Question

A solution of caustic soda $(\rm{NaOH})$ in water or when fused, conducts electric current. What is the similarity in these two cases?

Answer 1

Hint:Electrical conductivity is possible only if ions or electrons move from one end to another end. If there is no movement of electrons and ions, then electrical conductivity is not possible.

Complete answer:
Sodium hydroxide is an ionic compound. It contains sodium cations and hydroxide ions. Strong electrostatic forces of attraction are present between oppositely charged ions. These electrostatic forces of attraction between oppositely charged ions are called ionic bonds.
In solid state, sodium hydroxide cannot conduct electricity as the ions are not free to move. They are tightly packed in the crystal lattice and are localized. At the best, they can vibrate around their mean position. Since there is no movement of ions, there is no electrical conductivity in solid sodium hydroxide.
Fusion means melting. You heat a solid substance so that it melts, it changes its physical state from solid to liquid. A substance melts when it is heated to a temperature that is equal to its melting point. When you fuse sodium hydroxide, the molten liquid can easily conduct electricity because ions (sodium cations and hydroxide anions) can move freely throughout the liquid. An electric current is nothing but the passage of ions under the influence of an electric field.
In dilute aqueous sodium hydroxide solution, the ions are free to move throughout the solution. Since ions can freely move in the solution, they can conduct electricity.
Thus, a solution of caustic soda in water or when fused, conducts electric current. The similarity in these two cases is that ions are free to move throughout. You can consider ions as current carrying particles.

Note:
Based on electrical conductivity, you can classify the ions into three types, conductors, insulators and semiconductors. Conductors are good conductors of electricity. Insulators do not conduct electricity at all. Semiconductors have very little electrical conductivity. You can increase the electrical conductivity of a semiconductor by doping with suitable impurity.