Assertion (A) : Sodium metal is stored under kerosene. Reason (R) : Metallic sodium melts when exposed to air.

The assertion is true: metallic sodium is routinely kept immersed in kerosene (or similar hydrocarbon oils) to prevent contact with air and moisture, which would cause vigorous oxidation or reaction with water and can generate heat or fire [2]. The reason given is false: sodium does not melt merely on exposure to air. Its melting point is about 98 °C, so it remains solid at ambient conditions; in fact sodium becomes significantly more reactive when molten and can ignite at higher temperatures (~125 °C), which shows that melting is a consequence of heating, not exposure to air [3]. Thus R is not a correct explanation for A.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 4: The World of Metals and Non-metals > Activity 4.6: Let us investigate (demonstration activity) > p. 52
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > Activity 3.9 > p. 42
3. [3] https://www.britannica.com/science/sodium/Chemical-properties