Assertion (A) : Butter softens at room temperature. Reason (R) : Short chain fatty acids cause the melting point to decrease.

Assertion (A) is true as butter is a semi-solid fat that softens at room temperature due to its complex lipid composition. Reason (R) is also true and provides the correct explanation. The melting point of fatty acids is determined by chain length and degree of unsaturation. Butter contains significant amounts of short-chain fatty acids (SCFAs) like butyric (C4) and caproic (C6) acid. These SCFAs have much lower melting points (e.g., butyric acid melts at -7.9°C) compared to long-chain saturated fatty acids. A decreasing chain length reduces intermolecular Van der Waals forces, leading to a lower melting point. Consequently, the presence of these short-chain and unsaturated fatty acids in butter's triglyceride structure ensures it remains soft or liquid-like at room temperature rather than a hard solid.