and roughly linear.

Independent handout (15 min): Students complete the Student Handout: describe the association, draw a trend line, estimate its slope, and use the line to estimate a temperature.

Trend-line discussion (5 min): Have two students share their trend lines and slopes. Point out that reasonable lines differ slightly, and that a good line keeps the points close on both sides.

Closure and exit ticket (5 min): Students answer on a card: "As temperature goes up, does the dissolved oxygen in this creek go up or down?" Collect and check that students read the negative association correctly. If several miss it, review the direction of the trend before the next class.

Sculpin Creek Oxygen and Temperature: Scatter Plots and Trend Lines — Grade 8 Math
Name: ____________________________ Date: ______________

Student Handout

Sculpin Creek is a fictional creek. A monitoring team recorded its water temperature and dissolved oxygen on seven days. Dissolved oxygen is measured in milligrams per liter (mg/L). Fish in the creek need dissolved oxygen above 5 mg/L.

Temperature (degrees C) 10 13 16 19 22 25 28
Dissolved oxygen (mg/L) 11.0 10.2 9.3 8.5 7.6 6.8 5.9
  1. Plot the seven points with temperature on the x-axis and dissolved oxygen on the y-axis. Is the association positive or negative? Does it look close to linear?
  2. Are there any clear outliers, or do the points cluster along a line?
  3. Draw a straight line that fits the points as well as you can. Estimate its slope in mg/L for each 1-degree rise in temperature.
  4. Explain what that slope means for this creek.
  5. Using your line, estimate the temperature at which the dissolved oxygen would drop to 5 mg/L.
  6. Explain why warmer water usually holds less dissolved oxygen than cold water.

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: negative and roughly linear.
    As temperature rises, dissolved oxygen falls in a steady, nearly straight downward pattern, so the association is negative and close to linear.
  2. Answer: No clear outliers; the points cluster along a line.
    Every point sits close to the same downward trend, so there are no obvious outliers.
  3. Answer: about -0.3 mg/L per degree.
    A line that balances the points passes close to (10, 11.0) and (28, 5.9), so slope = (5.9 - 11.0) / (28 - 10) = -5.1 / 18, which is about -0.28. Any estimate from about -0.25 to -0.35 mg/L per degree is reasonable.
  4. Answer: Each 1-degree rise lowers oxygen by about 0.3 mg/L.
    The negative slope means that for every degree the water warms, the dissolved oxygen drops by roughly 0.3 mg/L.
  5. Answer: about 30 to 31 degrees C.
    Following the trend line down to 5 mg/L lands near 31 degrees. Using oxygen of about 13.8 at 0 degrees and a slope of -0.28, 5 = 13.8 - 0.28T gives T of about 31 degrees. Answers from about 30 to 32 are reasonable.
  6. Answer: Gases dissolve less easily in warm water.
    Warm water can hold less dissolved gas than cold water, so as the creek warms, less oxygen stays dissolved and the reading drops.

Differentiation

Support

Provide graph paper with the axes already labeled and scaled, and plot the first two points for students so they match the format. Give them a clear ruler and suggest they draw the trend line so about half the points sit above it.

Extension

Ask students to estimate the dissolved oxygen at a temperature not in the table, such as 30 degrees, using their trend line, and to explain why an estimate from a fitted line is less certain than a measured value.

ELL supports

Pre-teach "scatter plot," "trend," and "dissolved" with a labeled example graph and a picture of bubbles leaving warm water. Provide sentence frames such as "As temperature increases, oxygen ___," and "The slope means ___."

Extensions

If a local stream-monitoring group publishes temperature and oxygen data, have students plot a few of those real points and compare the trend with the fictional Sculpin Creek pattern, noting whether the real data are as tightly clustered.