Summary

In this lesson, students will work with a partner through a guided inquiry activity that will introduce, teach, and “solidify” the concept of precipitation reactions. In this multi-part lesson, students will review chemical and physical changes, identify spectator ions, perform small-scale precipitation reactions, view simulation-based heavy metal precipitation reactions, and identify a likely precipitate when combining two solutions. Students will also be introduced to writing net ionic equations.

Grade Level

High School

NGSS Alignment

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

• HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
• HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• Scientific and Engineering Practices:
• Using Mathematics and Computational Thinking
• Analyzing and Interpreting Data
• Engaging in Argument from Evidence

AP Chemistry Curriculum Framework

This lesson supports the following unit, topics and learning objectives:

• Unit 4: Chemical Reactions
  • Topic 4.1: Introduction for Reactions
    • 4.1.A: Identify evidence of chemical and physical changes in matter.
  • Topic 4.2: Net Ionic Equations
    • 4.2.A: Represent changes in matter with a balanced chemical or net ionic equation: a. For physical changes. b. For given information about the identity of the reactants and/or product. c. For ions in a given chemical reaction.
  • Topic 4.3: Representations of Reactions
    • 4.3.A: Represent a given chemical reaction or physical process with a consistent particulate model.
  • Topic 4.4: Physical and Chemical Changes
    • 4.4.A: Explain the relationship between macroscopic characteristics and bond interactions for: a. Chemical processes. b. Physical processes.
  • Topic 4.7: Types of Chemical Reactions
    • 4.7.A: Identify a reaction as acid-base, oxidation-reduction, or precipitation.

Objectives

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

• Differentiate between a chemical change and a physical change.
• Identify spectator ions and/or the ions that will chemically react in a precipitation reaction.
• Draw a particle diagram of a net-ionic precipitation reaction.
• Write a balanced net-ionic equation.

Chemistry Topics

This lesson supports students’ understanding of:

• Chemical Changes
• Physical Changes
• Spectator Ions
• Precipitation Reactions
• Net-ionic Equations

Time

Teacher Preparation: 15 minutes
Lesson: 90 minutes

Materials

• Printed copies of the student handout and assessment
• Well plates (or sheet protectors) for small-scale reactions
• Labeled Dropper bottles (or disposable Beryl pipets and small Erlenmeyer flasks) for each of the solutions below:
• Sodium chloride solution, 0.1 M
• Lithium nitrate solution, 0.1 M
• Sodium nitrate solution, 0.1 M
• Potassium iodide solution, 0.1 M
• Barium chloride solution, 0.1 M – Note: Barium chloride is highly toxic. Do not ingest the salt or solution. See teacher notes below for more information.
• Sodium sulfate solution, 0.1 M
• Calcium nitrate solution, 0.1 M
• Sodium carbonate solution, 0.1 M
• Access to computer simulation: https://interactivechemistry.org/IonicReactions/

Safety

• Barium chloride is highly toxic. Do not ingest the salt or solution.
• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• See SDS information for all chemicals used in this activity.
• All chemical reactions are being conducted by small-scale laboratory means, which requires only 1 or 2 drops of each chemical substance. This minimizes danger, waste, extra work, and clean-up.

Teacher Notes

• This is an inquiry activity which means that the learning process of each part will engage students to make connections through exploration, prediction, observation, and experiential learning. The role of the teacher is to distribute each part to students working in pairs and circulate the room to question the students to stimulate thinking. After each part, the teacher will bring the class together for discussion, teaching, reinforcement, correction, and summarization to make sure all students are on the right track.
• Since this is an inquiry-based, exploratory activity, there is no need for teachers to provide direct instruction on the topic before students engage in these activities. I incorporate this activity in my Chemistry Honors course after covering Intermolecular Forces (Physical Changes) and as we introduce the topic of Chemical Reactions. Since many Chemistry, Chemistry Honors, and AP Chemistry courses cover physical and chemical changes early on, this activity serves as a good foundation for delving into chemical reactions, solubility rules, and precipitation reactions.
• This lesson could be completed in one 90-minute block, or it could be broken up over two days if you have shorter class periods – for example, if you have 45 minute class periods, parts A, B, and C could be completed on Day 1 and parts D and E could be completed on Day 2.
• The solution particle diagrams in this lesson were created using Chemix, an online editor for drawing science lab diagrams. (Most lab equipment that would commonly be used in high school labs are free to use and download, with more advanced equipment available with a subscription.)

Part A - Let’s Get Physical…and Chemical (10 minutes)

• Working in pairs, allow students to predict the particle diagrams of a physical and chemical change as well as the definition of each.
• After students have completed their predictions, review as a class the difference between each change. It is good to use a molecular model kit to review the difference between breaking bonds and breaking attractions.

Part B - The Fly on the Wall (15 minutes)

• Distribute the four solutions – sodium chloride, lithium nitrate, sodium nitrate, and potassium iodide – and a reaction surface (either a well-plate or sheet protector) to each group.
• Since these substances will not produce precipitates, you can use distilled water for each substance and label them as the four solutions. This may save time and resources while still getting the point across about spectator ions.
• Allow students to mix the substances, write down their observations, draw particle diagrams, and identify patterns in which ions do not react. I always enjoy looking surprised when the students are confused that these substances are not reacting.
• Small-scale chemistry laboratories are relatively safe, time-efficient, easy to set up, and environmentally sound. I put the solutions in Flinn Polyethylene dropper bottles and clearly label each chemical solution. Small Erlenmeyer flasks that are clearly labeled with Beryl disposable pipets (clearly labeled as well) can also be used.
• After students have completed this part, review as a class what substances did not react in both reactions #1 and #2. Be sure to specify that these substances - nitrate and alkali metal ions - are almost always spectator ions. It is always good to use artistic performance or sports metaphors, or even a “fly on the wall” as examples of spectators that are not directly involved in the activity.
• Note that in this activity, students are led to the conclusion that nitrate and alkali metal ions are always spectator ions. As with most “always” statements, this is a generalization that, while almost always true, has a few exceptions – ex: lithium, an alkali metal, can form precipitates, such as insoluble lithium phosphate and slightly soluble lithium fluoride.
• Some students may wonder why they are doing Part B if it didn’t yield any positive results. This is a good opportunity to remind them that scientists don’t see negative results as failures but as essential steps in the scientific process. Negative results provide valuable information, helping chemists understand which substances do not react.

Part C - Make Sure you Precipitate in the Activity! (20 minutes)

• Distribute the four solutions – barium chloride, sodium sulfate, calcium nitrate, and sodium carbonate – and a reaction surface (either a well-plate or sheet protector) to each group.
• If you don’t want to use barium chloride due to its toxicity, consider using calcium chloride, which when mixed with sodium sulfate produces slightly soluble calcium sulfate and should produce a precipitate. Alternatively, you could choose other solutions that will form a precipitate, such as potassium hydroxide and magnesium chloride.
• Allow students to mix the substances, write down their observations, complete the two chemical reactions (utilizing the logical deductions made from Part B), and draw particle diagrams.
• After students have completed this part, review as a class their observations, chemical reactions, and the correct particle diagrams. Be sure to explain that the charges AND the atoms need to be balanced in a net-ionic equation. Also, emphasize that the net-ionic equation only shows what is chemically reacting and does not show the spectator ions.

Part D - The Simulated Solid (20 minutes)

• First allow students to predict the precipitate or solid that will form given the two solutions in Chemical Reaction #1 and #2.
• Use the simulation for each chemical reaction, allowing students to draw the particle diagram and write the net-ionic equation.
• Note: If students do not have access to devices for the simulation, the teacher can project it for the entire class. Students can then complete part D individually, in pairs, or as a whole class.
• Allow students to present and explain their net-ionic equation to the class.
• In Chemical Reaction #3, students have the freedom to choose their own salts, giving them the opportunity to "be the chemist." Encourage them to predict precipitates and deepen their understanding of net ionic reactions. This can be reviewed as a class or in smaller groups for a more interactive discussion.

Part E - Be part of the Solution…No, be part of the Problem…No, be part of the Precipitate! (20 minutes)

• Allow students in their groups of two to circle and identify the substances in each solution that will make a precipitate. Circulate the room asking questions and stimulating discussion. After students have completed this part in their pairs, review as the correct answers with the class.
• Allow students in their groups of two to write the net-ionic equation for each of the ten reactions, then review the chemical reactions emphasizing positive ions (cations), negative ions (anions), charges of each ion, balanced atoms and charges.
• Writing the net ionic equations can also be used as a homework assignment.

Precipitation Reactions Assessment

• There is a five-question assessment available for download to evaluate student understanding after they complete the inquiry activity. This can be assigned for homework, used as a quiz, or completed informally for students to check their own understanding.