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Question
How would you explain the following observation?
BeO is almost insoluble but BeSO4 in soluble in water.
Solution
BeO is almost insoluble in water and BeSO4 is soluble in water. Be2+ is a small cation with a high polarising power and O2– is a small anion. The size compatibility of Be2+ and O2– is high. Therefore, the lattice energy released during their formation is also very high. When BeO is dissolved in water, the hydration energy of its ions is not sufficient to overcome the high lattice energy. Therefore, BeO is insoluble in water. On the other hand, `"SO"_4^(2-)` ion is a large anion. Hence, Be2+ can easily polarise `"SO"_4^(2+)` ions, making BeSO4 unstable. Thus, the lattice energy of BeSO4 is not very high and so it is soluble in water.
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