Question

Molten sodium chloride conducts electricity due to the presence of:
(A) Free electrons
(B) Free ions
(C) Free molecules
(D) Free atoms

Answer 1

Hint: Electricity can only get conducted when there are charge carriers free to move around. When solid changes its state to liquid these carriers are now free from the shackles that once bound them.

Step-by-Step Solution:
Let us first try and understand what a good conductor of electricity really possesses which differentiates it from other substances which are not-so-good conductors of electricity.
Electricity is the phenomenon of flow of current through a body or medium. Current is the flow of charges across a potential difference. Charges are nothing but species that either lack electrons or have an excess of it. The former is referred to as a positive charge whereas the latter is called a negative charge. In actuality there is nothing called a positive charge, because the only entity that is shareable between atoms is the electron and they have a negative charge. So, less electrons than normal is conventionally called positive charge.
The wires that we use in our homes are good conductors of electricity because the materials used to build them have free electrons which behave as the charge carriers. The solution of sodium chloride in water is a good conductor of electricity because of the ions that act as the charge carriers. The ions are entities that derive themselves from neutral elements either by gaining or donating electrons. This completely depends on their individual tendencies.
So in the case of molten sodium chloride, if charge carriers are present then electricity is conducted or else it is not. Now, we know that sodium chloride is a salt and therefore it is fundamentally an electrovalent compound. An electrovalent bond is formed when the electronegativity of the atoms reacting, has a very significant difference. Here sodium atom is a metal belonging to group one while chloride is a non-metal belonging to group seventeen. They are practically the best examples of a typical metal and non-metal respectively.
This difference in electronegativity forms a crystalline solid structure of positively charged sodium ions and negatively charged chloride ions. The crystal is brittle and white in colour. When enough temperature and pressure is applied, the constituents melt into their liquid form. As the motility of individual atoms or molecules in liquid state is far greater than solid, the ions ($N{{a}^{+}}\And C{{l}^{-}}$) are free to move and therefore can act as charge carriers. Hence molten sodium chloride is a good conductor of electricity because of free ions.

The right answer is option (B).

Note: For the flow of electric current only the presence of charge carriers is not enough. They are a part of many such conditions that make current a reality. There has to be a potential difference between two points which are physically distant from each other. Potential difference occurs when charges are separated from each other. Along with that a suitable medium must also be present which will let the charge carriers flow.