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Something I know that leads to this question:

Water dissolved in ethanol.

It might sounds weird to say water as a solute. Textbooks always says that solvent is the one present in larger quantity. This leads to a contradiction with the saying that water is an universal solvent.

There can be water present in smaller quantity than ethanol in a water-ethanol solution. But which is the solute and which is the solvent?

How to identify the solute and the solvent in a solution besides locating the word "aqueous"?

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The textbook statement you gave ("solvent is always the one present in larger quantity") is simply a convention that makes it easier for us to talk to each other about mixtures of substances. We have other words for other types of mixtures. For example, in your ethanol-water example, you are correct in stating that it doesn't make much sense to talk about solute and solvent. So, we call these two liquids "miscible". They mix together in all proportions. A good demonstration that captures the essence of different types of mixtures is the phase diagram of phenol-water:

enter image description here

Here we have a feature that is called a "miscibility gap". There is a region, defined by the rainbow shape in the diagram above in which phenol and water separate into two distinct phases. If we had pure water and added just a little bit of phenol to it at room temperature, we would find that it would dissolve into the water up to about 7 or 8 wt%. So, phenol would be the solute and water the solvent. Adding more than this amount pushes the mixture into the miscibility gap so that now the phenol and water would mostly separate into two separate phases. Let's say we add enough phenol to get us to 40 wt%. If we heat the two phase mixture up to 67 C then it will suddenly become miscible. Now only one phase exists and we can add as much phenol as we want to it without having to worry about two phases forming again. There is no solute or solvent here. Just a mixture of two liquids that exist in a single phase.

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  • $\begingroup$ At that temperature you can put however much you want?! $\endgroup$ – most venerable sir Jan 9 '15 at 14:36
  • $\begingroup$ Don't you have to increase temperature as you add more to it. How can there be a solution without saturation state? $\endgroup$ – most venerable sir Jan 9 '15 at 14:37
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    $\begingroup$ As long as the temperature is above 67 C in the phenol-water mixture you can keep adding as much phenol as you want and never have to worry about saturation. It is a miscible mixture at this temperature, just like the ethanol-water mixture. Try your best to disengage from thinking in terms of a "solution" with a certain "solute" and "solvent" in this case. Its just two liquids that mix together in all proportions to form a single liquid phase $\endgroup$ – Qubit1028 Jan 9 '15 at 22:47
  • $\begingroup$ You are saying if we heat up to 67 C then ethanol should get disappeared bcz it has low boiling point in comparison with water then why water is getting disappeared? $\endgroup$ – Owaish Siddiqui Jun 22 '18 at 8:33
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I think you may be emphasizing the term "universal solvent" a bit too hard. Water is "universal" as a solvent simply because lots of molecules are at least somewhat polar and so dissolve in polar water. But otherwise I'd say your textbook is right: the solvent is generally in greater quantity than the solute by convention.

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I think you may be putting too much significance on the semantics of some mnemonics that are perhaps phrased more absolutely than they are intended to be used. That is particularly true with the phrase that "water is the universal solvent", which is not really a scientifically significant statement at all. Certainly gasoline, or mineral oil, will not dissolve in water, even in very small quantities, so why would it be considered "universal" in the sense you seem to be taking it?

To answer your question, the better rule of thumb is to use the statement that the solvent is always the component present in larger quantity .. but even that is not correct in all cases. For example in a multiphase extraction, there can be multiple solvents ... components that are polar will dissolve in the aqueous phase, so for those components, water is the solvent .. while components that are non-polar will dissolve in the non-aqueous phase, which will be the solvent for those components.

A final point is that for something like a 49-51% mixture of ethanol and water, the concept of a solvent becomes largely academic ... is the ethanol dissolved in the water, or is the water dissolved in the ethanol? The answer is: both, and neither .. they are miscible liquids, so it's a two component mixture, rather than a solution. The physical properties of a mixture depend rather strongly on the properties of both components, while the physical properties of a solution are more likely to be dominated by the properties of the solvent.

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  • $\begingroup$ You are saying that for non polar substances water is not solvent. Is it possible to have two liquids in the same ratio in a solution along with solute, if it is so which one would be the solvent. Plz make me understand I m not getting this.... $\endgroup$ – Owaish Siddiqui Jun 22 '18 at 8:27

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