On which conditions does determining solvent depend on? 1. the higher boiling indicates the solvent

  1. the material with the higher concentration is the solvent

  2. the solvent is kept first in a pot. Then the solute is added.

Here, pls identify the solvent and solute. I hope that it will clear all my doubts about them.

  • $\begingroup$ Note that solvent and solute are terms of convention and many cases are questionable, especially in case of unlimited miscibility. In some cases both can be considered solvent and solute at the same time, forming 2 separate phases. $\endgroup$ – Poutnik May 19 at 2:24
  • $\begingroup$ @Poutnik, then what is the point of defining the terms- solvent and solute? If they are just mere conventions , what is the absolute basis of their definition? $\endgroup$ – Habib May 19 at 2:34
  • $\begingroup$ For solid + liqud, it is easy. For 2 liquids, not so much. Lower melting point and higher abundance is just a guidance. Is ethanol in 70 % mixture with water solvent or solute ? It has lower melting point and higher mass ( but lower molar fraction ). Phenol with water forms 2 liquid phases, 1 with phenol majority, one with water majority. Which one is solvent ? What is considered solvent or solute is rather matter of their status then properties. Solute - substance of interest. Solvent - substance forming the environment for solutes. One may also consider solute=analyte, solvent= medium. $\endgroup$ – Poutnik May 19 at 4:47
  • $\begingroup$ 1/ Are traces of crude oil in water solvent and water solute ? 2/ Is a salt with extreme solubility in its saturated solution solvent ? 3/ If solid is put first, does it become a solvent ? $\endgroup$ – Poutnik May 19 at 4:55

Chemists need terminologies. NO terminology is perfect and you will always find exceptions. In general, in an n-component mixture, the component which has the highest concentration is the solvent.*

Therefore your point no. 3 is invalid because pouring does not determine the solute-solvent label. Only point 2 is relevant. Point 1 is irrelevant.

*Corollaries & Exceptions:

If we mix 50 mL of hexane and 50 mL of heptane, which one is the solvent and which one is your solute? It is up to you to label one as solvent and solute. Both components were liquids.

If I take 100 gram of water, I can dissolve about 140 grams of potassium iodide! The mixture remains a liquid. Now which one is the solute and which one is the solvent? Traditionally, you may call the liquid component as the solvent and the solid phase as the solute, because here water molecules are helping to solvate potassium and iodide ions.

The same goes with sugar in water.

So how to overcome these paradoxical labeling issues?

Physical chemists and analytical chemists prefer to use the term mole fraction x when they want to extra precise to what they are saying about mixtures. It is defined as "Amount of a constituent divided by the total amount of all constituents in the mixture. It is also called mole fraction. Amount fraction is equal to the number fraction: the number of entities of one constituent divided by the total number of entities in the mixture."

Just as nobody cares about "physical" vs. "chemical" changes taught in school, no one will chastise you for identifying one component as solvent and the other component as solute in complicated cases.

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