Ions, Moles and Molecules, Oh My: Identification of an Unknown Substance

                                                                        modified by Koni Stone on September 6, 2005

In this experiment you will identify an unknown substance (see the attached list.)  You will determine several of its physical properties and chemical reactions and compare these to samples of known substances.

Procedure:
Use as many of the following tests as necessary to eliminate substances from the list of possible compounds. It is very important that you confirm your identification of your unknown by comparing it's properties to the properties of authentic samples. This is best done by carrying out simultaneous measurements when you have narrowed down the possible candidates to a few.

Appearance: This can be helpful in narrowing a list of many entries to a few. Look carefully at the shape and color of the material. Is it colorless cubes? The color and form are listed in the Handbook of Chemistry and Physics which is found in the lab.

Effect of Heating: [How to light a Bunsen burner] In general, we find that ionic compounds have high melting points while polar compounds have lower melting points and non-polar compounds have an even lower melting points. First, place a very small amount of the solid on a spatula tip and place in a flame in the hood. Does it burn? Organic compounds usually do, inorganic salts usually do not. If carbon black is a product of the reaction, an organic compound is likely. Does it melt? The flame temperature is between 600 and 800oC. Once you are convinced that the compound will melt at a reasonable temperature, measure the melting point.

Compounds of sodium give off a bright yellow color when placed in a flame. Potassium compounds give a violet flame when heated in a flame; this can be viewed through a blue cobalt glass. If you are not sure, try a known compound to see what the flame color looks like. Be sure the wire is clean and fairly free from color before testing your unknown.

Density: Although this is listed in the Handbook, it is somewhat harder to measure quickly. We can, however, get an approximate value easily. If a substance floats on water, it must be less dense than water. The density must be less than 1.0 g/mL. By observing whether or not a substance floats or sinks in several liquids of different densities, we can narrow down the range of values of the density of the unknown and then compare to those listed. You should try to use a liquid which does not dissolve your unknown.

Solubility in water, acid and base:  Most sodium and potassium salts are soluble in water, a very polar solvent.

NaCl + H2O ---------> Na+(aq) + Cl-(aq)

KBr + H2O ---------> K+(aq) + Br-(aq)

Salts of organic acids are also soluble. Soaps are soluble in water because they are sodium and potassium salts.

e.g., H37C18COO-Na+(s) + H2O(l) ---------> H37C18CO2-1(aq) + Na+1(aq)

Organic compounds with about as many OH groups as carbons are generally soluble as well. The solubilities of compounds in several solvents are listed in the Handbook. Try to dissolve your unknown in water. Try a nonpolar solvent like hexane. Record your results.

We often find interesting results when we add dilute acid (1 M HCl) to a substance. Carbonate (and bicarbonate) salts bubble and evolve carbon dioxide.

e.g., Na2CO3 + 2 H+ --------> 2 Na+ + H2O + CO2(g)

Salts of organic acids form the acid by the equation:

CH3CO2- (acetate ion) + H+ ----------> CH3CO2H (acetic acid - smells like vinegar)

Many organic acids are insoluble in water so if we add acid (1 M HCl) to the corresponding salts, an insoluble material forms.

e.g., H37C18COO-Na+(aq) + H+(aq) ---------> H37C18COOH(s) + Na+(aq)

Adding base (1 M NaOH) to a sample reverses the effect of the acid. This means that an insoluble organic acid will dissolve when the base neutralizes the acid.

H37C18COOH(s) + NaOH(aq) ---------> H37C18COO-(aq) + Na+(aq) + H2O(l)

Benzoic acid and sodium benzoate are good examples of substances which can be distinguished using these reactions.

Ammonium compounds will react with base to generate ammonia gas (smell carefully).

e.g., NH4+(aq) + OH-(aq) --------> NH3(g) + H2O

Measure the solubility of your sample in water and the acid and base provided. What can you conclude?  The solubility values are at the end of this lab.     

Measure the pH of a solution of your unknown in pure distilled water. Acids will have a pH less than 7.0 and salts of certain acids will have pH's greater than 7.0. In particular, carbonates and bicarbonates are basic in solution and will thus have pH's of greater than 7. Solutions of carbonates will have higher pH's than solutions with equal concentration of bicarbonate. Test your unknown alongside known substances. Use exactly the same amounts of the known compounds as your unknown compound.  Do the tests side by side. Also test the deionized water alone.

Data:  Arrange your data in a table.  See the example below:

Test Result-what did you observe, measure What does this mean? Comments
Solubility in deionized water. flakes floated and then dissolved. My unknown is water soluble.   My unknown is less dense than water, because it floated.
Density compared to hexane. It sank. My unknown is more dense than hexane.  
Flame test       
pH      
Solubility in NaOH      

Results:
Summarize the results by making a table of the properties of your unknown and those of the likely compounds from the accompanying list.

Conclusion:

Restate your unknown number and what you determined it to be.  Support your conclusion by discussing your results.  Convince the reader that your identification is correct.

- - - - - - - - - -

Waste Disposal: Please use the following table to determine how to dispose of reagents and waste.

hexane solvents container
cyclohexane solvents container
HCl (0.1 M) sink, with running water
NaOH (0.1 M) sink, with running water
excess unknown trash
Table of Possible Unknowns
CHEM 1002
  Name MPoC solubility in water (g/100 ml)
1 Ammonium carbonate, (NH4)2CO3 d58* 100
2. Sodium carbonate, Na2CO3 851, -H2O,32 17
3. Potassium carbonate, K2CO3 891 112
4. Sodium bicarbonate, NaHCO3 - CO2, 270 7
5. Potassium bicarbonate, KHCO3 d100 22
6. Sodium acetate, Na+1[O2CCH3]-1 324, 3 H2O - 58 130
7. Ammonium acetate, [NH4]+1[O2CCH3]-1 114 150
8. Benzoic acid, C7H5O2H 122 0.27
9. Citric acid, HO2CCOH(CH2CO2H)2 -H2O, 70,153 133
10. Malonic acid, HO2CCH2CO2H 136 74
11. Urea, H2NCONH2 133 120
12. Dextrose (glucose), C6H12O6 146 84
13. Sucrose, C12H22O11 d186 >180
14. Sodium chloride, NaCl 801 209
15. Sodium benzoate, C6H5CO2-1Na+1 d198 20
16. Calcium carbonate, CaCO3 d825 0.0015
17. Sand ------- insoluble
* d = decompose -- note: For the table "-CO2, 270" means that at 270 oC the molecule reacts and produces a molecule of CO2.

last edited by Koni Stone on02/28/06