Increase Solubility of Sparingly Soluble Salts

Let's consider how to manipulate solubility of sparingly soluble salts.

230206 affect solubility

Under the concept of common ion effect, we have learnt explicitly that solubility of salts can be suppressed when a common cation or anion is added.

Check out this video lesson to learn all about common ion effect.

We can also increase solubility of salt via the following 2 methods.

1. Complex ion formation
2. Acid base reaction

Let's consider each one in detail.

1. Complex ion formation

We can differentiate aqueous halides by adding silver nitrate followed by aqueous ammonia.

The observations are as follows:

230206 differentiate aqueous halide test

When AgNO3 is added, the ionic product of silver halide is much higher than its solubility product since silver halides are sparingly soluble with very low Ksp values.

Hence silver halide precipitate is formed.

230206 form AgX ppt

When aqueous ammonia is subsequently added, diammine silver complex is formed which is very stable.

230206 form diammine complex

This decreases the concentration of Ag+(aq) which will shift the position of equilibrium for the dissociation of silver halide towards the right hand side.

230206 solubility of AgX increase

Hence more silver halide will dissolve and its solubility increases.

AgCl is the most soluble and requires only dilute ammonia to dissolve it.

AgBr is less soluble so will need concentrated ammonia to dissolve.

AgI is the least soluble and will not dissolve even with concentrated ammonia.

2. Acid base reaction

If the sparingly soluble salts are carbonates or hydroxides, we can add acid to react with the anions and reduce their concentrations.

230206 solubility of CaCO3 increase

230206 solubility of MgOH2 increase

In both cases concentration of carbonate and hydroxide decrease, position of equilibrium for dissociation of salt shifts to the right, more salt dissolves and solubility increases.

Topic: Solubility Product, Physical Chemistry, A Level Chemistry, Singapore

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