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Water molecules and various salt molecules are very different. However, it seems very difficult to separate the two. Once a salt is dissolved in water, an energy or chemical intensive method (like boiling) is required to separate the salt back out again. Why is this?

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This question appears to be off-topic because it is about chemistry rather than physics. Would chemistry.SE be a better place for this question? – Danu Jul 3 '14 at 13:26
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I don't think this is off-topic here. It might get more answers or exposure at Chemistry, but I think it's perfectly on-topic here. My suggestion would be to wait a bit (a day or two) and if there's no love here, ask to have it migrated. – tpg2114 Jul 3 '14 at 14:01
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@Danu: Whilst it is certainly on-topic for the chemistry SE, I believe it is also on-topic for the physics SE. In addition, in many aspects, physics subsumes chemistry. – JamalS Jul 3 '14 at 14:10
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This question describes a physical (rather than chemical) reaction as being of interest, seems on topic to me. – Kyle Oman Jul 3 '14 at 14:32
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In fact, it may even be more physics than chemistry in the sense that the reason for salt and water to be hard to separate is partly the same as why it is hard to separate different gases in a mixture. – Ruslan Jul 3 '14 at 16:56
up vote 3 down vote accepted

$"$If we put a crystal of salt in the water, what will happen? Salt is a solid, a crystal, an organized arrangement of "salt atoms."

Strictly speaking, the crystal is not made of atoms, but of what we call ions. An ion is an atom which either has a few extra electrons or has lost a few electrons. In a salt crystal we find chlorine ions (chlorine atoms with an extra electron) and sodium ions (sodium atoms with one electron missing). The ions all stick together by electrical attraction in the solid salt, but when we put them in the water we find, because of the attractions of the negative oxygen and positive hydrogen for the ions, that some of the ions jiggle loose.

enter image description here

In Fig.we see a chlorine ion getting loose, and other atoms floating in the water in the form of ions. This picture was made with some care. Notice, for example, that the hydrogen ends of the water molecules are more likely to be near the chlorine ion, while near the sodium ion we are more likely to find the oxygen end, because the sodium is positive and the oxygen end of the water is negative, and they attract electrically.$"$ $_1$ $-\text{Feynman}.$

Feynman has done well in explaining you the process in atomic point of view. Now, comes your complexity of separating salt from water in a salt solution. During the process of boiling, the intermolecular forces will be broken between water molecules, and also between ions and water molecules. Water molecules ($\text{H}$$_2$$\text{O}$) being less massive ($18.01528(33)$) than the other two individual ions ($\text{Na}$$=22.98976928(2)$-$\text{Cl}$$35.45(1) $), flies off easily leaving sodium and chlorine ions. These ions once again attract each other to form crystals. In other words energy is required to break the intermolecular forces and release ions from prison to join their partners.


Credits: $_1$ Feynman lectures on Physics-Volume 1-Page No.1-6. Page no's may vary depending on editions.

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If the energy required to break the ionic bonds is so high, how come we don't see that heat produced when the bonds are being formed, aka the salt being dissolved? – woojoo666 Jul 15 '14 at 17:00
    
@woojoo666, who says you don't see heat produced? Have you ever tried dissolving salt in water in a calorimeter? I haven't done it with salt, but I did it in a high-school science class with sodium hydroxide. We dissolved anhydrous NaOH at room temperature in distilled water at room temperature in a styrofoam cup, and the temperature rise was substantial. I assume that the same thing (but probably less dramatic) would have happened if we'd used NaCl instead. – james large Feb 3 at 17:54

In short, they are hard to separate because even though the molecules are very different, they have properties that attract them to each other.

Water is a polar molecule. the oxygen molecule oxidizes the two hydrogen molecules, creating a positive charge on the hydrogen side, and a negative charge on the oxygen side.

Meanwhile, salt is composed of Sodium, a positive ion, and Chlorine, a negative ion. The charges on the water molecule attract the oppositely charged ions and pull them off of the salt crystal, effectively breaking the salt apart at the molecular level.

Then energy that it takes to separate the particles back again is essentially what is needed to counteract these attractive forces.

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Visit http://www.science.uwaterloo.ca/~cchieh/cact/applychem/hydration.html You can see that the enthalpy of hydration is a two step proccess solvation and reverse crystallization, the ΔH(hydr) is actually positive so you have to give energy just to dissolve the NaCl in water, in order to seperate the water from the NaCl you need to account for the enthalpy of evaporation of water and the enthalpy of reverse hydrationEnthalpy of evaporation of water

The temperature is in Kelvins so going at around the 300 K mark you can see the ΔH evap is 44 KJ/mol substracting the ΔΗ hydr you get a lower ΔΗ meaning that the salt actually lowered the energy required to boil off the water>

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