CORNERSTONE -- ASSESSMENT: Removing Contaminants & Balancing pH of a Water Sample

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SWBAT remove contaminants (large organic molecules and/or metallic ions) and neutralize their unknown water samples in a cost and time efficient way.

Big Idea

We can apply chemistry concepts to solving real world problems, taking into consideration cost and time limits.

Why This Lesson?

Today students will clean and neutralize an unknown water sample.  This is a hands-on performance assessment that synthesizes all of the learning goals from several prior lessons.  Students will be not only judged on how well they clean and neutralize their sample, but also how quickly they finish and how few supplies they use.  I researched to get an accurate cost for each material available that students could choose to "purchase" in order to add to the real-world feel.  In reality, time limitations are also a consideration in determining success, so I also recorded finish time on assessments as they are turned in.  Students have bonus points available for spending low amounts of money on materials and for spending less time.

Students have already gained a background in how water quality is measured and will have participated in testing real samples from the community in two prior lessons: Water Quality: How is it Measured? and Water Quality: Testing Samples.  Then, students used nanoparticles to remove common contaminants like metallic ions and larger organic molecules (mimicking typical water contaminants we see in real life), engaging students in materials science.  They saw that the two different types of cleaning particles we use interact with the contaminant molecules/ions in different ways, leading to an understanding of both HS-PS1-3 (Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.) and HS-PS2-6 (Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.).  The two day lesson series included Water Quality: Removing Contaminants Day 1 and Water Quality: Removing Contaminants Day 2.  Finally, students spent two days learning about acid/base chemistry and how to neutralize pH in Water Quality: What is pH? and Water Quality: Neutralizing pH.

Treating water samples and exploring sources of poor quality water will help students meet Performance Expectation HS-ESS3-6: Use a computational representation to illustrate the relationships among Earth systems and how those relationships are being modified due to human activity.  Students will be cleaning samples, engaging in SEP 3: Planning and Carrying Out Investigations.  They will also be analyzing those results in order to modify and add additional treatment steps, participating in SEP 4: Analyzing and Interpreting Data.  

This performance assessment requires at least two days of class preparation time.  My students did not understand how to construct a decision tree or even what it is.  I needed to spend a day explaining the decision tree process before I could expect them to be able to create one.  The second preparation day was spent allowing students to clearly read the protocol for the assessment and to thoughtfully plan their approach.

The assessment also required a lot of preparation on my behalf.  I prepared 6 different types of unknown samples using the description below.  

  • A: Yellow #6 food coloring, enough HCl to lower pH to about 6
  • B: Yellow #6 food coloring, enough NaOH to raise pH to about 8
  • C: Copper sulfate, enough HCl to lower pH to about 6
  • D: Copper sulfate, enough NaOH to raise pH to about 8
  • E: Yellow #6 food coloring, copper sulfate, enough HCl to lower pH to about 6
  • F: Yellow #6 food coloring, copper sulfate, enough NaOH to raise pH to about 8

There need to be enough samples for each pair of students to have one.  I limited my general chemistry students to a sample from A-D, with honors students able to have any sample A-F.  Students would be in "competition" with each group within the same sample--for example, students with sample A that finished first would get the fastest time bonus and were not in competition with students having samples B-F.

I also needed to place pipettes, cups, and vials around for each group.  The supplies that students could use for their procedure also needed to be prepped.  I needed several 1/8 teaspoon samples of activated carbon and zeolite in vials ready for students, filter paper, syringes and screw tip filters, baking soda, and citric acid.  Also, students needed pH paper with rainbow scales for reading pH and phenolphthalein.

Distribution of Assessment Packet & Instructions

15 minutes

During the previous day (which is not included as a separate lesson write-up), I guided students through the rules of today's assessment and assigned them a decision tree to write using Improving Water Quality ASSESSMENT planning document.

Student Samples of Decision Trees:


Today, I begin by instructing students to take out their decision trees and then remind students of the instructions.  In order to keep the class orderly and support students in being able to finish the assessment in a timely manner, I give clear instructions of where to line up with order forms and how to get the necessary materials.

Then, I distribute Improving Water Quality FINAL ASSESSMENT packet, one per assessment pair (since students work in pairs).  I spend several minutes to clearly explain where to get materials and go over the expectations for today's assessment.  For more details, see the video included in "Communicating Procedural Steps" reflection.

Performance-Based Assessment

35 minutes

As students bring me their order forms, I give them the requested supplies and stamp the order sheet.  Pairs of students work to clean their samples, deciding together what steps to follow and what materials to use.

I display a large pace clock (which I borrowed from Swim, as I'm the coach), but any clock will do for timing.  As students complete their cleaning, they will turn in their clean sample in a labelled vial with their assessment packets stapled to their previously completed decision trees.  I write the time at the top of the packet to document how long they have spent on the task.

Sample Student Work:

Student Pair #1

This student pair only needed one order of supplies and spent the least in their sample group (they had sample C).  They turned in their cleaned sample at 24 minutes, 20 seconds elapsed, making them the second from the sample C group to turn in.  This gained this pair bonus points for both money and time consideration.  The decision trees that they turned in were acceptable, but did not account for all possibilities and lost 1 point.  Everything else was spot on.

Student Pair #2

This student pair used two orders of supplies.  They had sample E and had trouble neutralizing the sample.  Otherwise, they did spend the least on supplies in the sample E group.  They were the last in this sample group to turn in, and did not get time bonus points.


Student Pair #3

This pair used four different orders because they were only buying supplies as they needed them in order to maximize their budget.  They did end up using a lot of time, though, and as a result only earned a money bonus.  They had difficulty neutralizing the solution and the RGB did not read the solution as colorless.  This pair worked through the challenges well, though, and continued to persevere. 


5 minutes

As students finish and turn in their packets and cleaned samples, they are encouraged to clean up their lab areas.  This involves cleaning up their equipment and making sure that everything gets put in its place.  In order to encourage clean up, I praise and acknowledge those students who are helping, as described in the reflection video here.

Tomorrow, we will evaluate the success of this assessment in class by using the Lab Master RGB spectrophotometer to check for color and pH paper to check for neutralization.