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Using Qualitative Analysis to Identify Household Compounds (7 Favorites)

LAB in Observations, Physical Properties, Solubility, Chemical Change, Physical Change, Precipitate, Identifying an Unknown, Indicators, Chemical Properties. Last updated November 23, 2021.


Summary

In this lab, students will be introduced to common laboratory techniques, safety procedures, lab reagents, and terminology, all while identifying unknown household substances. Students will learn how to use qualitative analysis techniques as a systematic way to identify unknown materials. As part of this process, they will practice careful observation and documentation, as well as identifying relevant physical properties, such as solubility, and indicators of chemical changes, including color change, gas formation, and precipitation of solids.

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.
  • Scientific and Engineering Practices:
    • Analyzing and Interpreting Data
    • Engaging in Argument from Evidence

Objectives

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

  • Identify unknowns based on their physical and chemical properties using qualitative analysis techniques.
  • Identify and explain the difference between a chemical change and a physical change.
  • Make observations and accurately record their findings.
  • Correctly and safely use the Bunsen burner.

Chemistry Topics

This lab supports students’ understanding of:

  • Identifying unknowns using qualitative analysis
  • Chemical and physical properties
  • Chemical and physical changes
  • Lab techniques, terminology, and safety

Time

Teacher Preparation: 45-60 minutes
Lesson: 60 minutes

Materials

Unknowns (class set):

  • Place ~50 g of each “unknown” compound in a beaker labeled with the unknown number. Arrange 2-3 sets of the seven unknowns around the lab room for easy access.
  • Plastic spoon or metal scoopula in each beaker to avoid cross-contamination.
  • The seven unknowns are:
    • Cornstarch: C27H48O20
    • Baking soda: NaHCO3
    • Calcium supplement, crushed (see teacher notes)
    • Epsom salt: MgSO4
    • Washing soda: Na2CO3
    • Table sugar, granular: C12H22O11
    • Table salt: NaCl

Test Reagents (class set):

  • Prepare 6-8 containers of each of the following, either small dropper bottles or small beakers with disposable pipettes. (Fewer may be used with smaller class sizes or if groups are very well prepared and move efficiently through tests.) Amounts needed per group are provided below to help you determine how much of each solution you will need.

Lab Equipment (per group):

  • 1 Bunsen burner (with matches/striker/lighter)
  • 18 test tubes, large enough to hold approximately 25 mL of water
  • 1 test tube rack able to hold 18 test tubes (or 2 smaller ones)
  • 1 test tube holder
  • 1 graduated cylinder (10 mL would work best but larger size could be used)

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.
  • Always use caution around open flames. Keep flames away from flammable substances.
  • Always be aware of an open flame. Do not reach over it, tie back hair, and secure loose clothing.
  • Open flames can cause burns.
  • When lighting the match, be cautious with the flame.
  • An operational fire extinguisher should be in the classroom.
  • Exercise caution when heating test tubes in the Bunsen burner. Always point it away from people and constantly move the test tube in within the flame to allow the entire solution to be heated evenly. If the water begins to boil, remove it from the flame immediately.
  • When working with acids and bases, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
  • Do not consume lab solutions, even if they’re otherwise edible products.
  • Food in the lab should be considered a chemical not for consumption.

Teacher Notes

  • Pre-lab question #3 asks students to read the lab procedure and develop a “flow chart” of the procedure to be covered. I suggest you draw out the first step or two together as a class. Preferably, time will be given in class for partners to work together to draw out the remainder of the flow chart. Alternatively, you could have them complete the chart for homework.
  • Before the lab begins, the students should be shown how to light and adjust Bunsen burners, and you may want to let them practice first. The instructor should also show how to properly hold the test tube in the flame, concentrating on the open end pointing away from all others, and moving the test tube in the flame to evenly heat all of the liquid. If the liquid in the test tube begins to boil, it should be removed from the flame immediately. This step should be demonstrated by the instructor prior to the lab beginning. 
  • The “Calcium supplement” used in this lab can be any antacid tablet made with calcium carbonate. The tablets should be white, of course, and should be ground up with a mortar and pestle prior to starting the lab, so it has the same appearance as the rest of the samples.
  • If dropper bottles are available, bottles for phenolphthalein and iodine solutions can be made for future use in this lab and other activities. It is recommended that the vinegar and 0.5 M NaOH solution simply be placed in beakers with a dropper/pipette available. As it states in the “materials” list, 6 to 8 containers of each of these are preferable, but each individual class may need less, depending on the size and efficiency of the class.
  • Whenever the procedure calls for a “new sample”, it is expected that the next test (for example adding vinegar) is done by adding the reagent directly to a new pea-sized sample of the solid powder in a clean test tube. If it is desired for the “new sample” of the solid to be again dissolved in water, the instructions will make that clear.
  • It’s easy for test tubes to be contaminated with some type of base to give a positive (bright pink) result in the phenolphthalein test. The washing soda will be a very obvious positive test.
  • The vinegar + baking soda reaction is one that many are familiar with, and it provides a good introduction to acid/base chemistry, double replacement reactions, decomposition reactions, and the production of a gas as an indicator of chemical change. However, students might assume that all reactions that produce a gas are producing CO2, so it is important that they don’t leave with this misconception. The Stop & Go Gases resource in the AACT library is a good demo to use to provide a counterexample where one reaction produces CO2 and extinguishes a flame, while another reaction produces O2 and reignites the flame.
  • This lab provides students with an early year introduction to acid/base indicators, both for this class, and some applications in everyday life, such as working with a hot tub, pool, or aquarium, or testing soils in which plants will be grown.
  • This lab is also a good early-year introduction to the everyday concept of solubility. Students see that not all substances dissolve in water to the same extent, and substances as commonplace as sugar and salt will dissolve differently, even both are water soluble. Since many household white powders are soluble, seeing that the crushed-up calcium supplement and cornstarch are not very soluble are good counterexamples.
  • All of these powders are household compounds, and this lab serves very well to make students aware of the chemistry that is found in every corner of their house.
  • Another AACT resource that has students identify unknown household powders is As Easy As Pie, except they have to come up with their own tests and procedures. It has a “who done it” aspect to it and requires students to apply what they learned about these substances in this lab to develop their own procedures, which could make it a fun follow-up activity.
  • Here is a summary of what should be seen for each of the white powders in this lab:

Unknown Powder Expected Result From Test
Cornstarch Does not readily dissolve in water; forms a dark blue complex when iodine solution is added
Calcium supplement Does not readily dissolve in water; forms bubbles when vinegar is added
Washing soda Dissolves in water; as a base, it should turn a vivid pink when phenolphthalein is added, bubbles when vinegar is added to a new sample
Epsom salt Dissolves in water; forms a cloudy precipitate when NaOH is added
Baking soda Dissolves in water; forms bubbles when vinegar is added
Table sugar Dissolves in water; noticeably dissolves well (is not cloudy) when a lot is added to water and the mixture is heated
Table salt Dissolves in water; does not dissolve as well when a lot is added and it is heated above a flame

For the Student

Lesson

Introduction

Every chemical compound has its own unique set of physical and chemical properties, and these properties can be used to identify unknown substances. In this experiment, you will use qualitative analysis techniques to identify 7 common household compounds that are similar in appearance but have different properties that will allow you to distinguish between them.

Materials

  • Samples of 7 unknown substances (class sets)
    • Substances are cornstarch, calcium supplement, washing soda, Epsom salt, baking soda, table sugar, table salt
  • Test reagents (class sets)
    • Iodine-potassium iodide solution, phenolphthalein, vinegar, 0.5 M sodium hydroxide
  • 1 Bunsen burner (with matches/striker/lighter)
  • 18 test tubes
  • 1 test tube rack
  • 1 test tube holder
  • 1 graduated cylinder

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.
  • Always use caution around open flames. Keep flames away from flammable substances.
  • Always be aware of an open flame. Do not reach over it, tie back hair, and secure loose clothing.
  • Open flames can cause burns.
  • When lighting the match, be cautious with the flame.
  • Exercise caution when heating test tubes in the Bunsen burner. Always point it away from people and constantly move the test tube in within the flame to allow the entire solution to be heated evenly. If the water begins to boil, remove it from the flame immediately.
  • When working with acids and bases, if any solution gets on your skin, immediately alert your instructor and thoroughly flush the area with water.
  • Do not consume lab solutions, even if they’re otherwise edible products.
  • Food in the lab should be considered a chemical not for consumption.

Pre-Lab Questions

  1. Define physical change. Provide 3 examples of physical changes.
  2. Define chemical change. Provide 3 indications that a chemical change may have occurred.
  3. Read through the procedures below. On a separate sheet of paper, create a flow chart diagram to visually represent the steps of the procedure for this lab. Be sure to include the materials required for each step, the expected results, and how they will help you identify each unknown substance.

Procedure

Before you begin testing substances, label 7 test tubes 1 to 7 to correspond to the unknown compound #’s. If you are asked to take a “new sample,” label another clean test tube with the unknown # you are testing and add another pea-sized amount of the unknown. (Do not add water unless specifically instructed to do so.)

Record detailed observations of the tests you conduct at each step in the observations table below and identify the unknowns in the results table.

  1. Test each solid for water solubility by taking a pea-sized sample and placing it in its corresponding test tube with 10 mL of water. Determine which 2 are the least soluble. (They will remain the cloudiest.) The two cloudy samples will be the cornstarch and the calcium supplement. Save all solutions!
  2. To the 2 test tubes containing the least soluble compounds, add 2 drops of iodine solution. The iodine will react with the cornstarch to form a dark blue color. At this point, you should be able to identify the cornstarch. Record the identity of the unknown in the results table.
  3. To confirm that the other insoluble substance is the calcium supplement, place a new sample in another test tube (labeled with the same unknown #) and add 1 mL of vinegar. The vinegar will react with the calcium supplement to form carbon dioxide gas. Record the identity of the unknown in the results table.
  4. To the remaining 5 solutions from step 1, add 2 drops of phenolphthalein. The washing soda will turn a VIVID pink, while the others will remain clear or faint pink.
  5. To confirm the washing soda, add 1 mL of vinegar to a new sample. The washing soda and the vinegar will react to form carbon dioxide gas. Record the identity of the unknown in the results table.
  6. At this point, you should have identified the cornstarch, calcium supplement, and washing soda. Take new samples of the remaining 4 substances, and dissolve in 10 mL of water. To each solution, add 1 mL of sodium hydroxide solution. A white precipitate will form with the Epsom salt, but not with any of the others. Record the identity of the unknown in the results table.
  7. Obtain new samples of the 3 remaining unknown substances. Add 1 mL of vinegar to each. The vinegar will react with the baking soda to form carbon dioxide gas. Record the identity of the unknown in the results table.
  8. If everything has been correctly identified at this point, only table salt and sugar remain. Place 15 mL of water into 2 clean test tubes. Add enough of each compound to its test tube so there is a large amount (½ to 1 inch deep) of undissolved solid on the bottom of the test tube. Heat each test tube gently over the Bunsen burner. The sugar is much more soluble than the table salt and will form a clear solution. The salt solution will remain slightly cloudy. Record the identities of the final two unknowns in the results table.

When finished, wash all test tubes thoroughly, rinse with lots of water, and remove any tape. Wipe down the lab benches and wash your hands!

Observations

Record observations for all changes in the following steps
Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Step 7
Step 8

Results

Powder Unknown Number
Cornstarch
Calcium supplement
Washing soda
Epsom salt
Baking soda
Table sugar
Table salt

Analysis

For each step:

  1. Identify whether a physical change or chemical change was observed.
  2. If it was a physical change, describe the change that occurred. If it was a chemical change, list the indications that led you to this conclusion.
  3. Describe how the change you observed in this step helped you narrow down the list of unknown substances or identify one of the unknowns.

Step 1

Step 2:

Step 3:

Step 4:

Step 5:

Step 6:

Step 7:

Step 8: