A very important physiochemical characteristic of sugar is its ability to form crystals.
Impurities slow the rate of crystallization.
Some impurities increase the viscosity and thereby decrease the transfer of sugar to the crystal surface. Others effect the crystallization by adhering to the sugar molecule and thereby slowing the rate at which it is incorporated. An example of sugar crystallization is in hard candy. It is the sugar and fat crystals that one sees. It is due to sugar crystallization that hard candies can be produced.