Why is BeO insoluble in water whereas BeSO4 is soluble in water?
BeO is almost insoluble in water and BeSO4 is soluble in water because Be2+ is a small cation with a high polarising power and O2 is a small anion. 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, sulphate ion is a large anion. Thus, the lattice energy of BeSO4 is not very high and so it is soluble in water.