Correct Answer:

In order to explain the function of a salt bridge in the voltaic cell, it is important to understand what a salt bridge is. A salt bridge is a U shaped bridge composed of an inert salt (e.g. NaCl or KCl) solution and is usually placed on filter paper or within a glass tube. The filter paper or glass tubing is used to provide a solid surface for the salt bridge solution to remain. The use of a relatively inert salt will prevent the salt bridge from participating in the chemical reaction, while allowing it to still conduct. It is used to connect each of the reaction vessels by facilitating the free flow of electrons. The salt bridge is also the main reason each metal and its corresponding metal ion can be housed in its own reaction vessel.

Answering the Question:

In order to answer this question it is important to understand the process taking place. The process of moving electron from one substance capable of donating them, to another substance capable of accepting them is termed redox (reduction-oxidation). In the above diagram Mg(s) is giving up electrons (being oxidized) and Ni²⁺ (aq) is accepting them (being reduced). This is nothing more than the transfer of electrons from one substance to another. However it is important to note that the salt bridge in each of the solutions is the only item linking Mg(s) to Ni²⁺ (aq). Therefore, even if you are unfamiliar with what a salt bridge or a voltaic cell is, then by looking at the above equation in terms of electron transfer you should still be able to identify its purpose.

That purpose is to facilitate the flow of electrons.