019 Six Chemicals

Expt 019 -- Six Chemicals

Six known chemicals are studied by mixing them, two at a time. A set of unknowns is created by covering the labels on the knowns with identifying marks. On the basis of their reactions, the unknown chemicals are to be relabelled.

Safety

Solutions of nitric acid stain flesh and damage clothing. Solutions of silver nitrate are toxic and stain flesh and clothing. Wear goggles and apron. Avoid contact with silver nitrate and nitric acid. Wash immediately with water if contact occurs. Do not ingest chemicals. Wash hands after the experiment.

Procedure

Chemicals will be mixed systematically, two at a time, to discover the behavior of each combination. The chemicals may be mixed either in a 96-well plate or as puddles on an acetate sheet. Acetate sheets are recommended.
To remove a sample from a pipet storage device, hold the tip of the pulled pipet inside the top of a well or above an acetate sheet, making sure not to contaminate the pipet by touching the tip to the well, sheet, or drops of other chemicals. Squeeze very gently. Individual drops will emerge.

!!!Click here to See Movie.
Since order of addition doesn't matter in this experiment, and adding a chemical to itself rarely gives any useful information, a smaller number of combinations may be used, as shown in the diagram.

!!!Click here to See Picture.
96-well plates are labeled. Rows are designated with letters. Columns are designated with numbers. Each well's coordinates are embossed on the bottom of the plate; it is difficult to read. When using acetate sheets, it is best to make a labeled grid on a piece of white paper, then paper clip the acetate sheet to the paper and place the drops inside the appropriate boxes, just as with the labeled wells in a plate.
One way to be systematic is to add one drop of silver nitrate solution to wells (or boxes) (A2, A3, A4, A5, A6). Since order of addition is unimportant in this experiment, only one mixture of each reagent is necessary. Add carbonate down and then across the grid(A3,B3,C4,C5,C6).

!!!Click here to See Movie.
Continue adding both across and down. Carefully record observations of all mixtures. No reaction will be observed with several mixtures.
Whether using a 96-well plate or an acetate sheet, discard the contents into a disposal jar provided by the instructor.
The task comes in two parts. In the first part, combine all of the solutions two-at-a-time to discover which give detectable reactions. Decide how to organize the experiment, which observations are important, and how to record the data.

!!!Click here to See Picture.
The second part of the experiment is to work on unknowns. First, create the unknowns.
Cover each chemical container with a 2" x 3/4" strip of electrician's tape. Place the tape so that the label is unreadable. Place all six containers in a box or bowl. Randomly select one of these. Place a small piece of masking tape over the electrician's tape, and mark it with a 1. Set it aside. Select a second container. Use the same procedure to mark it with a 2, and so on for all six containers. Although the containers are numbered using a systematic procedure, they were drawn randomly.
Practice with just four chemicals -- Ag⁺, Cl^-, CO₃^2-, and H⁺.
Study the unknowns. Mix the chemicals two-at-a-time making careful observations. Based upon these observations and the knowledge from the studies of the knowns, decide the relationship between the number that was arbitrarily assigned and the identity of the chemical. Write down an assignment for each unknown. Remove the electrician's tape and check the answers. Repeat the experiment if there were any misidentifications.

Questions

Suppose that a 4-chemical problem is being studied: Ag⁺; Cl^-; CO₃^2-; and H⁺. Write down all of the possible combinations in which sets unknowns can be created from these four chemicals.
Suppose that a 4-chemical problem is being studied: Ag⁺; Cl^-; CO₃^2-; and H⁺. Chemical-1 is mixed with chemical-2, and bubbles are observed. List all possible combinations that remain for the unknown.
Suppose that a 4-chemical problem is being studied: Ag⁺; Cl^-; CO₃^2-; and H⁺. Chemical-1 is mixed with chemical-2, and no reaction is observed. List all possible combinations that remain for the unknown.

Handout

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 019 Six Chemicals

Watch the movies to see the knowns.

Watch the movies.

Record observations of the known reactions.

Curriculum-

This experiment involves logical deductions. The questions focus on design issues. Most students at first think that, when two chemicals are mixed and there is no reaction, little information is created. Systematic evaluation shows that, for the 4-unknown problem, fully half of the combinations are eliminated by that result. Although the teacher can wait until the reactions are better understood -- in terms of precipitation, acid-base, solutions formation, etc. -- the experiment works any time. It gives the opportunity for a creative thinking workout.

Safety-

Time-

Teacher Preparation: 20 minutes

Class Time: 30 minutes

Materials-

Prepare 1 mL of each reagent per group of students.
- 0.2 M silver nitrate -- (Dissolve 3.40 g silver nitrate (AgNO₃) in enough water to make 100 mL solution.)
- 0.2 M sodium chloride -- (Dissolve 1.17 g sodium chloride (NaCl) in enough water to make 100 mL solution.)
- 0.2 M sodium carbonate -- (Dissolve 2.12 g sodium carbonate (Na₂CO₃) in enough water to make 100 mL solution.)
- 0.2 M nitric acid -- (Add 6.7 mL of 3 M HNO₃ to enough water to make 100 mL of solution.)
- 0.2 M sodium bromide (Dissolve 2.06 g sodium bromide (NaBr) in enough water to make 100 mL solution.)
distilled water
96-well plate or acetate sheet
2 cotton swabs
12" black vinyl electrician's tape
6" masking tape
Data sheets recommended (see handout above).

Disposal-

Treat the contents of the disposal beaker with NaCl. Filter. Save the solid for recovery of silver. Neutralize the filtrate using NaHCO₃. Discard the neutralized solution at the sink with large volumes of acid.

Lab Hints-

Acetate sheets may be used in place of 96-well plates. Laminates in which a paper sheet of a matrix with some light and dark areas has been laminated serves the purpose well. Inserting a grid in a plastic sleeve intended for use in a 3-ring binder works well, too.

Answers-

Q1. Suppose that a 4-chemical problem is being studied: Ag⁺; Cl^-; CO₃^2-; and H⁺. Write down all of the possible combinations in which sets unknowns can be created from these four chemicals. A1. All possible combinations include (numbers arbitrary): 1. Ag⁺ Cl^- CO₃^2- H⁺ 2. Ag⁺ CO₃^2- H⁺ Cl^- 3. Ag⁺ H⁺ CO₃^2- Cl^- 4. Ag⁺ Cl^- H⁺ CO₃^2- 5. Ag⁺ CO₃^2- Cl^- H⁺ 6. Ag⁺ H⁺ Cl^- CO₃^2- 7. Cl^- Ag⁺ CO₃^2- H⁺ 8. Cl^- CO₃^2- H⁺ Ag⁺ 9. Cl^- H⁺ Ag⁺ CO₃^2- 10. Cl^- Ag⁺ H⁺ CO₃^2- 11. Cl^- CO₃^2- Ag⁺ H⁺ 12. Cl^- H⁺ CO₃^2- Ag⁺ 13. CO₃^2- Ag⁺ Cl^- H⁺ 14. CO₃^2- Ag⁺ H⁺ Cl^- 15. CO₃^2- Cl^- Ag⁺ H⁺ 16. CO₃^2- Cl^- H⁺ Ag⁺ 17. CO₃^2- H⁺ Ag⁺ Cl^- 18. CO₃^2- H⁺ Cl^- Ag⁺ 19. H⁺ Ag⁺ Cl^- CO₃^2- 20. H⁺ Ag⁺ CO₃^2- Cl^- 21. H⁺ Cl^- Ag⁺ CO₃^2- 22. H⁺ Cl^- CO₃^2- Ag⁺ 23. H⁺ CO₃^2- Ag⁺ Cl^- 24. H⁺ CO₃^2- Cl^- Ag⁺
Q2. Suppose that a 4-chemical problem is being studied: Ag⁺; Cl^-; CO₃Û^-; and H⁺. Chemical-1 is mixed with chemical-2, and bubbles are observed. List all possible combinations that remain for the unknown. A2. Possible combination in which the first two chemicals mix to produce bubbles: 17. CO₃^2- H⁺ Ag⁺ Cl^- 18. CO₃^2- H⁺ Cl^- Ag⁺ 23. H⁺ CO₃^2- Ag⁺ Cl^- 24. H⁺ CO₃^2- Cl^- Ag⁺
Q3. Suppose that a 4-chemical problem is being studied: Ag⁺; Cl^-; CO₃^2-; and H⁺. Chemical-1 is mixed with chemical-2, and no reaction is observed. List all possible combinations that remain for the unknown. A3. Possible combination in which the first two chemicals mix but produce no reaction: 3. Ag⁺ H⁺ CO₃^2- Cl^- 6. Ag⁺ H⁺ Cl^- CO₃^2- 8. Cl^- CO₃^2- H⁺ Ag⁺ 9. Cl^- H⁺ Ag⁺ CO₃^2- 11. Cl^- CO₃^2- Ag⁺ H⁺ 12. Cl^- H⁺ CO₃Û^- Ag⁺ 15. CO₃^2- Cl^- Ag⁺ H⁺ 16. CO₃^2- Cl^- H⁺ Ag⁺ 19. H⁺ Ag⁺ Cl^- CO₃^2- 20. H⁺ Ag⁺ CO₃^2- Cl^- 21. H⁺ Cl^- Ag⁺ CO₃^2- 22. H⁺ Cl^- CO₃^2- Ag⁺

Handout Ans.-

Not operational for web version

Key Words 1-

reaction, design of experiments

Elements-

Ag Cl C N H Br