Electrovalent compounds normally ------------

Analyze the question: It asks about a general property of electrovalent (ionic) compounds, such as NaCl and MgO.

Concept: Electrovalent compounds form when electrons are transferred from a metal to a non-metal, producing positive and negative ions. These ions are held in a crystal lattice by strong electrostatic forces of attraction.

Evaluate each option:

• A. Have low boiling points. False. The electrostatic forces between ions are strong and need a lot of energy to overcome, so ionic compounds have high melting and boiling points. (Low boiling points are typical of covalent molecular compounds.)
• B. Have mobile electrons. False. Mobile electrons are a feature of metals. In ionic compounds the electrons are held by the ions, and none are free to move.
• C. Conduct electricity in the solid state. False. In the solid, the ions are fixed in the lattice and cannot move. They conduct only when molten or dissolved in water, when the ions become free to move.
• D. Dissolve in polar solvent. True. Polar solvents like water have partial positive and negative ends. These attract the ions and pull them out of the lattice (hydration), so ionic compounds typically dissolve in water.

Answer: D

Tip: Remember "ionic = high melting point, soluble in water, conducts only when molten or in solution." Also remember "like dissolves like": ionic and polar substances dissolve in polar solvents, while non-polar substances dissolve in non-polar solvents such as kerosene or benzene.