# Why does adding water to a saturated solution increase the number of ions present in the solution?

In my book, it is stated that when some water is added to a test tube having a saturated $\ce{Ag2SO4}$ solution, with its solid at equilibrium, the number of $\ce{Ag}$ ions in the solution increases.

I don't understand it. If $\ce{Ag2SO4}$ solution is saturated, it means that no more solute can dissolve, and if we add water, the amount of dissolved solute should not change.

Also, if the amount of dissolved $\ce{Ag}$ increases, would the $K_\mathrm{sp}$ value also increase?

• The concentration is fixed. Not the amount of dissolved ions present in a solution. – Avnish Kabaj Mar 8 '18 at 14:53
• $\pu{K_{sp}}$ is a constant at a given temperature. It doesn’t increase with increase in $\ce{[Ag+]}$ ions. – MollyCooL Mar 8 '18 at 14:57
• Related question here – MollyCooL Mar 8 '18 at 15:07
• The number of $\ce{Ag}$ ions increases, but the number of $\ce{Ag}$ ions per volume of water does not increase. – Curt F. Apr 10 at 23:23

## 1 Answer

Consider a saturated solution of $\ce{Ag2SO4}$. The following equilibrium is attained. $$\ce{Ag2SO4 <=> 2Ag+ + SO4^2-}$$

Adding water accounts to increasing the volume of the solution. Hence more amount of solute can be dissolved since solubility of a salt depends on the amount of salt dissolved per unit volume. Therefore, there is an increase in the no. of ions produced in the solution. But as Avnish Kabaj mentioned, $[\ce{Ag+}]$ remains the same.

Also as MollyCooL said, $\pu{K_{sp}}$ doesn't change as it is a constant at a particular temperature.

• We have to add a fixed amount of solute to a fixed amount of water to get saturated solution so if we add more water to solution and don't change the amount of solute so the amount of dissolved ion should not change – Marva Jami Mar 8 '18 at 15:34
• @MarvaJami - It depends. Is there more of the solid $\ce{Ag2SO4}$ or not? So if 1 liter dissolves 0.1 grams of something, then 2 liters would dissolve 0.2 grams (total). – MaxW Mar 8 '18 at 16:20
• Will if the solution is saturated it means we have 0.1 gram dissolved ions in 1 liter so if I add 1 liter water and don't add more agno3 the number of ions would remain the same am I right? – Marva Jami Mar 9 '18 at 3:21
• Yes it wont change then, but what if there is a solubility equilibrium established where you have both the sakt and the ions, then on adding water, the equilibrium will be disturbed and more amount of salt can be dissolved – Bastardane Mar 9 '18 at 4:53