The process - a competition
"Dissolution" is the noun for the process of dissolving.
Just what is this phenomenon of an ionic salt dissolving in water? Let's chat ....
[In the animations in this video (sodium chloride as crystalline solid, melting, and dissolving in water), the sodium ions are represented by the smaller grey spheres, and chloride ions by the larger green spheres.]
Animations included with permission: © VisChem (VisChem.com.au)
The accepted model of a solid ionic salt is a solid in which there is a lattice network of cations and anions distributed among each other in an orderly way.
A representation of the lattice structure of sodium chloride (NaCl), for example, deduced from x-ray cystallography, has six negatively charged chloride ions as immediate neighbours around each sodium ion, and six positively charged sodium ions around each chloride ion.
A representation of the structure of part of a sodium chloride crystal. The smaller spheres represent sodium ions, and the larger spheres chloride ions.
From R.B. Bucat (Ed.) Elements of Chemistry: Earth Air Fire and Water, Australian Academy of Science, 1984.
Because of this interlocking structure over the whole of a crystal, the ions are tightly bound. This is evidenced not only by its state as a solid (the ions are locked in place), but the high melting point of 801 °C.
This then begs the question of why the ions separate when the solid is shaken in water, and the solid quickly dissolves.
This phenomenon is attributed to the evidence that water molecules are dipoles: they behave like a rod with a positive electrostatic charge at one end (on the side of the hydrogen atoms) and a negative charge at the other end (on the oxygen atom).
In conjunction with the ability of water molecules to freely rotate, we can imagine that water molecules are attracted to the ions on the surface of the crystal – with negative ends of the dipole pointing toward the sodium ions, and positive ends toward the chloride ions.
These water molecules are also attracted back into the bulk of the water (by dipole-dipole attractions to other water molecules around them).
The result is a competition for the ions on the crystal surface – between inner ions pulling them back into the crystal, and water molecules trying to drag them away from the crystal and into the aqueous phase.
The experimental fact that sodium chloride is soluble in water tells us that the water molecules win the tug-of-war.
The species in the solution are (i) sodium ions surrounded by water molecules, and said to be aquated, or hydrated, and (ii) aquated (or hydrated) chloride ions. – and there is no identifiable species that we call sodium chloride.
Schematic representations of an Na+(aq) ion, and a Cl-(aq) ion. Note the different orientations of the water molecules toward the positive ion and the negative ion.
From R.B. Bucat (Ed.) Elements of Chemistry: Earth Air Fire and Water, Australian Academy of Science, 1984
This process is represented by the chemical equation:
We would draw similar conclusions for other soluble salts such as potassium nitrate and magnesium sulfate.
In the case of insoluble ionic salts (like silver chloride, barium sulfate, and calcium carbonate) we can conclude that the inner crystal ions win the competition against the polar water molecules.
A concise video portrayal, at the molecular level, of the dissolution of sodium chloride in water has been produced by VisChem (VisChem.com.au) and is published here, with kind permission ....
Published with permission: © VisChem (VisChem.com.au)
For the best textbook discussion of this subject, you might like to go to …….
Mahaffy, P.G., Bucat, B., Tasker, R., and others. “CHEMISTRY: Human Activity, Chemical Reactivity” (Nelson Education), pages 178-182 (both International and Canadian editions).
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