Experiment 6

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. This 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 sand and calcium carbonate are examples of these. If a substance dissolves, then it can't be sand or calcium carbonate (or another insoluble substance) and these can be eliminated from the list of unknowns. The reverse also applies.

Two other applications of solubility in water are 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.

Finally, if a solid unknown contains one insoluble compound and one soluble one, then by adding water, the two can be separated for subsequent analysis.

Tests for ions such as CO₃^-2, OH^-1 or Mg²⁺ can be designed similarly.

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 bicarbonate ions to form carbon dioxide gas which bubbles out of solution.

CO₃^-2(aq) + 2H⁺(aq) ---> H₂O + CO₂(g)

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.

A good test for ammonium compounds is to add a basic solution such as a solution of sodium carbonate. This causes evolution of ammonia gas (smell very cautiously). The results of this test can be confirmed by placing wet pH paper above the solution. Ammonia gas will cause the pH paper to change color because of the high pH.

The Soap Test. 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 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 carbonate (CO₃^-2) being the most basic. Test solutions of known compounds to compare with your unknown.

How to light a Bunsen burner

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., sand 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. Bicarbonates (for example, NaHCO₃) often "sputter" off of the spatula. Again, comparison with known substances is especially helpful.

It is convenient and helpful in many cases to measure thet 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.

Flame tests. Several of the positive ions can be detected and identified by the color they emit when placed in a flame. Dip a ware attached to a glass rod 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⁺¹ -- persistent yellow

K⁺¹ -- 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

comments to: j byrd jim@chem.csustan.edu or m perona mike@chem.csustan.edu

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This page was last modified February 03, 2004
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