Magnesium sulfate is soluble in water. Its solubility is $\pu{26.9 g}/\pu{100 mL}$ at $\pu{0 ^\circ C}$ $(\pu{35.1 g}/\pu{100 mL}$ at $\pu{20 ^\circ C})$.[1] However, both calcium sulfate $(\pu{0.21 g}/\pu{100 mL}$ at $\pu{20 ^\circ C})$ and strontium sulfate $(\pu{0.0135 g}/\pu{100 mL}$ at $\pu{25 ^\circ C})$ are much less soluble in water. [2] and [3]

Moreover, magnesium hydroxide is less soluble than calcium hydroxide and strontium hydroxide.

Since the magnesium ion has a smaller radius, shouldn't magnesium sulfate be less soluble than calcium sulfate and strontium sulfate?

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    $\begingroup$ To determine solubility we have to consider both Lattice energy and Hydration Energy but the problem is that they both decrease down the group, so we cannot predict trends without knowing the data. All we can do is remember. $\endgroup$ – Nisarg Bhavsar Apr 1 at 2:46

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