Question

How do we describe the solution behaviour of `PbCl_2(aq)`?

Answer 1

Lead(II) chloride is very insoluble in aqueous solution. The filtrate is saturated with respect to `Pb^(2+)`.

Explanation:

A saturated solution is a solution in which the concentration of the solute is equal to THAT WHICH WOULD BE IN EQUILIBRIUM WITH UNDISSOLVED SOLUTE:

`PbCl_2(s) rightleftharpoons PbCl_2(aq)`, i.e.

`PbCl_2(s) rightleftharpoons Pb^(2+) + 2Cl^-`

The supernatant solution (the solution that lies atop undissolved solute) is said to be saturated with respect to lead chloride. Normally, a temperature is specified because a hot solution can usually hold more solute than a cold one.

AS for any equilibrium, we can express the reaction in terms of an equilibrium constant:

`K_(sp) = [Pb^(2+)][Cl^-]^2` `=` `"A small number"`. The subscript `"sp"` stands for solubility product. These are extensively tabulated for a range of sparingly soluble and insoluble salts; normally, laboratory temperature, `298` `K` is specified. So if `[Cl^-]` is artificially increased (by adding `Cl^-`), the ion product, `[Pb^(2+)][Cl^-]^2` will be momentarily GREATER than `K_(sp)`. In order to obey the `K_(sp)` expression, lead chloride will precipitate in order to reduce `[Pb^(2+)]` and `[Cl^-]`.

I acknowledge that I have gone on a long time about nothing to the power of less. The important definition is the following:

A saturated solution is a solution in which the concentration of the solute is equal to THAT WHICH WOULD BE IN EQUILIBRIUM WITH UNDISSOLVED SOLUTE.

An examiner would be quite justified in rejecting a definition of saturated solution as a solution in which the solvent holds all the solute that it can .