Final answer:
Ionic compounds are hard, brittle, have high melting and boiling points, and conduct electricity when dissolved in water due to their strong ionic bonds and the mobility of ions in liquid state.
Step-by-step explanation:
The kind of compounds that tend to be hard, brittle, have high melting points, high boiling points, and conduct electricity when dissolved in water are known as ionic compounds. These compounds are formed from the electrostatic attraction between oppositely charged ions, commonly a cation and an anion. Ionic compounds have high melting and boiling points because the ionic bonds that hold the ions in a crystal lattice are very strong. While these compounds do not conduct electricity in their solid state due to their rigid structure, they do conduct electricity when either melted or dissolved in water because the ions are free to move and carry an electric current.
For example, common table salt (sodium chloride, NaCl) is an ionic compound. It is hard and brittle, has a high melting point of about 801°C, and it conducts electricity when dissolved in water, making it an electrolyte.