Tests involving the reagent aqueous NaOH solution TABLE 1. COLOURED SOLUTIONSIon First addition of reagent Reagent in excess CommentsPale green (Fe 2+)

Dirty green ppt Fe2+ (aq) + 2OH- (aq) à Fe(OH)2 (s)

No further change, however at the top of the mixture it may turn brown on standingThis is due to oxidation of iron(II) hydroxide to iron(III) hydroxide

 

Yellow (Fe3+) Rusty brown ppt Fe3+ (aq) + 3OH- (aq) à Fe(OH)3 (s)

No further change  

Blue (Cu2+) Pale blue pptCu2+ (aq) + 2OH- (aq) à Cu(OH)2 (s)

No further change  

Bright green (Cr3+)

Blue or blue green pptCr3+ (aq) + 3OH- (aq) à Cr(OH)3 (s)

Ppt dissolves forming a bright green solutionCr(OH)3 (s) + 3OH- (aq) à[Cr(OH)6]3- (aq)

 

Pink(Co2+)

Blue ppt which turns pink on standingCo2+ (aq) + 2OH- (aq) à Co(OH)2 (s)

No further change  

Pale pink but usually colourless(Mn2+)

White or cream pptMn2+ (aq) +2OH- (aq) à Mn(OH)2 (s)

No further change, however ppt may turn brown on standingThis is due to oxidation of Mn(II) hydroxide to Mn(III)oxide

 

 TABLE 2. COLOURLESS SOLUTIONSIon First addition of reagent Reagent in excess CommentsCa2+ No ppt No ppt  NH4

+ No pptOn warming, a colourless pungent gas is evolved which turns blue litmus to redNH4

+(aq) +OH- (aq) à NH3 (g) + H2O(l)

No ppt  

Mg2+ White pptMg2+ (aq) + 2OH- (aq) à Mg(OH)2 (s)

No further change  

Ba2+ Little or no ppt No further change  Al3+ White ppt

Al3+ (aq) + 3OH- (aq) à Al(OH)-3 (s)

Dissolves to form a colourless solutionAl(OH)3 (s) + 3OH- (aq) à[Al(OH)6]3- (aq)

All three hydroxides of Zn, Al and Pb are amphoteric and form a complex ion:- zincate, aluminate and plumbate respectively.Zn2+ White ppt

Zn2+ (aq) + 2OH- (aq) à Zn(OH)2 (s)

Dissolves to form a colourless solutionZn(OH)2 (s) + 2OH- (aq) à[Zn(OH)4]2- (aq)

Pb2+ White pptPb2+ (aq) + 2OH- (aq) à Pb(OH)-2 (s)

Dissolves to form a colourless solutionPb(OH)2 (s) + 2OH- (aq) à[Pb(OH)4]2- (aq)

   

 Tests involving the reagent aqueous ammonia solution TABLE 3. COLOURED SOLUTIONSIon First addition of reagent Reagent in excessPale green (Fe 2+)

Dirty green ppt Fe2+ (aq) + 2OH- (aq) à Fe(OH)-2 (s)

No further change, however at the top of the mixture it may turn brown on standingThis is due to oxidation of iron(II) hydroxide to iron(III) hydroxide

Yellow (Fe3+) Rusty brown ppt Fe3+ (aq) + 3OH- (aq) à Fe(OH)-3 (s)

No further change

Blue (Cu2+) Pale blue pptCu2+ (aq) + 2OH- (aq) à Cu(OH)-2 (s)

Ppt dissolves to give a deep blue solutionCu(OH)2 (s) + 4NH3 (aq) à [Cu(NH3)4]2+ (aq)

Bright green (Cr3+)

Blue or blue green pptCr3+ (aq) + 3OH- (aq) à Cr(OH)-3 (s)

Ppt dissolves forming a royal blue solutionCr(OH)3 (s) + 6NH3 (aq) à[Cr(NH3)6]3+ (aq)

Pink(Co2+)

Blue ppt which turns pink on standingCo2+ (aq) + 2OH- (aq) à Co(OH)-2 (s)

Ppt dissolves to form a brown solution which then darkens on standing Co(OH)2 (s) + 6NH3 (aq) à[Co(NH3)6]2+ (aq)The darkening is due to the oxidation of Co(II) to Co(III)

Pale pink but usually colourless(Mn2+)

White or cream pptMn2+ (aq) +2OH- (aq) à Mn(OH)2 (s)

No further change, however ppt may turn brown on standingThis is due to oxidation of Mn(II) hydroxide to Mn(III)oxide

      TABLE 4. COLOURLESS SOLUTIONSIon First addition of reagent Reagent in excess CommenetsCa2+ No ppt No ppt  NH4

+ No ppt No ppt  Mg2+ White ppt

Mg2+ (aq) + 2OH- (aq) à Mg(OH)2 (s)

No further change  

Ba2+ Little or no ppt No further change  Al3+ White ppt

Al3+ (aq) + 3OH- (aq) à Al(OH)-3 (s)

No further change  

Zn2+ White pptZn2+ (aq) + 2OH- (aq) à Zn(OH)2 (s)

Dissolves to form a colourless solutionZn(OH)2 (s) + 4NH3 (aq) à[Zn(NH3)4]2-

A complex ion occurs in excess ammonia solution diammine zinc(II) ion

Pb2+ White pptPb2+ (aq) + 2OH- (aq) à Pb(OH)2 (s)

No further change  

      

TABLE 5. Tests with various reagents other than sodium hydroxide and aqueous ammoniaTest Observation Inference CommentsSilver nitrate solution in the presence of dilute HCl or HNO3 Then followed by aqueous ammonia

White ppt means ppt dissolves to give a colourless solution

Cream pptppt partially dissolves Yellow pptppt remains

Cl- presentAg+ (aq) + Cl- (aq) à AgCl (s) AgCl (s) +2NH3 (aq) à[Ag(NH3)2]+

  Br- presentAg+ (aq) + Br- (aq) à AgBr (s) I- presentAg+ (aq) + I- (aq) à AgI (s) 

Partial dissolution is observed by carefully looking at the amount of ppt before and after addition of the aqueous ammonia. This is usually difficult to discern but the colour of the ppt and fact that it does NOT totally dissolve in aqueous ammonia would allow one to make the distinction between chloride and bromide ions.     

Barium chloride or barium nitrate solution

Then followed by dilute HCl or HNO3

White ppt

ppt dissolves to give a colourless solution  ppt remains

SO32- or SO42- present

SO32- presentSO32- (aq) + 2H+ (aq)  àSO2(g) + H2O (l) 

SO42- present

Dilute nitric or hydrochloric acid must be present to prevent the precipitation of carbonates.

Addition of HCl or any source of chloride ionssometimes an additional test may be to warm the mixture if a ppt is formed and then allow to cool

White ppt

ppt dissolves fully when mixture is warmed and recrystallises to form needle shaped crystals when cool

Pb2+

Pb2+ (aq) + 2Cl- (aq) à PbCl2 (s)

lead(II) chloride is more soluble in hot water than cold water. Therefore one would see a total reduction in the amount of ppt present when mixture is warmed. As it cools, crystallization occurs and the true shape of the crystals are seen.NB lead(II) bromide gives a similar observation but it is only partially soluble in hot water while lead(II) chloride is completely soluble in hot water

Addition of aqueous potassium iodide solutionsometimes an additional test may be to warm the mixture if a ppt is formed and then allow to cool 

Bright yellow ppt     Red-brown solution which may have black particles.

Pb2+

Pb2+ (aq) + 2I- (aq) à PbI2 (s)    Potassium iodide is a reducing agent and if a colour change is seen, then a redox reaction has occurred and the sample has oxidizing properties.

lead(II) iodide is more soluble in hot water than cold water. A partial reduction in the amount of ppt present when mixture is warmed would be seen. As it cools, crystallization occurs and the true shape of the crystals are seen. Often the crystals exhibit a “glittery” effect.

Conc. sulphuric acid to a solid halide e.g. sodium chloride, sodium bromide, sodium iodide

Vigourous effervescencewhite mixtureColourless, pungent gasTurns blue litmus red 

Vigourous effervescenceorange/red mixturebrown, pungent gasTurns blue litmus red then white   Vigourous effervescencedeep purple/black mixturepurple, pungent gasTurns blue litmus red then white  

Cl- presentGas is acidicGas is hydrogen chloridedisplacement of hydrogen chloride by sulphuric acidNaCl + H2SO4 à HCl + NaHSO4

Br- presentBromide ions are strong reducing agents and a redox reaction occurs and bromine is producedgas is acidic and it also bleaches which implies the halogen is being producedH2SO4 + 2H+ + 2Br- à Br2 + SO2 + 2H2O  I- presentIodide ions are strong reducing agents and a redox reaction occurs and iodine is producedgas is acidic and it also bleaches which implies the halogen is being produced H2SO4 + 8H+ + 8I- à 4I2 + H2S + 4H2O

Concentrated sulphuric acid is a powerful oxidizing agent

Addition of acidified potassium manganate solution

Decolourisation i.e. from purple to colourless

Since acidified potassium manganate is an oxidizing agent, the sample tested must have reducing properties

 

Addition of acidified potassium dichromate solution

Goes from orange to green

Since acidified potassium dichromate is an oxidizing agent, the sample tested must have reducing properties

 

Addition of dilute acid to a solid sample either dilute HCl or HNO3

EffervescenceSolid dissolves No effervescenceSolid dissolves 

Gas producedsample must be either a carbonate or a sulphite Sample must be either an oxide or hydroxide

 

Addition of Devarda’s alloy to sample in presence of NaOH solution and warm

EffervescenceColourless, pungent gas evolved turns red litmus blue

Gas is basicGas is ammoniaNO3- ion present3NO3- + 8Al + 5OH− + 18H2O → 3NH3 + 8[Al(OH)4]−

 

Addition of conc H2SO4with copper turnings present and

EffervescenceBlue-green solutionBrown, pungent

Gas is acidicNO2 gas produced (brown gas)NO3- ion present3Cu(s) + 8HNO3(aq) -->

The conc sulphuric acid reacts with the nitrate ion to form nitric acid. The nitric acid then reacts with the copper turning to form nitrogen monoxide which is oxidized to

warm gas evolvedTurns blue litmus red

3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)

nitrogen dioxide

Addition of freshly prepared iron(II) sulphate solution or crystals and then add conc H2SO4slowly

Brown ring is formed

NO3- present NO3

- + 3Fe2+ + 4H+ → 3Fe3+ + NO + 2H2O[Fe(H2O)6]2+ + NO → [Fe(H2O)5(NO)]2+                                       (brown ring)