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In my textbook “solutions” are defined as follows:

Homogenous mixtures of two or more substances are known as solutions.

Should the two substances always be non-reacting?

The definition has no indication of this, buts it’s just something that came to my mind. Won’t the solution still be homogeneous so shouldn’t we still call it a solution?

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    $\begingroup$ Your textbook definition is good enough to answer your question. If the substance reacts with the solvent yielding soluble product, then you got a solution. Of course, there is no definite answer as to wheter the product(s) will always be soluble, you need to look at the concrete example. $\endgroup$ – andselisk Jun 6 at 18:46
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    $\begingroup$ Please never use MathJax's math mode for emphasis. $\endgroup$ – andselisk Jun 6 at 18:47
  • $\begingroup$ @andselisk Thanks! I completely neglected the possibility of precipitate formation after the reaction! So yes...it should depend on the reaction taking place. $\endgroup$ – Aditya Jun 6 at 18:50
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    $\begingroup$ The reasons against using MathJax's math mode for emphasis: 1. It's just wrong; 2. It's semantically incorrect to use math mode for highlighting the text; 3. Even if you were to use MathJax for emphasis you should've used $\textit{…}$ instead; 4. Every time MathJax is introduced, the web page loads corresponding JS scripts which are in general resource-heavy; 5. Did I mention it's wrong? Also, see Is it OK to abuse Mathjax for emphasis? $\endgroup$ – andselisk Jun 6 at 18:58
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    $\begingroup$ Example: Should bromine water be called a solution? $\endgroup$ – Nilay Ghosh Jun 7 at 3:11
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This is what Encyclopaedia Britannica definition of a solution in chemistry:

Solution, in chemistry, a homogeneous mixture of two or more substances in relative amounts that can be varied continuously up to what is called the limit of solubility. The term solution is commonly applied to the liquid state of matter, but solutions of gases and solids are possible. Air, for example, is a solution consisting chiefly of oxygen and nitrogen with trace amounts of several other gases, and brass is a solution composed of copper and zinc.

A solution consists of solutes (at least one) and a solvent. The solute is define as the substance that is dissolved in the solvent. In other words, for solutions with components in the same phase, the substance(s) present in lower concentration are solutes while the major substance present in highest abundance is the solvent. For example, 190-proof alcohol is a liquid-liquid solution, which is the mixture of 95% ethanol and 5% water by volume. The major substance in this mixture is ethanol. Therefore, it is the solvent. The 5% water is the solute. Brass is a solid-solid solution of copper and zinc metals. General composition of brass is 65% copper and 35% zinc by weight. Hence, in brass, copper is the solvent and zinc is the solute. Air is a solution of multi-gases. In air, 72% nitrogen and 20% oxygen, and rest is other gases including $\ce{CO2}$ and $\ce{H2}$. Thus, the solvent of air is $\ce{N2}$ while $\ce{O2}$ is one of its solutes. Saline solution, on the other hand, is a solid-liquid solution. In a saline solution, solid salt ($\ce{NaCl}$) is the solute dissolved in water, which is the solvent (liquid).

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    $\begingroup$ A few days ago a senior member was arguing with me here that air is not a solution. I disagreed. Now Encyclopedia Britannica says the same that air $is$ a solution. Thanks for sharing. $\endgroup$ – M. Farooq Jun 7 at 1:36
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    $\begingroup$ @M.Farooq Na, sorry. A solution must be a condensed phase. Britannica is inconsequent in its definition, because it is per se impossible that two gases have any kind of solubility limit. You change temperature or pressure, the componentes in a gas mixture condense totally independent of each other. No interaction -> not a solution. $\endgroup$ – Karl Jun 7 at 7:52
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    $\begingroup$ @Karl, It is ultimately semantics. There is no restriction on miscibility. ACN and water are miscible in all proportions. If we freeze a solution of water and NaCl, water freezes separately to some extent. Sorry, I don't buy this argument. Can you provide a solid reference which says that a solution must be a condensed phase? In microwave spectroscopy, there is a technique called chiral tagging where a enantiomer is exposed to a chiral alcohol in the gas phase and neon is used a "reaction medium". They form weak complexes in gas phase. Who says there can be no interaction in gases? $\endgroup$ – M. Farooq Jun 7 at 13:19
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    $\begingroup$ @Karl, just a clarification, chiral tagging in microwave spectroscopy is done at millitorr pressures in a chamber. It is neither supercritical state nor a high pressure experiment. Do solid solutions have colligative properties? I am saying that this is all semantics. You must read the poem "Five Blind Men of Indostan" en.wikipedia.org/wiki/Blind_men_and_an_elephant $\endgroup$ – M. Farooq Jun 7 at 17:13
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    $\begingroup$ That's a valid point. Perhaps worth bringing up in meta if no one has already. $\endgroup$ – Buck Thorn Jun 7 at 18:37
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Should the two substances always be non-reacting?

Short answer yes. If they two substances react, you cannot recover the original components which defies the definition of a mixture.

However, if you think deeply, what is the meaning of non-reacting? Sometimes words are not enough they can only be an approximation of a reality.

Let us take the example of a $\ce{I2}$ (iodine solid) and dissolve it in hexane or carbon tetrachloride, we get a purple solution. If I dissolve it in acetone, we get a brown solution. Chemists are okay to call it a solution. Thinking deeper, the purple color or the brown color indicates that the "species" of iodine is not the same into solvents. It means somehow the solvent is indeed interacting with iodine molecules. That interaction must be very weak, because the moment we evaporate the solution, we will get the exact amount of iodine and the solvent back!

So you can see there is no such situation where there is no interaction. Even there is some interaction between two strangers by gravitational forces, but it is extremely extremely weak.

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    $\begingroup$ You don't need to get into subtleties. Like every reaction you run in lab is in solution. Does it stop being solution when reaction starts? No, it doesn't. $\endgroup$ – Mithoron Jun 6 at 21:08
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    $\begingroup$ From a physical chemist's perspective, those reaction mixture will no longer be a binary component solution, it will have multiple solutes (=reactants and products). $\endgroup$ – M. Farooq Jun 6 at 21:15
  • $\begingroup$ Chemists tend to loosely use the word solution e.g., dissolving HCl gas in water yields HCl solution. Is it a mixture of HCl gas and water- Nope! $\endgroup$ – M. Farooq Jun 6 at 21:16
  • $\begingroup$ Come on, no chemist says "HCl solution"! It´s hydrochloric acid, acide chlorhydrique, Salzsäure! :-) $\endgroup$ – Karl Jun 6 at 23:10
  • $\begingroup$ @Karl, Yes, but the usage HCl solution for hydrochloric acid solutions is not uncommon. I just did quick Google Scholar search scholar.google.com/… $\endgroup$ – M. Farooq Jun 6 at 23:40

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