Question

Liquid butane & iso butane mixture

Answer 1

Butane boils at -0.5 C, while propane boils at -42 C. The difference in boiling point (BP) is due to the difference in masses of the molecules and the different intermolecular forces between the molecules. This is all determined by the size and shape of the molecules. The detailed chemical construction of the molecule plays a significant part in this, but we will ignore that for the present. With these as the extremes, the 'boiling point' of a mix of these two liquids will of course be somewhere in between.