Assertion(A): Graphite is used as a lubricant in machine industry. Reason (R) : Graphite is soft and a good conductor of heat.

Assertion (A) is true as graphite is widely used as a dry lubricant in the machine industry, particularly in high-temperature environments where liquid oils might fail [t2][t7]. Reason (R) is also true; graphite is physically soft due to its layered hexagonal structure held by weak van der Waals forces, allowing layers to slide over each other easily [t2][t4][t5]. Furthermore, graphite is a good conductor of heat (thermal conductivity) and electricity [t5][t7]. However, the reason (R) is not the correct explanation for the assertion (A). While softness (lubricity) is essential for its use as a lubricant, its property as a 'good conductor of heat' is not the primary reason why it functions as a lubricant. The lubricating ability is specifically due to its layered crystal structure and weak inter-layer bonding [t3][t8].

Sources

1. [1] https://www.sciencedirect.com/topics/materials-science/solid-lubricant
2. [2] https://en.wikipedia.org/wiki/Dry_lubricant