western
Physics and Astronomy

                                   
 
 
Mystery Solutions
Objective

The team will be given twenty minutes to identify the ion type in each vial.

Participants A team of up to six members.
Caution This event requires signiicant training and practise beforehand. Do not enter this event without any preparation, because you will have a poor experience, and will waste the chmeicals that have been prepared for you.
Materials Teams will be provided with a specified set of testing reagents in dropper bottles, any necessary glassware, equipment and a flame source. Eight unknown solutions will be provided in separate numbered test tubes.
Dress Code Students who do not meet the dress code will not be allowed to participate.
Students must:
  1. wear long pants.
  2. have no exposed midriffs.
  3. have socks and shoes that completely cover their feet.

We provide the safety glasses.

Review the section on Safety below.
Rules
THE EVENT

The team will be given twenty minutes to identify the ion type in each test tube.

The team will have 5 minutes at the end of the event to clean their glassware and workstation.

Students may bring into the laboratory:
  • one 8.5"x 11" sheet of paper with any notes they wish written using both sides, either typed or handwritten.
  • the Rules for the event.

UNKNOWNS:

Unknown solutions in three categories identified as

a) anion only (3 unknowns)

b) cation only (3 unknowns)

c) both cation and anion (2 unknowns)

Teams will be given about 15 mL of each solution and the individual tests should be run on samples of about 1 mL. The test tubes provided are 18 by 150 mm. 1 mL will be just a bit less than 1 cm in the bottom of a test tube. Fresh solution should be used for each new test. Once all of the reagents are added to the unknown in the test tube, shake well to mix.

1. ANION UNKNOWNS

You will be provided with about 15 mL of 3 solutions and you will be asked to determine the anion present in each. Each of these anions will be one of acetate, bromide, carbonate, chloride, iodide, or phosphate as their sodium salt as shown below.

sodium acetate CH3CO2Na
sodium bromide NaBr
sodium carbonate Na2CO3
sodium chlorine NaCl
sodium iodide NaI
sodium phosphate Na3PO4

It is up to you to devise the quickest scheme to do this using the reagents and procedures which follow :


Cl      Br        I

Solution is mildly acidic. pH 4 - 6

 

CH3CO2      CO3 2–      PO4 3–

Solution is somewhat basic, pH = 7-10

The anions can be conveniently broken into 2 categories based upon the pH of the solution when they dissolve. Use pH paper, not litmus paper, if available.

Individual anions

CHLORIDE Cl

Add about 1 mL of silver nitrate (AgNO3) solution. A white precipitate of AgCl forms.

BROMIDE Br

Add about 1 mL of a silver nitrate solution. A cream coloured precipitate of AgBr forms.

IODIDE I

Add about 1 mL of silver nitrate solution. A yellow coloured precipitate of AgI forms.

Note: The three halides (chloride, bromide and iodide) all give precipitates with silver ions but these can be distinguished by their colours. With a little practice you should have no trouble telling these apart.

To further distinguish between bromide and iodide:
Add about 1 mL of paraffin oil and 1 mL of chlorine water. An orange colour in the paraffin oil layer indicates bromide; a violet colour indicates iodide.

ACETATE CH3CO 2

This solution will have a pH of about 7. Add about 1 mL of silver nitrate solution. A white precipitate will form slowly. This precipitate will redissolve upon the addition of about 1 mL of a nitric acid solution (HNO3). The halide's precipitate will not redissolve.

CARBONATE CO32–

Add about 1 mL of a nitric acid solution and look for the formation of bubbles of carbon dioxide (CO2).

PHOSPHATE PO43–

Add 1 mL of nitric acid and warm almost to boiling. (Set the test tube into the beaker of warm water sitting on the hot plate.) Then add 1 mL of ammonium molybdate solution [(NH4)2MoO4]. A yellow coloured precipitate will form.

2. CATION UNKNOWNS

You will be provided with about 15 mL of 3 solutions and you will be asked to determine the cation present in each. Each of these cations will be present as either the chloride or nitrate as listed below.

aluminum chloride AlCl3
ammonium chloride NH4Cl
barium nitrate Ba(NO3)2
copper (II) nitrate Cu(NO3)2
iron (III) nitrate Fe(NO3)3
sodium chloride NaCl
strontium nitrate Sr(NO3)2
zinc chloride ZnCl2

Your task is to identify the specific cation present in each solution as quickly as possible using the reagents and procedures which follow:

ALUMINUM Al 3+

Add a few drops of a sodium hydroxide solution (NaOH). A white precipitate will form. Then add about 1 mL of an ammonia solution, NH3 (really ammonium hydroxide, NH4OH). The white precipitate remains. Compare this result with the zinc ion.

AMMONIUM NH4+

Add about 1 mL of a sodium hydroxide solution. Warm the test tube in the hot water bath provided. Immediately test the vapours emitted from the test tube with moist litmus paper. (Hold the litmus paper with tweezers.) A basic pH will be observed - the red litmus paper will turn blue.

BARIUM Ba 2+

Add about 1 mL of a sodium sulfate solution (Na2SO4). A white precipitate will form. Both Ba2+ and Sr2+ give this result. Barium ions can be confirmed using a FLAME TEST (See below).

COPPER (II) Cu 2+

Add a few drops of a sodium hydroxide solution. A gelatinous pale blue precipitate will form. Then add about 1 mL of an ammonia solution. The precipitate dissolves and the solution becomes deep blue in colour. Copper (II) can also be confirmed by a flame test.

IRON (III) Fe 3+

Add a few drops of a dilute sodium hydroxide solution. A copious brown precipitate should be observed.

SODIUM Na +

There is no simple test for sodium ions. Use a flame test.

STRONTIUM Sr 2+

Add about 1 mL of a sodium sulfate solution. A white precipitate will form. Both Ba2+ and Sr2+ give this result. However, Sr2+ gives a flame test quite different than Ba2+.

ZINC Zn 2+

Add a few drops of a sodium hydroxide solution. A white precipitate will form. Then add about 1 mL of an ammonia solution. The white precipitate will redissolve. Compare this result with the aluminum ion.

3. COMPOUND UNKNOWNS (BOTH CATION AND ANION)

You will be given about 15 mL of 2 solutions and you will be asked to identify both the cation and the anion present in each and thus deduce the identity of the compound that has been dissolved.
The two compounds will be composed of the cations and anions from the previous tests. You should use the tests presented earlier for the anions and cations separately.

4. FLAME TESTS FOR CATION IDENTIFICATION

Soak a wooden splint in the solution so that some of the solution adheres to it. Adjust the bunsen burner/butane torch so as to produce a blue flame. Apply the unknown solution to the tip of the blue flame. Do not burn the splint as this will affect the colour of the flame.

RESULTS

Barium Green
Copper Green
Sodium Intense yellow/orange (colours whole flame)
Strontium Bright red

You must try these tests with known compounds in order to become familiar with the colours, as words cannot describe the colours adequately.

 

5. A LIST OF REAGENT SOLUTIONS AND NECESSARY APPARATUS

The following is a list of reagent solutions and apparatus that will be available on the day of the contest. You should design your schemes with this in mind. If you have a problem locating specific reagents, do not hesitate to contact the coordinator.

REAGENT SOLUTIONS

Ammonium hydroxide 6.0 M
Ammonium molybdate 0.5 M
Chlorine water Cl2 / water
Nitric acid 6.0 M
Silver nitrate 0.2 M
Sodium sulfate 1.0 M
Sodium hydroxide 1.0 M
Paraffin Oil  

APPARATUS

  • Test tubes and test tube rack
  • Glass stir rods and pH paper
  • Red litmus paper and tweezers
  • Bunsen burner/Butane torch
  • Hot Plate with beaker
  • Wooden splints

Disposable pipettes not needed - for each test simply pour the unknown into the test tube. Remember, the test tubes provided are 18 by 150 mm. 1 mL will be just a bit less than 1 cm in the bottom of a test tube.

6. PREPARATION INSTRUCTIONS FOR REAGENTS AND UNKNOWNS

After you develop a scheme and procedure for analyzing the possible unknowns you will receive, it wouldn't hurt at all to practice. This means that you must prepare your own unknown test solutions and reagent solutions. The following list should prove helpful. About 10 mL of each of these should prove more than sufficient for any practice round.

REAGENTS

Ammonium hydroxide 6 M

Add about 4 mL of concentrated ammonium hydroxide to 6 mL of distilled water. Be careful with concentrated ammonium hydroxide for it can burn you and put holes in your clothes.

Ammonium molybdate 0.5 M

Add about 1 gram of the solid to about 10 mL of distilled water.

Cl2 water

This must made and used in a fumehood, so students should wait and try it at the competition.

Nitric acid 6 M

Add about 4 mL of concentrated nitric acid to 6 mL of distilled water. Be careful with concentrated nitric acid for it can burn you and put holes in your clothes.

Silver nitrate 0.2 M

Add about 0.4 grams to 10 mL of distilled water. If this solution is cloudy add a few drops of dilute nitric acid. Don't get this solution on your hands as they will turn black.

Sodium hydroxide 1 M

Add about 0.4 grams of sodium hydroxide pellets to 10 mL of distilled water. The pellets may be a little slow to dissolve but just give it time.

Sodium sulfate 1 M

Add about 1.4 grams of solid to 10 mL of distilled water.

Unknowns

To make solutions of your unknowns, add about 0.5 grams of solid to 10 mL of distilled water. The solids may be a little slow to dissolve. A little shaking, stirring, and gentle warming will help but so will patience.

The following compounds are recommended for making up unknowns but other soluble salts can be used.

Aluminum chloride
Ammonium chloride
Barium nitrate
Copper (II) nitrate
Iron (III) nitrate
Sodium acetate
Sodium bromide
Sodium carbonate
Sodium chloride
Sodium iodide
Sodium phosphate
Strontium nitrate
Zinc chloride

Safety

Safety is the first priority in all lab work.

The University safety policy, devised by Occupational Health and Safety, mandates “maximal skin coverage” for all laboratory personnel, including students. Ankle-length pants and socks that cover the ankle are required. No shorts or capris are permitted. Exposed midriffs are not allowed. Long hair must be tied back. Shoes must completely cover your feet: toes, sides, heel and the entire upper foot. Running shoes are generally acceptable. Sandals, open-toed shoes, ballerina shoes, open-top shoes, and shoes with cut-outs or openings are not acceptable.

Safety glasses will be provided. Make sure you wear safety glasses at all times in the laboratory.

Read and understand all instructions prior to starting the procedure. If you do not understand anything ask the supervisor.

If you spill any materials or break glassware contact the supervisor before attempting a cleanup.

Do not carry out unauthorized experiments.

Judging

Teams will be judged on the number of anions and cations determined correctly.

Sponsor Department of Chemistry, Western University.
Source London District Science Olympics. This event was created by Duncan Hunter. It has been updated by Sandra Zakaria Holtslag.

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