Summary

In this lab, students will identify 3 unknown acids by using the solubility rules. They will be given a list of materials and will design their own procedures for identifying the unknowns. For each combination of reactants, they will predict whether a product forms and, if it does, write complete and net ionic equations for those reactions.

Grade Level

High School

NGSS Alignment

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

• MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
• 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.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Planning and Carrying Out Investigations
  • Constructing Explanations and Designing Solutions

Objectives

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

• Identify 3 unknown acids as either hydrochloric acid, acetic acid, or sulfuric acid.
• Use solubility rules to determine if a precipitate is expected.
• Write complete and net ionic equations.

Chemistry Topics

This lab supports students’ understanding of:

• Experimental design
• Chemical reactions
• Solubility Rules
• Double Replacement reactions
• Complete and net ionic equations

Time

Teacher Preparation: 20 minutes
Lesson: 40 minutes

Materials (per lab group)

• 10 mL of 1 M hydrochloric acid
• 10 mL 0f 1 M sulfuric acid 
• 10 mL of 1 M acetic acid
• 10 mL of 0.20 M lead (II) nitrate
• 10 mL of 0.50 M barium nitrate
• Well plate
• Disposable pipettes
• Periodic table and solubility rules table

Safety

• 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
• When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• When diluting acids, always add acid to water.

Teacher Notes

• This lab is designed to be performed after instruction on chemical reactions, double replacement reactions, solubility rules, complete ionic equations, and net ionic equations.
• The SDS information for all of the substances used in this lab are linked in the materials list, above. Of particular note are the sulfuric and hydrochloric acids, which are both strong acids, so even though they are not particularly concentrated at 1 M, they have very low pH values and students should be instructed to be extremely cautious when handling their acid samples. (Flinn even notes “Treat all sulfuric acid solutions with the same caution you would have for concentrated sulfuric acid. For example, the pH of 0.05 M sulfuric acid solution is 1.2.”)
• Additionally, depending on your state/local waste disposal requirements, you may need to dispose of the lead- and barium-containing products as hazardous waste using a licensed hazardous waste disposal firm. See Flinn’s Disposal Methods #26c, #27f, and #27h, and always consult your local authorities before disposing of lead or barium compounds in the trash or down the sink. To minimize hazardous waste products, use the smallest amounts of reagents possible.
• Students can be in groups of 2 or 3.
• Students can use a large classroom solubility table or individual ones. Solubility rules can be found in most chemistry textbooks in the form of a table or chart, such as this one.
• If multiple classes will be reusing the well plates, be sure students thoroughly rinse them with lots of water when they are done. If there are still some leftover ions in the wells from the previous group, it could cause contamination for the next lab group which would result in more precipitates than expected.
• This lab requires students to design their own procedures for determining the identities of the unknown acids, which, even for a fairly simple experiment like this one, can be very challenging for students if they are used to being told exactly what to do. It may take longer than you expect, but it is a very valuable scientific skill to develop! If you have lower-level classes or wish to focus more on the data analysis aspect of this lab, students can be given the following instructions and data table if they struggle to develop their own procedures:
  • Place 5 drops of each unknown acid in 2 wells on the well plate.
  • Place 5 drops of lead (II) nitrate solution with each unknown acid.
  • Place 5 drops of barium nitrate solution with each unknown acid.
  • Record which reactions form a precipitate and which ones do not form a precipitate.