Question

why butter solid and mustard oil is liquid at room temperature

Answer 1

Butter has a higher proportion of straight (i.e. hydrocarbon chains are unbent) saturated fatty acids (all carbons bonded to each other by single bonds), which can pack together very tightly, and thus have a higher melting point (i.e. they will be solid at a higher temperature). As a plant oil, mustard seed oil is higher in unsaturated fatty acids (some carbons connected via carbon-carbon double bonds), which pack together in a "looser" fashion than do saturated FA's. Unsaturated FAs tend to bend or kink around those double bonds--this makes them less stable at a given temperature, and thus they have lower melting points (i.e. they remain liquid at lower temperatures).