Identify Unknown Solutions

Question

You have been prepping for a chem. lab and the safety officer has discovered some
mysterious white solid on the floor of the corridor outside of the lab. She has asked you if
you could figure out what it is. Based on the exercises that were run in the lab recently,
you have narrowed it down to one of 4 possible compounds:

 

silver nitrate
salicylic acid
sodium bicarbonate
potassium chloride

 

How would you go about identifying which of the 4 chemicals it is? Assume you
have access to any chemicals you need.  show the
relevant chemical equations 

Answer 1

Examining what the following tests produce will identify the substance;

 

Thermal Decomposition of Silver Nitrate:

Silver nitrate(aq) decomposes(g) when heated:

2 AgNO3 → 2 Ag + O2 + 2 NO2

 

 The decomposition of silver nitrate produces elemental silver.

 

Thermal Decomposition of Sodium Bicarbonate:

 

Above 50 °C, sodium bicarbonate gradually becomes sodium carbonate, water and carbon dioxide. The conversion is quick at 200 °C

2 NaHCO3 → Na2CO3 + H2O + CO2

Continued heating converts the carbonate into the oxide (over 850°C)

Na2CO3 → Na2O + CO2

Mix the white Potassium Chloride powder in water containing Cl- ions:

 

Potassium Chloride can react chloride ion when mixed into water. it will precipitate insoluble AgCl(s)

KCl(aq) + AgNO3(aq) → AgCl(s) + KNO3(aq)

 

Decomposition of Salicylic acid by rapid heating produces  a white solid phenol and CO2 gas.

 

salicylic acid---> Phenol + CO2

 

 

Answer 2

This is probably NOT the answer you're supposed to give, but there's no need to actually make solutions in real life. Just grab a black light and an acid dropper bottle and some chemwipes and head out to the hallway. Salicylic acid should glow greenish under a black light. Silver nitrate will leave brown stains on basically anything you wipe it up with, probably the floor would already be stained. Bicarbonate will fizz if you put a drop of acid on it. If it doesn't do any of those 3 things, it's the KCl.

Answer 3

Although very good answers have been provided ( see other answers) I would like to propose a simple and fast procedure and of course a preferred instrumental approach if instrumentation is available.

First the instrumental procedure.
An emission plasma spectrophotometer capable of a simultaneous elemental determination will provide the identification and quantitation of all metals present, Ag, Na, and K.
salicylic acid will be the answer if none of the metals are present in a quantity sufficient to account for the mass of the white matter. Less expensively an atomic absorption spectrophotometer will also be satisfactory and more expensively a mass spectrometer will do a complete and accurate work.
Alternatively the anion can be determined rapidly and inexpensively using an automated colorimetric analyzer or an ion chromatograph. None of these technique, however, will determine the organic compound salicylic acid. If a direct determination of this compound is needed a high pressure liquid chromatograph or a gas chromatography apparatus will do it.
But if the high tech instrumentation is only a dream in your lab.
The following procedure is fast, inexpensive and accurate:

A. Dissolve about 500mg of the sample in 25ml of distilled water.
If the sample appears to be insoluble or slightly soluble, only the presence of salicylic acid is possible. This conclusion can be confirmed by decanting the water and adding acetone. The organic acid will dissolve rapidly.

B. If the sample is completely soluble in water, add a few ml of a 0.1 N HCl solution.
B1. The bicarbonate will effervesce with development of carbon dioxide:
NaHCO3 + HCl = CO2 (g) + NaCl

B2. The silver nitrate will react with the chloride ion with the formation of a white precipitate insoluble in acids:

AgNO3 + HCl + AgCl ( s) + HNO3
The presence of Ag can be confirmed by decanting and adding ammonium hydroxide the precipitate will dissolve completely because of a formation of silver –ammonia complex soluble in water:

AgCl (s) + 2NH3 = Ag (NH3 )2 Cl (aq)
B3. No reaction with HCl. Only KCl can be present. It can be confirmed with a flame test ( se answer of Elisabeth W. ).

Regards.
Edmondo C.

Answer 4

1. First  dissolve a sample of the unknown solid in water and measure pH with pH paper. Measure the pH of pure water as a control (this is your neutral).

 

NaHCO3 will be basic. Salicylic acid will be acidic. If the the solution is neutral, or rather, the pH of the pure water, you have either AgNO3 or KCl. 

 

 

2. Dissolve a sample of the unknown solid in water.

Test for the presence of Ag+ by adding a solution of NaCl in water to the unknown solution. If a white ppt forms, you have made AgCl, and this is positive for AgNO3:

 

AgNO3(aq) + NaCl -> AgCl(s) + NaNO3(aq)

 

If no ppt, then you have KCl. To confirm, test the sample for chloride by adding an aqueous solution of AgNO3:

 

KCl(aq) + AgNO3(aq) -> AgCl(s) + KNO3(aq)

 

To further confirm, perform a flame test with the unknown. Dissolve a sample in water, dip a Q-tip in the solution, place into the flame of a bunsen burner. K+ ion will produce a pale violet flame.

 

Hope this helps!