For example, ethanol is soluble in water, but doesn't react with it. On the other hand, H2SO4 is said to be soluble because it dissociates (reacts) in water.

Would ethanol be soluble in HCl (aq)? It won't react with the HCl, but it can form hydrogen bonds with the water, right?

How about ethylamine or ethyl methanoate? Would they be soluble in HCl (aq)? They react with the HCl, so I guess they will be. Is this correct?


Summary, TL;DR:
No, A reacting with B does not mean that A is soluble in B. Solvation and chemical reaction are two different processes, although the distinction can in some cases seem less than perfectly clear. All of the example compounds you discussed are soluble or miscible with both water and hydrochloric acid. The IUPAC definition of a chemical reaction is "A process that results in the interconversion of chemical species", which is fundamentally different from solvation, and does not apply to any of the examples in the question.

As you suggest, solvation and chemical reactions are two fundamentally different concepts. For example, if compound A reacts with water to form a homogeneous liquid solution, that is a different processes than if compound A were to simply be dissolved by water.

Solvation is defined by the IUPAC Gold Book as follows:

Any stabilizing interaction of a solute (or solute moiety) and the solvent or a similar interaction of solvent with groups of an insoluble material (i.e. the ionic groups of an ion-exchange resin). Such interactions generally involve electrostatic forces and van der Waals forces, as well as chemically more specific effects such as hydrogen bond formation.

Whereas the IUPAC Gold Book defines a chemical reaction as:

A process that results in the interconversion of chemical species.

One aspect of solvation vs. reaction that may seem confusing is the solubility of ionic species. Even though $\ce{NaCl}$ becomes $\ce{Na+}$ and $\ce{Cl-}$ in an aqueous solution, this does not constitute a chemical reaction as defined above, and we say that $\ce{NaCl}$ is soluble in water. The same is true of your example of $\ce{H2SO4}$; even though it dissociates in water, it is not converted to a new chemical compound. One way to think of this is if you remove the solvent, the solute should typically resume to it's original form. From our previous example, if we evaporate the water from our aqueous solution of $\ce{Na+}$ and $\ce{Cl-}$, we just get the solid $\ce{NaCl}$ back.

Your examples of solubility in hydrochloric acid are a bit complex because that is a two-component system of water and $\ce{HCl}$. All of the compounds you discuss are water soluble and it doesn't matter if the $\ce{HCl}$ is there or not. One slight exception in your examples is ethylamine. Ethylamine itself is miscible with water, but many higher molecular weight amines are not water soluble, but are soluble in hydrochloric acid. In this case, the $\ce{H+}$ from $\ce{HCl}$ protonates the amine, making the hydrochloride salt. This is still an example of solubility however, as once you remove the solvent, you are left with the original compound, in this case the amine.

  • $\begingroup$ Thank you for your answer! Could you give me an example of a compound which reacts with water but does not dissolve in it? And why is the salt obtained from protonating Ethylamine soluble in water? Lastly, would the results change if we used liquid HCl rather than aqueous? $\endgroup$ – John Apr 28 '17 at 21:50
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    $\begingroup$ Metallic sodium is not water soluble, but undergoes a chemical reaction with water to form a water soluble species ($\ce{NaOH}$) and a pretty insoluble gas, $\ce{H2}$. The result is that it appears that the sodium has dissolved, when in fact it has undergone "a process that results in the interconversion of chemical species" (from definition in answer). Regarding ethylamine: amines have a basic lone pair on the amine nitrogen that readily accepts a proton from a strong acid. This gives you a charged species, which is much more polar and generally water soluble than neutral species. Continued.. $\endgroup$ – airhuff Apr 28 '17 at 22:06
  • $\begingroup$ I hope the amine protination explanation made sense. Again, amines themselves are somewhat polar, and a small compound like ethylamine is polar enough to be water soluble. I just extended that to the example of large hydrocarbons where the amine moiety itself isn't polar enough to make the compound water soluble. In this case, ionic species are so polar that protonating even an otherwise large, aliphatic amine can make it soluble. Regarding pure HCl: it boils at about -85C, so it is very much a gas at room temperature. The gas readily dissolves in water, and the strongest HCl soln is ~38%. $\endgroup$ – airhuff Apr 28 '17 at 22:14
  • $\begingroup$ Thanks, it's mostly clear now. But I meant to ask would Ethylamine, Ethanol, Ethyl Methanoate be soluble in a pure (liquid) acid, not if HCl would be soluble in a water. $\endgroup$ – John Apr 28 '17 at 22:22
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    $\begingroup$ Unfortunately, every acid has it's own very distinctive properties, so that such a question (is compound X soluble in pure acid) is impossible to fully answer. It's almost like asking if compound X is soluble in liquids. But for the sake of argument, H2SO4 can be almost a pure solution. Ethylamine would be very soluble in H2SO4. Ethanol would undergo a dehydration reaction to ethene gas and water. I suspect that ethyl methanoate would be fairly soluble, though probably less so than in water (~10% soluble in water). The situation would be very different though in, say, HNO3, or other acid. $\endgroup$ – airhuff Apr 28 '17 at 22:38

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