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.
  • Iodine-Potassium Iodide solution, 4 drops per group. (It is likely this solution or already made dropper bottles are available from a biology storage area.) If you would like to prepare this solution yourself, you can use the first set of instructions in this video.
  • Phenolphthalein, 10 drops per group
  • Vinegar, 5 mL per group
  • Sodium Hydroxide, 0.5 M, 4 mL per group

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: