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

an ion that is found in a wide variety of compounds
an ion that is present in every sample of pure water
an ion that is found dissolved in most aqueous solutions that are widely used in labs
an ion that is present in an equilibrium system and a compound added to the system

The correct answer is " an ion that is present in an equilibrium system and a compound added to the system."

For example, AgNO₃ when added to AgCl in equilibrium, here Ag⁺ is the common ion.

AgCl --> Ag⁺(aq) + Cl⁻(aq)

The common ion (Ag⁺) will reduced the solubility or dissociation of AgCl. Ag⁺ will favor the backward reaction or the association of Ag⁺ and Cl⁻ causing the formation of AgCl.

a pan containing water is placed in a positio n where it is exposed to a 10 mile per hour wind. the pan contains a uniform depth of 1.27 cm of water, is quite wide, and has a length of 2.44 m in the wind direction. the water in the pan is at a constant temperature of 15.5 °c.