0905 Dissolution of ionic salts in water

Module 0905
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Dissolution of ionic salts in water: a competition

When we dissolve salt in water, it "disappears". Does it still exist?

What is happening (at the level of ions and molecules) when an ionic salt dissolves in water?

What is the structure of ionic salts?

Why are some salts soluble in water, and others not?

Water molecules as active participants

The dissolving process as a competition. Who wins the tug-of-war? 

​Aquation (or hydration) of ions
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​Dissolution is the noun for the process of dissolving.

Just what is this phenomenon of an ionic salt dissolving in water? Watch Aussie have a few Aha! moments with the gentle nudging of Prof Bob .......

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)

KEY IDEAS - Dissolving ionic salts in water

May your understanding become crystal clear.

The role of water molecules - A preface

The big take-home message of this module is:

When substances dissolve (or don't dissolve) in water, the water molecules are not just onlookers in the separation of the ions in the crystal lattice: they are active participants in the process.

The structure of ionic salts: A lattice of cations and anions

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.
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​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.

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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.

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So why do the ions separate from each other in water?

Why do the cations and anions separate from each other when an ionic salt is shaken in water, and the solid dissolve?

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.
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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.


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Aquation (or hydration) of the cations and anions

​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.
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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.

Think about it .......

Salt dissolves in water ..... or     Water dissolves salt?

Salt dissolves in water ..... or     Salt is dissolved by water?

Video portrayal

How do chemists visualise the process, at the molecule/ion level, of dissolution of sodium chloride in water? What do they think we might see if we could put a super-powerful microscope into water while sodium chloride is dissolving? Click HERE to view a video portrayal of what it might look like.

Video animation produced by ©VisChem (VisChem.com.au) 
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Extension: All chemical reactions are competitions

The notion discussed in this module that dissolution of ionic compounds in water as a competition process can be extended to all chemical reactions - except that we can classify different types of reaction according to the question "What is being competed for?" or "What is competing for what?" See Module 0508 Chemical reactions as competitions  and Module 1101 Visualising dynamic chemical equilibrium.

External reference
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).

SELF CHECK: Some thinking tasks

Have you got the notion of the dissolving process as a tug-o-war competitive process? Check ....

Assess whether the following statements are correct or incorrect.
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1. Sodium ions are grey, and chloride ions are green.
2. Chloride ions are bigger than sodium ions.
3. In a sodium chloride crystal, each chloride ion has six other chloride ions as immediate neighbours.
4. In water, the water molecules are free to rotate in any direction, depending upon nearby forces of attraction (or repulsion).
5. When a crystal of sodium chloride is placed in water, sodium ions at the surface are attracted toward water molecules with their oxygen atoms directed toward the sodium ions.
6. Potassium nitrate is soluble in water. If solid potassium nitrate is placed in water and shaken up, the attraction from water molecules to potassium ions and nitrate ions on the surface “wins” the competition against attraction from ions within the crystal.
7. If a crystal of calcium carbonate is placed in water and shaken up, the attraction of water molecules to calcium ions and carbonate ions on the surface “wins” the competition against attraction from ions within the crystal.
8. The symbolism (aq) means aqueous.
9. In a solution made by dissolving solid potassium sulfate (K2SO4) in water, potassium sulfate molecules are surrounded by water molecules.
10. The following three statements are eqauivalent: (i) Water molecules are attracted to sodium ions, (ii) Sodium ions are attracted to water molecules, and (iii) Water molecules and sodium ions are attracted to each other.
11. If 5 g of solid sodium nitrate is shaken up in 100 mL of water until it all dissolves, there is no species that we call sodium nitrate in the solution.
12. The term “aquated nitrate ions” (or “hydrated nitrate ions”) refers to nitrate ions surrounded by a “shell” of water molecules with their oxygen atoms directed toward the nitrate ions.
13. In a solution of magnesium sulfate in water, the only species present (apart from water molecules) are aquated magnesium ions and aquated sulfate ions.
14. If solid crystalline calcium carbonate is shaken in water, calcium ions and carbonate ions on the surface of the crystal are attracted by both (i) other ions of opposite charge within the crystal, and (ii) water molecules. The ions within the crystal “win” the competition. So calcium carbonate is soluble in water.
15. When we dissolve sodium chloride in water, we cannot see it. But it still exists.
    

     

Answers

1. Incorrect. Colour is a property of substances, not of individual atoms or ions. The colours of the spheres that represent ions in the animation are chosen randomly so that we can distinguish them.
2. Correct, as is the case for most anions (negatively charged ions) compared with cations. This is partly due to the fact that anions have more electrons than the parent atoms, while cations have less electrons than their parent atoms.
3. Incorrect.
4. Correct.
5. Correct.
6. Correct.
7. Incorrect. Calcium carbonate is essentially insoluble in water.
8. Incorrect. The symbol (aq) means aquated (or hydrated): that is, to be surrounded by a “shell” of strongly attracted water molecules. The terms aqueous means that the solvent is water. For example, a solution of sodium chloride in water is an aqueous solution. And the species present are aquated sodium ions and aquated chloride ions.
9. Incorrect.
10. Correct.
11. Correct.
12. Incorrect.
13. Correct.
14. Incorrect.​
15. Incorrect. There is no such substance as sodium chloride in the solution. The ions have been pulled apart by the attraction of water molecules, and the species present are aquated sodium ions, Na+(aq), and aquated chloride ions, Cl-(aq).
    

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