Question

What is the definition of a common ion as it applies to Le Chatelier’s principle?

Answer 1

The definition of a common ion as it applies to Le Chatelier's principle is an ion that is present in an equilibrium system and a compound added to the system. This is the case since in Le Chatelier's principle, it is based on an equilibrium system and where the reaction shifts to the left or to the right; towards the products or the reactants side.  

Answer 2

Answer: D) an ion that is present in an equilibrium system and a compound added to the system

Explanation: Any change in the equilibrium is studied on the basis of Le-Chatelier's principle.

This principle states that if there is any change in a equilibrium reaction, the equilibrium will shift in a direction so as to minimize the effect.

Thus when a common ion is introduced to an equilibrium reaction, the equilibrium will shift in a direction where the concentration of common ion is decreasing.

For example: In a equilibrium reaction for dissociation of acetic acid:

$CH_3COOH\rightleftharpoons CH_3COO^-+H^+$

If HCl is introduced wth $H^+$ as common ion.

$HCl\rightarrow H^++Cl^-$

The equilibrium will shift in backward direction where the concentration of $H^+$ is decreasing.