Question 1

How does the Baeyer test detect carbon-carbon unsaturation?

Accepted Answer

Cold, dilute alkaline potassium permanganate is decolourised by an alkene, and a brown precipitate of MnO2 forms. Saturated alkanes and simple alcohols do not give this decolourisation under the standard test conditions.

Question 2

Why are both bromine water and Baeyer reagent used for unsaturation?

Accepted Answer

Both give a clear decolourisation with an alkene, so a matching observation from two tests supports C=C unsaturation. Baeyer reagent also produces brown MnO2, whereas decolourisation of bromine water is the main observation in the bromine test.

Question 3

How are Group I, Group II, Group III and Group IV cations separated?

Accepted Answer

Dilute HCl precipitates Group I chlorides. Passing H2S through the acidic filtrate precipitates Group II sulphides. Next, NH4OH in the presence of NH4Cl precipitates Group III hydroxides, and H2S in the ammoniacal medium then precipitates Group IV sulphides.

Question 4

Why is dilute HNO3 used before AgNO3 in the halide test?

Accepted Answer

The solution is first acidified with dilute HNO3 to remove interfering ions without introducing chloride. AgNO3 then gives white AgCl, which dissolves in dilute NH3, cream AgBr, which dissolves only in concentrated NH3, or yellow AgI, which is insoluble in NH3. Using HCl beforehand would itself add chloride ions.

Question 5

What observations identify carbonate ions?

Accepted Answer

Carbonate reacts with dilute acid to release CO2: CO3^2- + 2H+ → CO2 + H2O. The gas first turns limewater milky by forming insoluble CaCO3, but excess CO2 clears the milkiness because soluble calcium hydrogencarbonate forms.

Qualitative Analysis Bench — Tests, Gases & Coloured Precipitates

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