Purpose

You are the first to arrive at the crime scene.  There is a mysterious white powder by the base of the left foot of the body.  What is it? Is it a pure substance or a mixture?

In this experiment you will determine the composition of the unknown substance that was collected at the scene of the crime. The possible substances are on the list shown in the Procedure section.  Did the victim choke on antiperspirant or was the death caused by baking powder mixed in chalk?  Please note: each person will have their own unknown but you are encouraged to work as a team.

References

 Chemistry, 4^th ed, by J. McMurry and R. Fay. Prentice Hall, 2004, Sections 2.8 - 2.10, 4.3 - 4.5, 15.6 - 15.7

Background

The method used to separate and identify the unknown substances is to perform a series of tests on the unknowns and compare the results of those tests with results of the same tests performed on known samples and/or comparison with properties reported for these known substances. More detailed descriptions of these tests are found in the following paragraphs.

Solubility  Chemical compounds have a wide range of solubilities in distilled water and water containing acids or bases. The solubility can be tested by adding a small amount of solid sample to a small amount of solvent in a test tube and stirring. If solid remains then we would say the substance is insoluble or only slightly soluble in that solvent. Note in the list of chemicals attached, substances such as calcium carbonate are examples of these insoluble substances. If a substance dissolves, then it can't be calcium carbonate (or some other insoluble substance) and these can be eliminated from the list of possible compounds. The reverse also applies. That is, if the substance doesn't dissolve, then it can't be one of the soluble compounds.  

A good rule of thumb is that if the solubility of a substance is less than 0.5 g/100 mL, then it can be considered insoluble.  A good way to test solubility is to add about 0.05 gram of the substance to 10 mL of water in a test tube and stir to see if it all dissolves.  You can warm it gently to speed up the process but check it at room temperature since the solubility depends on temperature.  Do not test in 100mL of water!

Two other applications of solubility in water are also important. Soluble ionic compounds are separated ions in solution; however, if two ions which can combine to form an insoluble solid are present in the same solution, the solid will form. For example, calcium chloride dissolved in water is present as Ca⁺²(aq) and Cl^-1(aq) ions which are formed according to the reaction:

CaCl₂•H₂O ---> Ca⁺²(aq) + 2Cl^-1(aq)

and for sodium sulfate:

Na₂SO₄•H₂O ---> 2Na⁺¹(aq) + SO₄^-2(aq)

If the two solutions are mixed, the ions which can form the insoluble compound CaSO₄ are present and it forms as a precipitate:

Ca⁺²(aq) + SO₄^-2(aq) ---> CaSO₄(s)

This can be used as a test for the presence of either Ca²⁺ or SO₄^-2 as ions in a solution. To a solution which may contain SO₄^-2, add some CaCl₂ dissolved in water. If a precipitate forms, then an ion which forms an insoluble calcium salt (e.g., SO₄^-2 or CO₃^-2) is present.  Tests for ions such as CO₃^-2, OH^-1 or Mg²⁺ can be designed similarly.  By adding a solution containing appropriate ions to a solution of an unknown substance, the presence or absence of a substance can be indicated.

If a solution contains a soluble ionic compound it will conduct electricity.  You can easily test this with a conductometer.  Be sure the electrodes are clean before you do this test. 

Finally, if a solid unknown contains one insoluble compound and one soluble one, i.e., a mixture, then by adding water, the two can be separated for subsequent analysis.  The soluble substance will be in the aqueous solution and the insoluble substance will be  visible as a solid.

Effect of adding acid or base. A simple test for the presence of CO₃^-2 or HCO₃^-1 is to add an acid such as acetic acid. This reacts with the carbonate or hydrogen carbonate ions to form carbon dioxide gas which bubbles out of solution.

Many insoluble carbonate compounds (e.g., CaCO₃) will dissolve and evolve CO₂ gas when the solid is sprinkled into a beaker containing vinegar (dilute acetic acid) or another acid. Test insoluble solids in this way.  Many insoluble compounds containing hydroxide ions are, not surprisingly soluble in acids.

If a basic solution is added to a solution containing ammonium ion, ammonia gas is evolved.  Its presence can be confirmed by carefully testing for the characteristic odor. 

The Soap Test. In hard water, soap does not foam and a precipitate is formed with the positive ions present in the hard water (Ca²⁺ and Mg²⁺ ). Add a soap solution to your unknown solution in a test tube. Test for foaming by shaking and look for cloudiness caused by precipitation. A clear solution which has foam indicates the absence of Ca²⁺ and Mg²⁺ ions. This test is best used to confirm the results of other tests.

Solution pH. The acidity of a solution can be measured with pH paper. Use a stirring rod to deposit a drop of the solution to be tested on a small piece of pH paper. Compare the color to the pH paper color chart to determine pH. A high pH (>7) indicates a basic solution. The higher the pH, the more basic. Carbonate- and bicarbonate-containing solutions are basic with the carbonates being the most basic. It is necessary that a sample be soluble in water to use this test. Compare solutions of known compounds with solutions of your unknown at similar concentrations.

Effect of Heat. The solid unknowns have a wide range of melting or decomposition temperatures (see Table). Place a small amount of unknown solid on the tip of a clean spatula and put in the flame of a Bunsen burner. Compounds which do not decompose and have a high melting point (e.g., KCl or NaCl) will be unchanged in the flame. You can often detect the decomposition of compounds such as NaHCO₃ or Ca(OH)₂ by the changes which occur. Carbonate compounds (for example, Na₂CO₃) often "sputter" off of the spatula. Sugars will burn with a characteristic odor.  Again, comparison with known substances is especially helpful.

It is convenient and helpful in many cases to measure the temperature at which melting occurs and to compare this value with the with the values found for known compounds. This can be done using the melting point apparatus as demonstrated by your instructor.  It is only practical if the substance melts below about 400 ºC.

Flame tests. Several of the positive ions can be detected and identified by the color they emit when placed in a flame. Dip a Nichrome wire into a beaker of water and place in a flame until it glows red-orange. Do this several times until the wire is clean. Dip it into a concentrated solution of either a known or unknown substance. Observe the color of the flame when the liquid is evaporating and compare with the list below.

Na⁺ -- persistent yellow

K⁺ -- violet, purple-red through blue glass (this color is masked by sodium so the wire must be clean)

Ca⁺² -- yellowish red, greenish through blue glass

NH₄⁺ -- feeble green

          Li⁺  --  carmine red, violet through blue glass

         Al^{+3 --    colorless}

Procedure

This experiment will e done twice. On the first day, your unknown will be a single compound.  On the second day, you will have a different unknown, this time, a mixture of two compounds.  

Obtain an unknown for each day and record the sample ID code in you notebook.  No one else knows which unknown you have, so, without an ID, you can not get credit for this experiment.  Be sure to note if it is a single compound or a mixture.

If your unknown is a single compound, you can apply the tests described above in any order that you wish.

If your unknown is a mixture of two compounds, you must separate the two in order to test pure samples of each.  Add water to some of your sample.  If a solid remains then this is an insoluble component of your unknown,  one.  If there is a soluble component, it is in the supernatant solution.  You should test the insoluble substance and the supernatant solution separately.   You can also test a dry sample of the mixture to identify likely candidates.  Remember, you can isolate a soluble substance from an aqueous solution, as a solid, by gently boiling the solution to remove the water by evaporation.  If the supernatant contains a soluble ionic substance, the solution will conduct electricity. Introduction to Basic Functions in Spreadsheet

Assuming this unknown contains two compounds that differ greatly in their solubility in water, the first step in your analysis is to use this to separate the two compounds. Take approximately one-third of your unknown sample and put it into a medium test tube. Add water and mix thoroughly using a stirring rod. Centrifuge this mixture and remove and save the clear solution. This is the soluble component of your unknown. Wash the remaining solid by adding water and again mix thoroughly and centrifuge. Remove the clear liquid and discard it. Repeat this washing step at least one more time. The remaining solid should be a pure sample of the insoluble component of your unknown.  This washing step is really important because if your solid sample is contaminated with the solution containing the soluble unknown, you will get very misleading evidence.

Use a combination of tests from above to narrow the possibilities, by elimination, to a small, more manageable, set that you can compare to authentic samples.  Use a flowchart to keep track of the compounds you  have eliminated with your tests and those that remain as possibilities.

Candidates for Unknowns

Results

Record the results for each of the tests on your unknown samples along with the results for the authentic compounds when tests on these were also done. You will need the record of these results to help to eliminate the unlikely compounds and to refer to in the Conclusion section of your write-up of this experiment.  Write a balanced equation and a balanced net ionic reaction for each test that involves a chemical reaction.  The more confirmatory results, the better your conclusions would hold up in court.

Conclusions

Be certain to include the unknown number.  For your conclusions identify the unknown substances in each of your samples.   Explain your conclusions by referencing the data in your Results tables.  This is important!  Your grade depends on:

The more clearly (and neatly) you present your concluding arguments, the easier it will be to convince a jury (and your instructor) of the composition of your unknown sample.  The best way to support your conclusion is to refer to a neatly drawn flowchart of your results.

09.28.05