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Investigating Precipitate Formation (0 Favorites)

LESSON PLAN in Solubility, Precipitate, Solubility Rules, Predicting Products. Last updated April 13, 2022.


Summary

In this lesson, students will learn about lead and the contamination of drinking water. Through collaboration, students will then consider strategies for decontaminating water, and have the opportunity to perform small-scale precipitation reactions as a method of extracting metal ions from a water sample. Finally, students can conduct research and reflect on their experience to propose a possible solution for decontaminating drinking water.

Grade Level

High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

  • HS-PS2-6. Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
  • Scientific and Engineering Practices:
    • Developing and Using Models
    • Analyzing and Interpreting Data
    • Constructing Explanations and Designing Solutions

Objectives

By the end of this lesson, students should be able to:

  • Describe the difference between a soluble and insoluble compound.
  • Observe (some) ions that precipitate in aqueous solutions.
  • Propose a chemical that could be added to water to remove lead ions.
  • Explain how drinking water sources become contaminated and its impact on society.

Chemistry Topics

This lesson supports students’ understanding of:

  • Solutions
  • Solubility
  • Precipitation
  • Solubility Rules
  • Chemical Reactions
  • Predicting Products
  • Ionic Compounds

Time

Teacher Preparation: 45 minutes (solution preparation)
Lesson: 90 minutes (or more depending on the depth of the research you’d like students to complete)

Materials

  • Well plate or precipitate sheet (laminated paper with ½ inch black dots printed—see below)
  • Beakers with solutions:   
  • Pipettes/dropper bottles

Safety

  • Teachers should consult the information provided in the associated SDS for each of the solutions (each linked in Materials section) used in the lab portion of the lesson.
  • Lead Nitrate has notable hazardous properties. Teachers can replace the use of lead nitrate with magnesium nitrate to yield the same experimental results. See the Teacher Notes section for more information.
  • When using microscale amounts of solution, proper clean-up consists of soaking up droplets with a paper towel and disposing in the trash. Avoid rinsing substances down the drain.
  • Always wear safety goggles when handling chemicals in the lab.
  • Students should wash their hands thoroughly before leaving the lab and at any time if skin comes in contact with solutions.

Teacher Notes

  • I suggest completing the following activities as part of this lesson (in the order outlined below). However, teachers could certainly choose specific parts to complete or omit given time constraints or student needs:
    • First watch the ACS Reactions Video, “How Lead (maybe?) Caused the Downfall of the Roman Empire” (approximately 6 minutes).
    • Next watch a video from the Nature Resources Defense Council, “Fighting for Safe Water in Flint” (approximately 13 minutes).
    • Following the videos teachers can ask students to respond to the following questions:
      • What is a societal challenge being experienced in this video?
      • What solutions have been proposed or tried?
      • How do you think chemistry relates to the challenge shown in this video?
    • It should be noted that both videos support the following NGSS Performance Expectations:
      • HS-ETS1-1: Analyze a major global challenge to specify qualitative and quantitative criteria and constraints for solutions that account for societal needs and wants.
      • HS-ETS1-3: Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.
    • After the videos, students should be asked to hypothesize ways that we can get contamination (dirt? sand? salt? lead and other minerals?) out of water. Using a Google Jamboard for students to add anonymous sticky note to share ideas. If Jamboard isn’t available, teachers could use a white board or poster board to support a similar collaboration.
    • A Water Treatment infographic from Compound Interest could be shared, and students asked to again comment through Jamboard sticky notes or another collaborative platform in response to questions like, “What do you notice?” and “What do you wonder?” about this model of the water treatment process.
    • Next students will complete the attached lab (student handout available for download) and perform several small-scale reactions in order to learn about one method of extracting ions from drinking water.
    • At the end of the lab students will be asked to propose a solution that could be added to drinking water to precipitate out lead ions and then could go on to research the benefits and drawbacks of the ions contained in their proposed solution.

Lab portion:

  • Teachers should prepare the laboratory solutions in advance. Students will only need a couple drops of each solution, so 100mL of each solution will suffice for approximately 100 students working in small groups.
  • Solutions should be placed in labeled dropper bottles or small beakers labeled, each with labeled pipettes (prepare enough for each group).
  • As a safer alternative to using lead (II) nitrate due to its potential toxicity, teachers can replace it with magnesium nitrate and it will yield the same results. Teachers should note that since lead is obviously important to the overall lesson goals, if magnesium nitrate, or any other alternative solution is used instead of lead, the students should still carry out the lab (and answer the related questions) as if lead nitrate solution was used. 
  • Many other solutions can react to precipitate a solid. Consult a solubility table to find additional combinations of ions that would result in a precipitate if alternatives are preferred. 
  • An Answer Key document has been provided for teacher reference.
  • It is suggested that students complete the lab in small groups of 2-4 students.
  • If you don’t have a well plate, I suggest printing 5x7” index cards to use in place of the well plate with a grid of the black circles (examples shown below) and then laminating it—it works very well to showcase precipitates. 
Lab investigatingprecipitateformation teachersnotesimage
  • If solutions are not concentrated enough, precipitates will be difficult to see.  You can double the concentration of the solutions if needed. Do not use higher than 1.0M NaOH as it is a strong base. It is suggested that teachers test each of the reactions prior to using them with students.
  • Consider having students tour each other’s models of subatomic particles developed for each reaction on the student handout. Consider having them draw a model on a large whiteboard and having a 10-minute showcase where other students point out things they like about the models they see. 
  • Consider having students perform a flame test to verify the ions in solution. One downside of this Investigating Precipitate Formation lab is that they can't see soluble ions (by definition), but they can test for them!

For the Student

Lesson

In many communities around the world, people are at risk of consuming unsafe amounts of some minerals (ions) in drinking water. One way to purify drinking water is to add more ions, which “pull” other ions out of solution by coagulating them. In chemistry, this is called precipitation.

You have observed soluble and insoluble ionic solids in the chemistry lab. You have created a solubility table documenting precipitate formation involving different cations and anions. Today you will conduct 3 reactions with the aim of modeling what happens on the atomic level when a precipitate does (or does not) form.

Materials

  • 1 well plate (or laminated sheet with black dots)
  • 3 beakers with solutions and pipettes:   
    • 1.0M NaOH, 0.10M Pb(NO3)2 , 0.10M K2CO3

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
  • Sodium hydroxide (NaOH) is a strong concentrated base; wash hands with soap and excess water if you touch NaOH.

Macroscopic Observations

Procedure

Take care not to contaminate the solutions, as that could lead to confusing results!

  1. Put 2 drops of NaOH into a well plate.
  2. Carefully and slowly, 2 drops of Pb(NO3)2 into the same well plate. Do NOT touch the pipette tip to the glass (avoid contamination).
  3. Record your observations.
  4. Repeat the steps above for other solutions listed below.

Observations

Reaction
Solutions
Observations
1
NaOH and Pb(NO3)2
2
NaOH and K2CO3
3
Pb(NO3)2 and K2CO3

Clean-Up

  1. Organize all materials on your lab table.
  2. Dispose each experiment into a paper towel and then throw it in the trash.
  3. Wash your goggles with soap and water, dry them, and return them to the goggle cabinet.

Analysis: Reaction 1

  1. Symbolic Representation: Write the balanced chemical equation for Reaction 1 below:
  2. Submicro Representation: 
    1. Draw a particle diagram of the reaction. Label appropriate ions AND water molecules to show what you think Reaction #1 would look like on the atomic level. Your teacher will ask you and your peers questions about your model and how it relates to the balanced equation you wrote above. 
Lab investigatingprecipitateformation studentsectionimage
    1. Write a short statement beside each container above, describing what’s going on inside.
  1. Where were the lead ions before this reaction?  Where were they after?
  2. What is left dissolved in the water after the reaction? Are any of these ions harmful? You may need to research!

Analysis: Reaction 2

  1. Symbolic Representation: Write the balanced chemical equation for Reaction 2 below:
  2. Submicro Representation: 
  3. .Draw a particle diagram of the reaction. Label appropriate ions AND water molecules to show what you think Reaction #2 would look like on the atomic level. Your teacher will ask you and your peers questions about your model and how it relates to the balanced equation you wrote above. 
Lab investigatingprecipitateformation studentsectionimage
    1. Write a short statement beside each container above, describing what’s going on inside.
  1. Why didn’t a precipitate form in this reaction?
  2. Research how you could test to prove that Na+ ions and K+ ions are in a solution (even though you cannot see them with your eyes).

Analysis: Reaction 3

  1. Symbolic Representation: Write the balanced chemical equation for Reaction 3 below:
  2. Submicro Representation: 
  3. Draw a particle diagram of the reaction. Label appropriate ions AND water molecules to show what you think Reaction #3 would look like on the atomic level. Your teacher will ask you and your peers questions about your model and how it relates to the balanced equation you wrote above.
Lab investigatingprecipitateformation studentsectionimage
    1. Write a short statement beside each container above, describing what’s going on inside.
  1. Does adding NaOH to water contaminated with lead ions help to precipitate out the lead ions? Cite evidence from your observations.
  2. Does adding K2CO3 to water contaminated with lead ions help to precipitate out the lead ions? Cite evidence from your observations.
  3. Think of two ways you could separate the precipitate from the solution around it.  Think of an advantage and a disadvantage to each way you propose.  Could be related to cost, materials, environment, sustainability, smell—be creative!