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My textbook [Chemistry Live, Declan Kennedy] states that ''Ethanol is said to be infinitely soluble in water''. It then says ''verify this for yourself by mixing some ethanol with water in a test tube''.

Both water and ethanol are colourless. How can I observe that they are infinitely soluble ?

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    $\begingroup$ I think what they mean by infinitely soluble is they are miscible. And when they are miscible, any concentration will be mixed to form a homogeneous solution. See here the first paragraph is explaining it. $\endgroup$ – Desai Jan 29 at 16:54
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    $\begingroup$ That you don't see a difference is the proof. Add some naphtha and difference is visible. $\endgroup$ – Mithoron Jan 29 at 16:55
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    $\begingroup$ @Mithoron What's wrong with this question? I think it is a very natural one. How would you know the difference, if you never mixed two liquids before? $\endgroup$ – Ivan Neretin Jan 29 at 17:10
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    $\begingroup$ @IvanNeretin Well, not asking yourself what would happen otherwise, lack of research in homework-type question... And even observing a slowly dissolving sugar in tea would show that lack of difference in color is no problem. $\endgroup$ – Mithoron Jan 29 at 17:18
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    $\begingroup$ @Mithoron There is a huge wall between any chemistry class and the life outside. The knowledge of sugar dissolving in tea can't be brought inside. Ditto for any other common knowledge. Likewise, the chemistry knowledge is of no use outside. At least that's what seems to be universally taught. $\endgroup$ – Ivan Neretin Jan 29 at 17:37
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The better term is miscible in all ratios. The idea behind the term is that you can have a mixture from 99.9999… % water and 0.0000…1 % ethanol to a mixture of 0.0000…1 % water and 99.9999…% ethanol. No infinity necessary.

The obvious opposite to miscible is immiscible (in some or all ratios). A common pair of household liquids that are immiscible are fatty oils and water. In the chemistry lab, hexane and water might be a better example.

To verify the miscibility of water and ethanol, set up a parallel experiment with water and hexane/a fatty oil such as sunflower oil. You will notice that a fully immiscible pair will never mix, there will always be two different bits of liquid visible. You can try a little oil/hexane and lots of water and vice-versa. On the other hand, a miscible pair will mix and you get a solution where you cannot tell one from the other.

For completeness, partially miscible: if you repeat the experiment with ether and water you will observe that the first couple of drops of water will dissolve in sufficient volumes of ether (and vice-versa) but if more is added you will again get two phases.

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  • $\begingroup$ Good answer. "All ratios" raises a (stupid?) question in my mind. Is a single 50-50 ratio experiment adequate? Or is it possible that some ratio like 1:10 or 1:100 would show immiscibility? In solids, different ratios can give different phases. $\endgroup$ – James Gaidis Mar 29 at 13:01
  • $\begingroup$ @JamesGaidis This phase diagram (hexane, nitrobenzene) suggests that at high temperatures, there is one phase at 50:50 but two at 60:40. people.cst.cmich.edu/teckl1mm/pchemi/chm351ch8af01_files/… $\endgroup$ – Karsten Theis Mar 29 at 15:25
  • $\begingroup$ @Karsten: Thank you. That suggests there may be many others if the two components are similarly disparate. $\endgroup$ – James Gaidis Mar 30 at 14:08
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Both water and ethanol are colourless. How can I observe that they are infinitely soluble ?

They have different refractive index. You will see Schlieren as you mix them, which will eventually disappear. If two solutions don’t mix, there will be a visible interface. If you have trouble seeing it, mix it and let it settle.

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"How to verify that ethanol is infinitely soluble in water?".

You cannot do it because it is WRONG. And not because it is false for ethanol, it is simply plain wrong conceptually and mathematically.

Solubility is the ratio between the amount of the specified substance and the volume of the phase solution.

As for a substance occupies its own volume, solubility cannot tend to infinity but at most reach the value dictated by its molar volume in the given P, T conditions.

For instance, let me take water - for which I remember the value - instead of ethanol. Its concentration as pure substance is about 55.5 mol/l.

This is aimed to correct a disturbing error in the text book rather than provide a practical answer.

For finding out if water and ethanol are miscible in any ratio - this should have been the question - the book is at least correct that a simple test tube suffices to get at least a reasonable idea. How and why is already in the other answers and comments and I assume it is clear to the OP as well, at this point.

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  • $\begingroup$ I would like to know why this has been downvoted. $\endgroup$ – Alchimista Mar 29 at 10:29
  • $\begingroup$ It is sad to see how somebody uses the downvote function. My answer does correct a terrible mistake of a chemistry book. Infinity soluble is understood in day life perhaps, but to even think of infinite solubility is completely wrong from too many viewpoints. $\endgroup$ – Alchimista Mar 29 at 12:08
  • $\begingroup$ It's interesting to see your viewpoint. The term "infinitely soluble" can indeed be misunderstood to mean that an infinite amount of solute could be dissolved in a finite amount of solvent. However, that is so farfetched that it would be glossed over. $\endgroup$ – James Gaidis Mar 29 at 12:57
  • $\begingroup$ @JamesGaidis the point is the it does not matter how farfetched it is. Is simply wrong and stupid. The very point is not even abstraction to infinity. It is not even the point. At most solubility tends to a finite amount (whatever depending on how S it is expressed) and such a wording isn't acceptable on a chemistry book or chemistry forum. I am not saying it can be misunderstood in practice, but conceptually yes it can be. Your comment is in that direction. Farfetched is something about mass or amount of substance, not about solubility or more generally conc. $\endgroup$ – Alchimista Mar 29 at 14:09
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    $\begingroup$ @Alchemista: Well, how much water is soluble in a drop of ethanol? Quite a lot, and if not infinity, then mighty close to it, because the amount is boundless. $\endgroup$ – James Gaidis Mar 30 at 14:13

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