This article is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Dr. Sue Ann Bowling is an Associate Professor of Physics at the Institute. There's a lot of physical chemistry involved in making old fashioned fudge.
Sugar dissolves far less readily in cold liquids than in hot.
There is no way that two cups of sugar will dissolve in a cup of milk at room temperature. Heating the sugar and milk mixture allows the milk to dissolve more and more sugar, and by the time the mixture is boiling, all the sugar is dissolved. When the milk has dissolved all the sugar it can hold, and there is still some undissolved sugar left, the mixture is said to be saturated. The higher the temperature, the more concentrated the saturated solution becomes. In general, a solid dissolved in a liquid makes it harder for the liquid molecules to escape.