Which one of the following substances conducts electricity by the movement of ions?

Electricity conduction depends on the presence of mobile charge carriers. In substances like metals, electrons are the mobile carriers, whereas in ionic compounds, ions serve as charge carriers when they are free to move.

Sodium chloride (NaCl) is an ionic compound composed of Na⁺ and Cl⁻ ions arranged in a rigid lattice. In its solid state, the ions are held tightly in place, so solid NaCl does not conduct electricity. However, when NaCl is heated to its melting point, it forms molten NaCl, and the rigid lattice breaks down. The ions become mobile, allowing them to move freely under an applied electric field.

In molten NaCl, Na⁺ ions move toward the cathode, and Cl⁻ ions move toward the anode, completing the circuit and allowing electric current to flow. This type of conduction is called ionic conduction and is characteristic of molten salts and aqueous solutions of electrolytes.

In contrast:

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  Graphite conducts electricity via delocalized electrons along its layers.

  
  


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  Copper and mercury are metals, and they conduct electricity through mobile electrons, not ions.

  
  


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  Solid NaCl cannot conduct because ions are fixed in the lattice and cannot move.

  
  

Understanding ionic conduction is crucial in inorganic chemistry, particularly for electrolysis, where molten salts or aqueous solutions are used to deposit metals or generate gases like chlorine and hydrogen. Molten NaCl electrolysis produces chlorine at the anode and sodium at the cathode, demonstrating practical applications of ionic conduction.

Thus, among the given options, molten sodium chloride (NaCl) is the substance that conducts electricity by the movement of ions