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

Question

The other day, when we were dealing with the chapter Solutions, our teacher asked us this:

If I add a drop of water, to a tin full of sugar (without mixing it in), what's the solvent here? The water or the sugar?

Naturally we were taken aback by the 'unorthodox' nature of the question. Seeing that none of us were in a position to answer that any time soon, he gave us the "answer":

It's the sugar. If you remember the definition we learnt ( solute + solvent = solution; between the solute and the solvent, the solvent is one present in a larger quantity and is in the same phase/state of matter as the solution ) you ought to have realized that, since there's more sugar than water, and at the end of the process the tin's filled with a solid.

Now that may seem like a clever way of testing our command of terminologies and definitions, but I saw a hitch there.

A solution is defined as a homogeneous mixture.

Adding a drop of water to a tin of sugar (without mixing it in) does not result in a homogeneous mixture. So as interesting as the question is, I feel it's flawed on this account... because what we're dealing with isn't technically even a solution.

Now I told this to my teacher, but he (quite deftly) side-stepped my query...a subtle way of indicating that he's not comfortable discussing this.

So I guess my question here boils down to this:

Was my teacher's "answer" correct? Or was the question seriously flawed to begin with?

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EDIT-

I wouldn't call this a duplicate. Sure, I mean, both my question and the other one that was linked with this was about identifying the solvent and solute but I feel my question is distinct because:

1) It isn't answered well enough in the other question (that answer was too generalized... I've given a specific instance here)

2) I also want to know if a drop of water added to a tin full of sugar (without mixing it) can be considered a solution.

Answer 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.

Answer 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.

Answer 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.

Answer 4

To be clear: the definition the Original Post (Question) gave is NOT one which is acceptable to me, or I'd guess, to a majority of the chemists of the world. The IUPAC definition clearly indicates that the "solvent" is assigned arbitrarily – the material in the mixture which may be "treated differently" depends on the CHOICE of treatment and so will vary depending on that choice – in fact can change from one instant to the next.

The question also suffers from vagueness: how much sugar is in a "tin"? Sugar is also ill defined. Table sugar is generally accepted to be the disaccharide sucrose. Sucrose is composed of the two hexoses fructose and glucose bonded together (their respective melting points are 103 °C and ~147 °C (but it depends on their stereochemistry) and sucrose has no melting point (as far as I know, I wonder if at high pressure this would still be true, but IDK), it decomposes at ~186 °C.

So, picture a container with molten (melted) fructose in it. There's no trouble adding a drop of water (a drop of water is often taken to be 1/20th of a gram since that's about what you get out of a glass eye-dropper (you get much less out of a plastic one, generally (due to less surface tension))). You'd not be wrong to claim you had a solution of water in a sugar. Allow the solution to freeze – does it suddenly stop being a solution? No – at least not necessarily. Take the solid and break it up into a powder. Now has it changed? Well, you have to be careful here. Often times grinding changes the chemistry (of the particles' surface), but if you've avoided that, you now have a granular solid solution of water in a sugar.

The problem is, as I've already said, that the definition your teacher gave you is inadequate in several ways. For instance what does "larger" mean? I can (easily) create a molecule (called a polymer) which weighs 100 grams (or 100 kg for that matter). If I add 1 molecule to 1,000,000,000,000,000,000,000 molecules of water, which is the solvent and which the solute? If I add 1 gram of water to 2 grams of tungsten which is the solvent? The water has a volume of 1 ml, the tungsten's volume is 0.1 ml so which is the "larger" amount the water or the tungsten? (Tungsten wouldn't dissolve in water at room temperature, so you'd have to heat this mixture as well). BUT. As an exercise in logic, given the definition you are supposed to rely on, then it is the water that is the solute.

I don't understand why you claim you weren't in a position to answer it. As far as I can see, you were. I also don't understand why you think that a homogeneous mixture MUST be continuous. I think you have confused two different concepts. A mixture of red marbles and blue marbles might be segregated or might be mixed randomly or might be in a well ordered array. Red-Blue-Red-Blue… (or some other order). The fact is, as you hopefully know, the atomic nature of matter means that matter is NOT continuous, but discrete (on a microscopic basis).