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

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 t.

Explanation:

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your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.