Chemistry - A drop of water in a tin of sugar: Which one's the solvent, the sugar or the water?

Solution 1:

You could imagine stirring the sugar enough for the water molecules to be uniformly distributed throughout - it would then be homogeneous.

However, even then, to refer to the mixture as a solution of water in sugar is unhelpful, not least because referring to a slightly damp solid as a solution will only confuse. A definition needs to be useful, as well as being a set of criteria to be met.

Consider adding more water, and stirring till uniformly distributed. At some point, the mixture will become a thick syrupy liquid. Add yet more, and it will eventually resemble an ordinary solution. At what arbitrary point do you say one is a solution of the other?

As a device to get students thinking about the interactions that take place when a solid dissolves in a liquid, or a liquid in another liquid, it's an excellent question, but not one that has either a right or a wrong answer, other than "neither", or indeed "both".

What is definitely wrong is to insist that there's only one right answer to the question.

Solution 2:

Both answers are right. In the IUPAC Gold Book it states

A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are called solutes.

So either way can be used, but it is usually more conventional to call the substance that is in larger amounts the solvent.

Solution 3:

The question is flawed since we do not know whether the resulting mixture consists of a single phase or not (I guess that there are two phases).

Moreover, the definition

solute + solvent = solution; between the solute and the solvent, the solvent is one present in larger quantities and is in the same phase/state of matter as the solution

seems to be a little bit impractical to me. E.g. a saturated solution of sucrose in water will always contain more than 64% mass of sucrose, but hardly anybody would call it a solution of water in sucrose. So I would prefer the IUPAC definition cited by Clangorous Chimera.