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The trend of solubility down group 2 increases for hydroxides, while it falls for sulphates. Why is that?

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If we consider the trend in solubility for a group II cation with a sulfate vs a hydroxide then we are looking at two very different things.

The trend for the solubility with a sulfate or carbonate is the trend for a larger dianion. If we consider the trend Be,Mg,Ca,Sr,Ba,Ra then the size of the dication is increasing.

If we have two large ion types (one cation and one anion) then these will pack better into the crystal lattice than a large and a small. As a result the energy given out when the ions are transfered from the gas phase into the solid is larger.

On the otherhand when we are considering the solubility of hydroxides and fluorides then we are dealing with small anions. Here the small cations such as magnesium two plus will pack better into the lattice with fluoride than the larger cations such as barium and radium.

The most important thing to consider is the difference in the solvation energy which is the energy given out when the gas phase ions are dispersed and dissolved into water and the lattice energy which would be given out when the gas phase ions are combined into a crystaline lattice.

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