Here, we’ll show you how the solubility of a compound can be changed. Altering Solubility #1 Introduction.

Here, we’ll show you how the solubility of a compound can be changed.

Altering Solubility

#1 Introduction

Here, we’re dealing mainly with low-solubility, or “slightly soluble” compounds. These include compounds identified as “Low Solubility” on the solubility table…

And all the compounds listed on the Ksp table on your data booklet.

Ksp Table

In any question dealing with altering solubility, we always start with two important things:

In any question dealing with altering solubility, we always:

First, we identify which compound has low solubility. We do this by consulting the solubility table or Ksp table. Remember compounds containing spectator ions, which are alkali metal cations and the nitrate anion, usually don’t have low solubility.

In any question dealing with altering solubility, we always:

1.Identify which compound has low solubility.

After we have identified the low solubility compound, we write the solubility equilibrium equation for it. This equation is central to our discussion.

In any question dealing with altering solubility, we always:

1.Identify which compound has low solubility.

2.Write the solubility equilibrium equation for it.

As an example, we’ll start by considering the low-solubility compound, barium sulphate.

Consider the low-solubility compound barium sulphate.

It’s solubility equilibrium equation looks like this, with the solid BaSO4 on the left and it’s dissolved ions on the right. In the animation to follow, we’ll represent the amount of each substance by the size of its text in the equation. Watch it closely.

Consider the low-solubility compound barium sulphate.It’s solubility equilibrium equation is:

4(s)BaSO 2(aq)Ba 2

4(aq)SO

Anything which causes this equilibrium to shift to the left…

Consider the low-solubility compound barium sulphate.It’s solubility equilibrium equation is:

Shift to the LEFT

4(s)BaSO 2(aq)Ba 2

4(aq)SO Shift Left

Like this

4(s)BaSO 2(aq)Ba 2

4(aq)SO Shift Left

Shift to the LEFT

4(s)BaSO

Results in both dissolved ions having a net decrease in concentration.

2(aq)Ba 2

4(aq)SO

Net DECREASE in

concentration

Net DECREASE in

concentration

Shifted to the LEFT

4(s)BaSO

And the undissolved solid having an increase in mass. Remember we cannot change the concentration of a solid, only the amount, or mass of solid present in the container.

2(aq)Ba 2

4(aq)SO

Net INCREASE in mass

Shifted to the LEFT

4(s)BaSO

So a shift to the left means there is more undissolved solid and less dissolved ions.

2(aq)Ba 2

4(aq)SO

A shift to the left means: there is more undissolved solid and less dissolved ions.

4(s)BaSO

This means a shift to the left causes the solubility to decrease. This is something you should always remember. Shifting to the Left, Decreases the solubility.

2(aq)Ba 2

4(aq)SO

A shift to the LEFT causes:The solubility of the compound to DECREASE

Now, lets consider the solubility equilibrium equation for barium sulphate again and see what happens when it shifts to the right…

Consider the low-solubility compound barium sulphate.It’s solubility equilibrium equation is:

Shift to the RIGHT

4(s)BaSO 2(aq)Ba 2

4(aq)SO

Shift Right

Like this.

Shift to the RIGHT

4(s)BaSO 2(aq)Ba 2

4(aq)SO

Shift Right

24(aq)SO 2

(aq)Ba

We can see that as a result of shifting to the right, the concentrations of the dissolved ions have increased…

4(s)BaSO

Net INCREASE in

concentration

Net INCREASE in

concentration

Shifted to the Right

24(aq)SO 2

(aq)Ba

And the mass of undissolved solid has decreased

4(s)BaSO

Net DECREASE in the mass of

Solid BaSO4

Shifted to the Right

24(aq)SO 2

(aq)Ba

So a shift to the right means there is less undissolved solid remaining and more dissolved ions in the solution. So in other words, as this equilibrium shifts to the right, more of the solid dissolves.

4(s)BaSO

Net DECREASE in the mass of

Solid BaSO4

A shift to the right means: there is less undissolved solid and more dissolved ions.

Net INCREASE in

concentration

Net INCREASE in

concentration

24(aq)SO 2

(aq)Ba

So we can say that when the solubility equilibrium shifts to the right, the solubility of the compound increases.

4(s)BaSO

A shift to the RIGHT causes:The solubility of the compound to INCREASE

So to summarize what we’ve done, when the solubility equilibrium shifts to the left, the solubility decreases, and more solid forms.

4(s)BaSO 2(aq)Ba 2

4(aq)SO

A shift to the LEFT causes:The solubility of the compound to DECREASE

24(aq)SO 2

(aq)Ba

And when the solubility equilibrium shifts to the right, the solubility increases and more of the solid dissolves. It is very important to remember these.

4(s)BaSO

A shift to the RIGHT causes:The solubility of the compound to INCREASE

4(s)BaSO 2(aq)Ba 2

4(aq)SO

A shift to the LEFT causes:The solubility of the compound to DECREASE