Ionic solids are hard and brittle. Ionic solids can be melted to form liquids that are an electrical conductors. Ionic solids that are water soluble, dissolve to form solutions that are electrical conductors. Why ionic substances are the way they are. Ionic solids are rigid because the multiple interactions between oppositely charged ions hold the charged particles in relatively fixed positions.
Electrical conductivity depends on ions being able to move freely.
The conduction of heat depends partly on the movement of electrons in a substance. This means there is poor mobility of charges and poor conductivity of electricity and heat. Ionic solids are brittle and hard because the electrostatic attractions in the solid again hold the ions in definite positions. The electrostatic attractions must be overcome to move the ions. When the ions are shifted by some very strong force the positions of ions shift so that like charged ions are close together.