Spot tests are simple chemical procedures which uniquely identify a substance. They can be done on small samples, even microscopic samples of matter with no preliminary separation.

In a typical spot test, a drop of chemical reagent is added to a drop of an UNKNOWN mixture.
If the substance in question is present, it produces a chemical reaction characterized by one or more unique observables.

These observables can include.....  gas bubbling, color change, odor, precipitation.
Occasionally the presence of one substance will interfere with the identification of another.  This is referred to as an.....Interference.  Alternative or additional tests must be used to get correct results.
...

IDENTIFYING an UNKNOWN

1.

CO3-2

  adding H+ produces effervescence (CO2)
2. SO4-2

acidified Ba+2 produces a white precipitate

3. PO4-3

(NH4)2MoO4 + HNO3 produces a yellow precipitate

4. CrO4-2

acidified H2O2 produces a blue solution + O2

5. SCN-

acidic Fe+3 produces a blood red solution

6. Cl-

acidified Ag+ gives a white curdy precipitate

7. CH3CO2-

acidification produces a vinegar odor

8. NH4+

heating with base produces a basic gas (NH3)

NOTES  (numbers refer to above ions)

1.  No heat necessary. To see effervescence may require shaking the tube.  This effervescence will also be seen in acidifying UNKNOWN for other tests.
2.  Acidify to prevent precipitation of BaCO3, Ba3(PO4)2 or BaCrO4.
3.  Heat may be necessary.  Must form yellow precipitate! Yellow solution indicates only a trace of PO4-3.
4.   Very characteristic test!
5.  May require many drops of Fe+3  if ions such as PO4-3 are present.
6.  If SCN- is present it must first be destroyed by oxidizing with boiling HNO3.
7.  Use H2SO4 to acidify because it is less volatile than other strong acids.  Caution your nose can become less effective at recognizing the vinegar odor after smelling it for a while.
8.  Heat solution and test vapors with moist red litmus paper to confirm presence of NH3.  Caution: placing  litmus paper in the test liquid will turn it blue due to the NaOH present. 

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PLEASE READ:

  1. In filling out your DATA SHEET, particularly for your UNKNOWN, enter enough information so that you could reproduce the experiment without using your Manual.

  2. Save your UNKNOWN until you get a grade on this experiment. It does not matter if it evaporates. The UNKNOWN itself is nonvolatile.
    If there is a mistake in making up your UNKNOWN, it can be corrected but only if it exists and we can test it.

R.W..Kluiber 2/25/98, 7/31/2010