regognition reactions on ions
NH4X(s) + NaOH(aq) (NH4OH) + NaX(aq)
(NH4OH) = NH3.H2O NH3(g) + H2O
The ammonia gas only escapes at heating, and can then be detected with humid litmuspaper (turns blue) or with HCl(g) where a white smoke can be observed of NH4Cl(s).
MCO3(s) + H3O+(aq) M+(aq) + (H2CO3)
(H2CO3) = H2O + CO2 H2O + CO2(g)
The carbondioxyde can be detected, leading the gas through lime water or a solution of Ba(OH)2:
CO2 + Ca(OH)2(aq) CaCO3(s) + H2O
The CaCO3(s) is white trouble.
MS(s) or MS2(aq) + H3O+(aq) M+(aq) + H2O + H2S(g)(bad smell!)
H2S-gas can be regognized with the smell, but also with a reaction with Lead acetate paper (white becomes black):
PbAc2(s) + H2S(g) PbS(s)(black) + 2HAc
MSO3(s) of SO32-(aq) + H3O+(aq) M+(aq) + (H2SO3)
(H2SO3) H2O + SO2(g)
SO2(g) can change the color of a solution of dichromate in acid environment from orange to green.
sulfite ions also can cause the same color change.
Ag+(aq) and Cl-(aq) regognize each other: AgCl(s)(white)
The appearing silver chloride (s) dissolves again in ammonia, producing the ion: silver diammoniun ion:
AgCl(s) + 2NH3(aq) Ag(NH3)2+(colorless) + Cl-(aq)
In the same way, also a bad soluble Copper(II)compound reacts; the Copper(II)ammonium complex is however dark blue.
Cu2+(aq) + 2NaOH(aq) Cu(OH)2(s)(pale blue) + 2Na+(aq)
Cu(OH)2(s) + 4NH3(aq) Cu(NH3)42+(aq)(dark blue)
Br-(aq) and I-(aq) form with Ag+(aq) again the precipitate, just like chloride ions.
A difference however is that AgBr and AgI are not white, but (pale)yellow.
Iron(II) and Iron(III) ions can be detected in the appearance of Berlins Blue(s)
Fe2+ + [Fe(CN)6]3- + K+ KFe[Fe(CN)6](s)
Fe3+ + [Fe(CN)6]42- + K+ KFe[Fe(CN)6](s)
Lead(II) ions (Pb2+(aq)) often are detected in diluted hydrochloric acid. Leas(II)chloride dissolves rather badly at room temperature in water, but rather well at 100ºC.
So the first formed precipitate dissolves at heating.
Nitrate ions can be detected with the 'Brown Ring reaction'.
The investigated liquid (about 2 ml) must first be mixed with a same volume of concentrated sulfuric acid. (be carefull please, and cool down the mixture).
Then in another test tube a saturated solution of Iron(II)sulfate is prepared. This solution is very carefully 'layd upon' the acid solution.
If nitrate was present, then in between the two solutions shows up a brown ring.