The explanation in Extra is definitely helpful & adds conceptual understanding to the definition. I can also understand how “when a chemical reaction has reached equilibrium” adds more meaning to a reader than “chemical equilibrium.” I’ve edited to have “Exclude pure solids and liquids” above your explanation in Extra since users would immediately see it first & then be able to read through the conceptual explanation. If it’s the opposite way, some readers won’t immediately see it & move onto the next card, but since it’s popped up on a couple AAMC materials, I think it’s worth having first in the Extra