Question

Electrovalent compounds are hard crystals but brittle?

Answer 1

Electrovalent compounds are hard because of tight packing of ions in lattices (structural arrangement of ions in crystals). The positive and negative ions are strongly attached among themselves. So, if mechanical pressure is applied to an Electrovalent structure then ions of similar charges may be forced to get closer to each other.
Now, by doing so, the electrostatic repulsion can be enough to split or disorient completely the lattice structure. Thus imparting the brittle character.