CHEM 1112: Separation and Identification of Unknown Substances

In this experiment you will determine the identity of several unknown substances. Each student will have his or her own unknowns. The first unknown will be a pure compound. It will have a cation and an anion or it will be sand. The second unknown will be a mixture of two compounds. One compound of the mixture is soluble in water (hence, it will have a cation and an anion) and the other is not (it may be sand or a cation and an anion).

Wastes - Solutions and soluble compounds can be run down the drain in this experiment - be sure the cold water is running for your safety in case acids are present. Solids can be put in the trash.

The method used to identify the unknown substances is to perform a series of tests on small samples of your 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 and you may find it useful to create a flow chart to help organize these analyses. 

One of the first tests on an unknown material is a test of its solubility in water. Ionic compounds that dissolve in water dissociate to form ions in solution. Identification of the compound can be achieved by separate analyses of the cations and anions using various chemical tests.

Compounds that do not dissolve in water are called insoluble. Generally, however, these materials can be dissolved in acidic or basic solutions.  Add some calcium carbonate to ~1 ml of 1M hydrochloric acid. Then add calcium carbonate to 1M sodium hydroxide.  Add calcium sulfate to 1M hydrochloric acid and then add some calcium sulfate to sodium hydroxide.  Then add some sand to 1M hydrochloric acid and to 1M sodium hydroxide.  Record your observations.  Write the net ionic equations for any reactions that occur.

Mixtures containing a soluble compound with an insoluble compound must be separated before further analysis can be done. Separation can easily be achieved by dissolving the soluble compound in water. The insoluble compound will remain at the bottom in solid form and the soluble compound can be removed by decanting the liquid into a separate test tube. Use a centrifuge to speed up this process.  The insoluble compound should be washed with water several times to remove all traces of soluble compound before subsequent analysis.  

Generally, cations and anions are identified separately using various chemical tests that rely on the differences in solubilities between various materials. For example, suppose an unknown contains either calcium or magnesium cations. To identify which cation is present, a solution of sodium sulfate can be added to the unknown. The solubility of calcium sulfate is 0.3 g/100 ml whereas, the solubility of magnesium sulfate is 71 g/100 ml.

Explore the following mixtures of soluble compounds.  Some will form precipitates and this information can be used to develop a plan to determine what ions you have in your unknown.

Make (and label) about 10 mL of a 0.5M solution of each of the following solutions:

Label each solution. Mix equal amounts (~1 mL, use pipets) of each solution and record what happens. Write balanced net ionic equations for each reaction that occurs.  You may want to make a chart to keep track of what solutions you have combined.

3. Solution pH

The acidity of a solution can be measured with pH paper. Use a stirring rod to deposit a drop of the solution onto a piece of pH paper. A high pH indicates a basic solution, characteristic of carbonates and bicarbonates. Which is more basic? Carbonate or bicarbonates? Why?  Test the pH of sodium bicarbonate and sodium carbonate. What is the pH of a sodium chloride solution? Sodium sulfate? Sodium acetate? Test the pH of all of the solutions that you have made. Why do you need to make sure that they are all the same molarity?

Write the balanced equations for the acid/base reactions.

4. Addition of Acid or Base

Add a small amount of calcium carbonate (solid) to a 1M solution of hydrochloric acid. What happens? Write the balanced net reaction.  Add sodium sulfate to 1M hydrochloric acid.  What happens?

All ammonium compounds are soluble and a good test for the presence of ammonium ion is the addition of sodium carbonate. This causes the evolution of ammonia gas which can be detected by placing wet pH paper above the solution.  Ammonia gas will cause pH paper to change color to indicate a change in pH. What color does the paper turn when you add a pinch of sodium carbonate to 1 mL of a 1M solution of ammonium acetate? Write the balanced net ionic reaction.

5. Flame Tests [How To Light A Bunsen Burner]

Several of the cations in these unknowns can be detected and identified by the color they emit when placed in a flame.  Dip a wire into a beaker of acid and place in a flame until it glows red-orange. Do this several times until the wire is clean. Dip it into a very 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. Test sodium compounds last because sodium is difficult to remove from the wire.

6. Soap test for hard water

In hard water, soap does not foam and also a precipitate is formed with the positive ions present in the hard water (Ca²⁺ and Mg²⁺ ). Add a soap solution to a small amount of your calcium chloride, magnesium chloride and sodium chloride solutions.  Test for foaming by shaking each test tube 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.  The abbreviated formula for soap is RCOO^-, write a balance net ionic equation for the precipitation reactions.

 7. Melting Points / Effect of Heat

If you suspect that your unknown is one of the compounds on the attached list with a melting point below 400 °C, you can use the melting point apparatus in the lab to verify the melting temperatures of your unknown and compare with the known compounds. Other compounds that have relatively high melting points or decomposition temperatures can be placed in a flame to observe any changes. Sand and sodium chloride will be unchanged. Bicarbonates often "sputter" off the spatula.

8. Some Further Comments

It would be very unusual for any single test to identify an unknown uniquely. Be certain to use other tests to confirm your identification if it is possible. If the tests do not permit you to distinguish between two or more choices, then explain this in your conclusion.

Make a table of the results and observations from your tests on both known and unknown compounds so that you can compare results and eliminate those substances which you know can't be present. It may be helpful to design a flow chart, so that you can eliminate or confirm possibilities to aid you in the decision making process. Each student will have their own unknown.  You must show a balanced net ionic equation for every reaction.

For your conclusions identify the unknown substances. Explain your conclusions by referring to the data in your Results section. Be certain to include the identification numbers that you get from the stockroom.

TABLE: Data for Unknown Substances*

"d" and "dec" mean decomposes. "subl" means sublimes. "NA" means Not Applicable and Not Available.

*Extracted from the 1975 Handbook Of Chemistry And Physics 56^th Edition by John Burt, April 2004. Additional information about the compounds can be found there.

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Last edited 11/30/2004 by Koni Stone