Oh it's too simple concept !

as we have learned cooking oil contain a mixture of triglycerides of both saturated and unsaturated fatty acids and hence decolourise bromine water.

ND kerosene oil.. on the other hand, is a mixture of only saturated hydrocarbon ND hence doesn't decolourise bromine water !

hope u got it !

Introduction
Kerosene oil and cooking oil are both commonly used in various applications. However, they have different chemical compositions, which affect their reactions with bromine water. In this explanation, we will explore the reasons why kerosene oil does not decolourise bromine water, while cooking oil does.

Kerosene Oil
Kerosene oil is a flammable hydrocarbon liquid commonly used as a fuel for engines, lamps, and heating. It is derived from petroleum and primarily consists of alkanes, which are saturated hydrocarbons. The molecular structure of kerosene oil lacks functional groups that can readily react with bromine water.

Cooking Oil
Cooking oil, on the other hand, is primarily composed of triglycerides, which are esters derived from fatty acids. These fatty acids contain unsaturated bonds, such as double bonds, in their carbon chains. The presence of these double bonds makes cooking oil more reactive towards bromine water.

Decolourisation of Bromine Water
Bromine water is a reddish-brown solution of bromine in water. It is commonly used as a test reagent to detect the presence of unsaturated compounds. Bromine water can undergo an addition reaction with alkenes, which are unsaturated hydrocarbons containing double bonds. This reaction results in the decolourisation of bromine water.

Reaction with Kerosene Oil
Due to the absence of double bonds in kerosene oil, it does not undergo an addition reaction with bromine water. The alkanes present in kerosene oil are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. As a result, kerosene oil does not have the ability to break the bromine-bromine bond and decolourise bromine water.

Reaction with Cooking Oil
Cooking oil, containing unsaturated fatty acids, can readily react with bromine water. The double bonds in the fatty acid chains of cooking oil provide sites for an addition reaction with bromine. This reaction leads to the cleavage of the bromine-bromine bond, resulting in the decolourisation of bromine water.

Conclusion
In summary, kerosene oil does not decolourise bromine water because it primarily consists of saturated hydrocarbons lacking double bonds. On the other hand, cooking oil, which contains unsaturated fatty acids with double bonds, can react with bromine water and cause its decolourisation. The differences in the chemical compositions of kerosene oil and cooking oil account for their varying reactions with bromine water.