Solution:
Concept- Electrolysis is defined as the process of breaking down ionic compounds into their elements by passing direct current through the compound in liquid form. Cations get reduced at the cathode and anions get oxidised at the anode.When concentrated aqueous sodium chloride is electrolyzed, water can also undergo reduction and oxidation reactions at different potentials. At the cathode, water can be reduced to hydrogen gas or sodium ions can be reduced to metallic sodium. So sodium is formed at the cathode. Therefore, option 1 is incorrect.
During the electrolysis of concentrated hydrochloric acid, chlorine gas is formed at the anode, resulting in a green colour. Thus, no green gas is formed at the cathode. So option 2 is incorrect.
During the electrolysis of dilute sulphuric acid, hydrogen gas is produced at the cathode and oxygen gas at the anode. Since both are colourless gas, option 3 is correct.
During the electrolysis of molten lead bromide, lead atoms are formed at the cathode and bromine molecules are formed at the anode. So option 4 is incorrect.
Hence, the correct answer is 3.
