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# Reactions Between Ions in Solutions

## Key Questions

• When ions are in solution, they have been pulled apart by the dissolving properties of water.

When salt (NaCl) dissolves in water, ions of sodium and chloride pull away from each other.

NaCl(s) -> NaCl(aq)

OR

NaCl(s) -> $N {a}^{+}$(aq) + $C {l}^{-}$(aq)

A good video on this topic can be viewed here:

One reason this concept is important is that many reactions take place between dissolved ions, for example the double replacement reactions which produce precipitates.

Why would you NOT want to know the mass, the quantity, of a given solute, in a given volume of solvent?

#### Explanation:

And solubilities are quantities that are determined by experiment.

For the quantitative treatment of solubilities, see this old answer for details of ${K}_{s p}$...

I think you yourself could advance many scenarios where the knowledge of solubility is important.

Well, you would have to perform chemical tests to be sure.

#### Explanation:

Some common ions, such as halide ions, copper ions, zinc ions, and ammonium ions can be easily tested.

Halide ions will react with silver nitrate $\left(A g N {O}_{3}\right)$ to give coloured precipitates of silver halides. For example, chloride ions will react with $A g N {O}_{3}$ to give out silver chloride $\left(A g C l\right)$, which is an insoluble white precipitate.

Copper ions are usually in their $+ 2$ state, and are blue in coloured solutions. To test for them, simply add some sodium hydroxide and a blue precipitate of copper(II) hydroxide $\left(C u {\left(O H\right)}_{2}\right)$will form.

Of course, there are many other tests, and these are the basic ones.