This complex mixture of fats gives butter its important characteristic of “spreadability.” The physical change of melting requires that the interactions between fatty acids to be disrupted by the addition of energy in the form of heat. When the butter temperature is low, the carbon chains of the fatty acids stack tightly against one another, held in place by a type of weak attractive van der Waals interactions called London forces. Heat provides enough energy for the forces holding the fats together to be disrupted as the fat begins to vibrate, rotate, and move more freely with the increased added thermal energy, and as more of the intermolecular forces are broken, the fat turns from a solid into a liquid oil.
The longer and straighter the fatty acid chain (saturated fatty acids are straight, while unsaturated are kinked or bent), the more contacts each chain makes with the other fatty acid chains and the more heat required to defeat the forces holding the fats in place.
A fat that is made from one type of triglyceride (a triglyceride is comprised of three fatty acids bonded to a glycerol backbone) will have a sharp melting curve—there will be a definable temperature at which the intermolecular forces holding the solid together give way and the solid melts.