In a solid, the particles (atoms, molecules, or ions) are held in fixed positions by strong intermolecular forces, such as covalent bonds, ionic bonds, or metallic bonds. These strong forces prevent the particles from moving freely and carrying electric charge. As a result, solids generally do not conduct electricity.

However, when a substance is melted, the intermolecular forces are weakened or broken, and the particles become free to move more easily. This allows the particles to carry electric charge, which is why molten substances can conduct electricity.

For example, sodium chloride (NaCl) is a solid at room temperature and does not conduct electricity. However, when sodium chloride is melted, the ions (Na+ and Cl-) are free to move and carry electric charge, allowing the molten substance to conduct electricity.

In summary, the difference in conductivity between solid and molten substances is due to the mobility of the particles. In a solid, the particles are fixed in place and cannot move freely, preventing the flow of electric charge. In a molten substance, the particles are free to move, allowing them to carry electric charge and conduct electricity.