Re: Which forms faster a sugar crystal or a salt crystal and why?

First of all a few basic concepts about crystallization. Crystals of any dissolved material won't form by themselves in solution unless the solution is saturated with the material. A saturated solution is defined as a solution in which the undissolved and dissolved material is in equilibrium with each other. In other words, the rate at which the material dissolves and the rate at which the material crystallizes out of solution are the same. Secondly, the crystallization is influenced by several atmospheric factors such as temperature, pressure, relative humidity, and the chemical nature of the material itself. Let us assume for the purpose of this question that the first 3 factors are fixed. The last factor will always be fixed since we cannot alter the nature of the compound itself. By nature of the compound I mean the kind of compound it is. Sugar for example is a molecular solid. It exists as single molecules bonded together by weak chemical forces such as hydrogen bonding. Most common sugars are molecular solids. Salt, or sodium chloride, on the other had is an ionic solid. It is composed of charged atoms (ions) of opposite charge (sodium is 1+ charge and chloride is a 1- charge). Many pairs of the two ions are held together in a salt crystal by ionic bonding which is just a formal way of saying that the opposite charges attract each other very strongly and so they are held together.

Now, to the answer:

We have a saturated solution of salt and a saturated solution of sugar at room temperature and atmospheric pressure and humidity. Which one will form crystals faster? What this really asks is which solution has the larger rate of crystallization relative to the rate of dissolution. I stated at the beginning that these two solution were inequilibrium so something has to change in order for the rate crystalization increase.

Lets us assume that over time some small amout of evaporation removes water from each solution. Now, the solutions will no longer be in equillibrium because the solutions have more material in them than they can hold (this is called a supersaturated solution). Which one will crystalize faster? The answer is the one that takes less energy to crystallize and reaches the lowest energy state. Without going in to a technical explanation of the energy involved, let's just say that it's more difficult for sugar to detach itself from water than it is for salt.

Each material has to do this in order to crystallize out of solution. Let us also say that salt in solid form is lower in energy than sugar in its solid form. Sugar has a greater affinity for water than does salt and so it easier for salt to let go of water and come out of solution. In doing so it also releases more energy than sugar. This is directly related to the ionic nature of salt and the molecular hydrogen bonding nature of sugar and is the key to the answer. Salt wins the race!!