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Name: Tanner C.
Status: N/A
Age: 11
Location: N/A
Country: N/A
Date: 5/22/2003


Question:
What makes sugar dissolve faster than salt?


Replies:
Rates of solution are very difficult to measure quantitatively. The rates depend upon so many factors in addition to the "basic" chemistry. Some of them are: temperature (e.g. if you are close to the melting point the faster something will dissolve), the size of the particles (the larger the surface area the faster something will dissolve), the stirring rate, and the list goes on. As a general rule, a non-ionic soluble substance will dissolve faster than an ionic soluble substance, because the forces holding the non-ionic substance is usually weaker than an ionic substance. However, that "general rule" can be overwhelmed by these other factors.

Vince Calder


Tanner,

Rather than thinking of sugar dissolving "faster" than salt, look at the situation as a matter of how much of each dissolves. In other words, a greater weight of sugar will dissolve in a fixed quantity of water than will salt in that same amount of water. Simpler still, sugar is more soluble than salt. Factors that influence the amount of each that will dissolve (and the speed at which they dissolve) are the nature of the solutes themselves, the temperature and purity of the water, and whether the solution is stirred during the dissolution process.

Salt is an ionic compound -- that is, the sodium and chloride parts are charged particles, Na+ and Cl-. The bonds that join these ions in the crystal are very strong. That's why salt is so very hard to melt. Sugar melts easily because the bonds that hold those molecules together in the solid (crystalline state) are much weaker.

What is true in the solid state is not necessarily evident when these materials are asked to dissolve in water. Both the ions comprising salt and molecules of sugar can interact with water once these particles are freed from their crystalline states by the dissolution process. Once dissolved and free to move within the solution, the bonds between sugar molecules and water are more numerous in number than those that can form between the Na+ and Cl- ions in salt. So, as sugar molecules dissolve from the crystal, they are immediately surrounded by water molecules that insulate them from recombining with the crystal. This also happens when the ions of salt are free of the crystal. Even so, the opportunities for bonding with water molecules are fewer with salt than with sugar because the ions are very small when compared to the size of sugar molecules.

The initially stronger ionic bonds in salt and the more numerous water-sugar bonds that form after dissolution gives sugar the solubility edge. Thus the greater solubility of sugar over salt.

Regards,
ProfHoff 674



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