Question

Which of the following is not a chemical test to distinguish unsaturated hydrocarbons from saturated hydrocarbons:

• A. Flame test
• B. bromine water test
• C. both of them
• D. none of these

Answer 1

The correct option is C both of them

Saturated hydrocarbons burn in air with bright blue flame whereas unsaturated hydrocarbons burn in air with sooty yellowish flame, because saturated compounds contain less percentage of carbon which easily oxidised completely in air but unsaturated compounds need more oxygen for complete burning which is not present in air.

Unsaturated compounds gives addition reaction with $B{r}_2$ water and decolorize it but saturated cmpounds does not give addition reactions. Hence they can distinguish by flame test as well as by bromine water test.

$B{r}_2$water is a brown coloured liquid.
(a) Unsaturated hydrocarbons give addition reaction with $B{r}_2$. So the colour of $B{r}_2$-water gets decolourized.
$R-C=C-R+B{r}_2\to R-BrC-CBr-R$
(b) Saturated hydrocarbons do not react with $B{r}_2$-water, so the colour of $B{r}_2$.-water does not get decolourised .