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Lab / experiment Grade 8 Science

Redbank Creek Jar Test: Finding the Best Coagulant Dose

A grade 8 science lab where students run a jar test, dosing muddy water with alum, rate each jar's clarity, and find the coagulant dose that clears it best.

Objectives

  1. Students will run a jar test, adding increasing coagulant doses to muddy water and rating each jar's clarity.
  2. Students will analyze their clarity data to identify the dose that produced the clearest water.
  3. Students will explain why treatment plants add a coagulant and how their results compare to a real turbidity standard.

Vocabulary

Coagulation
Coagulation is the first cleaning step at a drinking water plant. Operators quickly mix in a chemical that makes tiny bits of dirt stop pushing each other away so they can start sticking together.
Floc
Floc is the fluffy clumps of dirt and treatment chemicals that form in the water during cleaning at a treatment plant. Floc looks like tiny snowflakes drifting in the water.
Turbidity
Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.
Jar test
A jar test is a small-scale experiment operators run in the lab before treating a whole tank of water, testing different amounts of chemical in separate jars to see which dose clears the water best. It helps operators avoid wasting chemicals or under-treating the water.

Also: dose

Teacher overview

Before a treatment plant filters river water, it adds a coagulant such as alum, which makes tiny clay particles stick together into clumps called floc that are heavy enough to settle out. Deciding how much coagulant to add is a real operator problem, and plants find the answer with a jar test. In this lab, groups model the muddy water of the fictional Redbank Creek and run their own jar test to find the dose that clears the water best. Real plants must reach a turbidity of 0.3 NTU in nearly all samples, far clearer than a classroom can measure, so students use a simple 1-to-5 clarity scale to compare their jars. A common misconception is that more coagulant always means clearer water; students often find a best dose beyond which extra alum stops helping.

Redbank Creek Jar Test: Finding the Best Coagulant Dose — Grade 8 Science
Name: ____________________________ Date: ______________

Procedure

Prepare the source water. Make a muddy water batch by stirring soil into water until it is evenly cloudy; this is your fictional Redbank Creek sample. Fill five clear jars with 200 mL of the muddy water each. Keep the batch stirred so every jar starts about equally cloudy.

Set the doses. Label the jars 0, 1, 2, 3, and 4 for the number of coagulant doses each will receive, where one dose is a set number of drops of alum solution (or a small measured scoop of alum). Jar 0 gets no alum and is your control.

Add coagulant and mix. Put on goggles. Add the labeled number of alum doses to each jar. Stir each jar quickly for 1 minute to spread the coagulant, then slowly for 2 minutes to help floc form. Do not taste or drink any water in this lab.

Let the jars settle. Let all five jars sit without moving for 10 minutes. Watch for floc clumps forming and sinking toward the bottom.

Rate the clarity. After settling, rate each jar's clarity from 1 to 5 using the comparison card, where 1 is very cloudy and 5 is clear. Record each rating in the data table. Wash hands after cleanup.

Data & Analysis

Record your clarity ratings for each jar.

Jar (alum doses) Clarity rating after settling (1 to 5)
0 (control)
1
2
3
4
  1. Which jar (which dose) produced the clearest water, and what clarity rating did it earn?
  2. Compare the control jar (0 doses) to your clearest jar. By how many clarity points did the coagulant improve the water?
  3. Did adding more alum always improve clarity, or was there a best dose after which clarity stopped improving or got worse? Use your data to answer.
  4. Real plants must reach 0.3 NTU, a much clearer reading than your best jar. Could settling alone in your best jar meet that standard? Why or why not?
  5. Coagulation makes tiny particles clump into floc that settles. Using your results, explain why a plant adds a coagulant instead of just letting muddy water sit.
  6. If you could combine the best features of two of your jars, what dose and settling time would you test next to try for even clearer water? Justify your choice with your data.

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: The jar with the highest clarity rating.
    Accept the jar the group's data shows as clearest, with its recorded 1-to-5 rating. In most runs a middle dose (jar 2 or 3) rates highest, though the exact jar depends on the alum strength used.
  2. Answer: About 3 clarity points.
    Accept any value equal to the group's clearest rating minus the control rating; a typical improvement is about 3 points (for example, from a 1 or 2 up to a 4 or 5). The answer must match the student's own data.
  3. Answer: There is usually a best dose, not "more is always better."
    Accept answers showing that clarity improved up to a point and then leveled off or worsened, since too much alum can leave fine particles suspended. The claim must be supported by the group's recorded ratings.
  4. Answer: No.
    Settling alone cannot reach 0.3 NTU, because a real plant follows coagulation and settling with fine filtration and continuous testing; the jar test only removes the larger floc that settles, not the microscopic particles that make up a true turbidity reading.
  5. Answer: Coagulant makes particles clump and settle faster.
    Without a coagulant, the tiny clay particles are too light to settle in any reasonable time, so the water stays cloudy; the coagulant joins them into heavier floc that drops out, which is why the dosed jars cleared far more than the control.
  6. Answer: A dose near the best jar with a longer settling time.
    Accept any plan that builds on the data, such as testing a dose between the two best jars and allowing a longer settling time, as long as the student justifies the choice with the clarity results they recorded.

Differentiation

Support (below grade level)

Reduce the test to three jars (0, 2, and 4 doses) so the pattern is easier to see, and pre-make the muddy water batch for each group so class time goes to dosing and observing rather than setup.

Extension (above grade level)

Have students test a second variable, such as stirring time or water temperature, at their best dose, and write a short comparison of which change produced the clearer result and why.

ELL supports

Provide picture steps for the procedure (mix, dose, stir, settle, rate) and a labeled clarity card. Allow students to record results using the 1-to-5 numbers and simple phrases rather than full sentences, and pair each group with a partner for the analysis questions.

Extensions

Have students graph dose on the horizontal axis and clarity rating on the vertical axis, describe the shape of the curve they get, and mark the dose where adding more alum stopped helping, then connect that point to how an operator would choose a dose at a real plant.