Title:
The Identification of Unknown Samples
Procedure:
1.
All the samples were divided into solids and liquids and their colours were
noted. The characteristic smell of the NH4OH solution made it extremely easy
to identify, even at this point (colourless)
2.
There were three coloured compounds, 1 blue and 2 black. The blue
compound was identified as CuSO4 and was confirmed by giving a blue
solution when dissolved in water., and then also reacting it with NH4OH
giving a green and then a dark blue solution.
3.
Each of the black compounds were placed into 3 respective test-tubes. The 3
remaining liquids were then added to a solution of each. No reaction was
observed in 3 of the 6 test-tubes. The reaction in the sixth tube was between
CuO + H2SO4  giving a green/blue colour. MnO2 was thus the other black
compound (acid insoluble).
4.
The solubility of the remaining 10 solids was then recorded. It was noted that
2 of the 10 solids were insoluble. These were Al2O3 and ZnCO3. Al2O3 is
insoluble in acid and ZnCO3 is very soluble with CO2 evolution.
Effervescence was observed. (Acid used = H2SO4)
5.
The remaining 2 unidentified liquids were used to test the solubility of the 8
remaining solids. In one, all the solid dissolved and this was identified as H2O.
Thus the last liquid was AgNO3 (note silver specks when held up to light)
6.
The reaction of AgNO3 with the solids yielded 2 yellow precipitates, KI and a
white precipitate BaCl2. Confirmation of these results was obtained from the
flame tests. KI  violet colour. BaCl2  green colour
7.
At this point the other compounds were also flame tested with Ca(NO3)2
identified by a red colour.
8.
The two sulphur compounds were readily identified by their reaction with
H2SO4. The smell of the sulphur was acute and a yellow ppt was observed.
The Na2S2O4 was differentiated from Na2S2O4 (red). The Na2S2O3 had a
slower rate of reaction because it has 1 oxygen atom less.
9.
Distinguishing MgSO4 and (NH4)2SO4 was attempted by reacting them with a
solution of CuSO4 but no reaction was observed. No reaction occurred either
with Na2CO3, but both formed precipitates with BaCl2. Both are insoluble in
H2SO4. Reaction also took place with NH4OH and MgSO4 ( formed a ppt ).
No reaction was observed with (NH4)2SO4
Last Reactions:
(NH4)2SO4 (s) + 2 NH4OH (l)  (NH4)2SO4 (l) + 2 NH4OH (l)
MgSO4 (s) + 2 NH4OH(l)  Mg (OH )2 (s) + (NH4)2 SO4 (l)
This reaction distinguished between the 2 compounds because all of the NH4+
compounds are soluble always. However, not all Mg2+ compounds are and
Mg(OH)2 is only sparingly soluble.
Identification:
Na2CO3 + H2SO4  Na2SO4 + H2O + CO2
Pre-practical questions:
1.
A precipitation reaction is a reaction between 2 solutions in which an insoluble solid is
formed as the precipitate.
e.g.
BaCl2 + K2CrO4  BaCrO4 () + 2KCl
2.
Solubility decline in the order MgSO4, CaSO4, BaSO4.
The cations get bigger from left to right and hydration enthalpies are also lower.
The surrounding hydrate sphere of magnesium is strong due to hydrogen bonding while
Ca and Ba have a weaker second sphere.
3.
Ag NO3 + e-  Ag +NO3-
AgCl – no colour, soluble
AgI – bright yellow colour, soluble
AgBr – yellow colour, insoluble
4.
Bench AgNO3 (aq) is stored in a brown bottle because it is photosensitive. A standard
concentration cannot be maintained otherwise.
5.
Groups 1 and 2 are grey or light coloured.
The transition metal compounds are brightly coloured due to the metal to ligand covalent
bonding. M-L + L-M
charge transfers are in the visible spectrum.
--------------------------Paul Walsh – 2001
pwalsht@maths.tcd.ie