Why is solubility often listed as a chemical property? Isn't the action of dissolving just a physical change? [closed]








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I am making a list of basic physical vs chemical properties.
I have been taught that when salt dissolves in water it is simply a special kind of mixture called a solution.  

I have been taught that mixtures do not create a chemical change or reaction.
But I found a lot of high quality chemistry websites that list solubility as a chemical property of compounds.


Why isn't solubility a physical property if dissolving is simply forming a mixture and not causing a chemical reaction?




The classification of physical and chemical changes has done more harm to students than good. This topic was taught in the 19th century but it still lingered in modern general chemistry textbooks which serve to repel good students from taking up chemistry as their field. For example, if we call the charge of electron red, and the charge of proton blue, will it change any fundamental thing?


You can try to understand physical vs. chemical change as follows: If you are changing the *physical form (liquid, solid, gas)* of one or more component, then you can safely label it as a physical change provided you can bring the original form back.


You can melt NaCl crystals into a liquid, this is a physical change because we can cool it and bring back the original material. 


In the same way, solubility is just a *property* of matter. It is a very complex phenomenon. Whether you call it physical or chemical it does not matter in real life (most scientists will not worry about it and nobody will ever ask you these questions once you pass this course-so much so for the utility of this concept). For example, HCl is very soluble in water, but also reacts with water in such a way that it protonates water. Dry HCl gas is "neutral" but its solution in water is a powerful acid. There is no simple way to extract the gas back out of water. This solubility involved a chemical change.


On the other hand, salt NaCl when dissolved in water, it will not chemical react with water per se and you can recover the salt by evaporation. Should I call this a chemical change or a physical change? It does not matter.


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The distinction between physical and chemical change is actually not as cut and dried as one might think. Sure, what happens to copper in nitric acid is very different from what happens to sugar in water. But what about, say, the process that occurs when a steel strip is cleaned (basically removing carbon from the surface) in an atmosphere containing carbon dioxide?


In this process the carbon "reacts" with the carbon dioxide according to:


$\ce{C(s) +CO2 <=> 2CO}, \Delta H>0$


where we note the equilibrium; the gas can become "saturated usually with a lot of the carbon dioxide "solvent" left over. As indicated by the sign of the enthalpy change, this is an endothermic reaction, and more carbon reacts upon heating. If we were to allow the strip to cool the carbon could redeposit (it may also do so on cooler parts of the furnace). From that point of view the process begins to look more like sugar + water (which is also reversed by cooling) than copper + nitric acid. Moreover, we could interpret the carbon monoxide as carrying the carbon in a gas-phase solvated form, from which the carbon can be un-solvated again if precipitation is made favorable (e.g. by cooling), again something we would associate more with a physical change than the chemical one that the reaction above describes.


So is dissolution a physical or a chemical change? Depending on the system it could be either ... or both.


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Consider a solution of $\textrm{NaCl}$ in water and another solution of $\textrm{AgNO}\_3$ in water. If you mix these two solutions, $\textrm{AgCl}$ precipitates out. If you do not consider solubility as a chemical process/change, then it is very hard to explain this.


A mere physical process should be no more than juxtaposition of four different kind of ions in the above mixed solution. Why should two ions choose to combine and opt out of this free democratic existence? 


A chemical process OTOH, is conventionally understood as one in which bond formation and/or bond cleavage takes place. Exactly what has transpired in the above process. Isn't it?


 3

One simple way to look at solubility as a chemical property may be to ask...What will it dissolve in?


You're right. When salt dissolves in water you get a solution. 


But, salt does not dissolve well in oil. 


There are, in fact, mnemonics to help people remember all the different rules for solubility. 


Here's a link to a few solubility rules: <https://m.youtube.com/watch?v=AsCLuLS-yZY>


Physical properties are much more cut and dry. When you reach a certain temperature the substance melts. When you add even more heat the substance will eventually boil. 


I hope that helps to give you a way to think about solubility as a chemical property. 


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