Correct Option
Assertion (a) states that sodium metal is stored under kerosene. This statement is true. Sodium is an alkali metal characterized by its high reactivity. It reacts vigorously with atmospheric oxygen and moisture, often resulting in an exothermic reaction that can lead to ignition. Storing sodium under kerosene oil effectively prevents its direct contact with air and water, thereby inhibiting these undesirable reactions.
Reason (R) states that metallic sodium melts when exposed to air. This statement is false. Sodium has a melting point of approximately 97.8°C, which is considerably higher than typical room temperatures. While sodium readily reacts with air and moisture, this chemical reaction does not cause the metal to melt at ambient conditions. Melting is a physical phase change that requires reaching its specific melting point.
Therefore, Assertion (a) is true, but Reason (R) is false.
Incorrect Options
Options (1) and (2) are incorrect because they both premise that Reason (R) is true. As explained, sodium does not melt upon exposure to air; its melting point is significantly above room temperature, making R a false statement.
Option (4) is incorrect because it asserts that Assertion (a) is false. As established, sodium is indeed stored under kerosene due to its high reactivity with atmospheric components, confirming Assertion (a) as true.