(a) Give a chemical test to distinguish between saturated and unsaturated hydrocarbon.
(b) Name the products formed when ethane burns in air. Write the balanced chemical equation for the reaction showing the types of energies liberated.
(c) Why is reaction between methane and chlorine in the presence of sunlight considered a substitution reaction ?

Explanation

Bromine water test. The addition of bromine `(Br_(2))` gives addition reactions with unsaturated compounds (like alkenes and alkynes.) The addition of bromine is used as a test for unsaturated compounds. For this purpose, bromine is used in the form of bromine water. A solution of bromine in water is called bromine water. Bromine water has a red-brown colour due to the presence of bromine in it . When bromine water is added to an unsaturated compound, then bromine gets added to the unsaturated compound and the red-brown colour of bromine water is discharged . So, if an organic compound decolourises bromine water, then it will be an unsaturated hydrocarbon (containing a double bond or a triple bond), but saturated hydrocarbon (alkenes) do not decolourise bromine water.
(b) When ethane burns in air, carbon dioxide and water vapours are formed along with heat and light.
`underset("Ethane")(2C_(2)H_(6))+ underset("(Air)")(7O_(2)(g)) to underset("Carbon dioxide")(4CO_(2)(g))+ underset("Water")(6H_(2)O(g))+ "Heat " + "Light"`
This reaction is an exothermic reaction due to the evolution of heat.
(c) Methane reacts with chlorine in the presence of sunlight to form chloromethane and hydorgen chloride.
`CH_(4)+Cl_(2) overset("sunlight")to underset("Chloromethane")(CH_(3)Cl)+ HCl`
In this reaction, one 'H' atom of methane has been substituted (replaced) by a 'Cl' atom, converting `CH_(4)` to `CH_(3)Cl` . Hence, it is considered a substitution reaction.