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This is more of a technical/wording question, but is there a difference when they say that "at equilibrium, the concentrations are constant" vs "at equilibrium the compositions are constant"? I understand when they say that the concentrations are constant, but is composition the same thing?
According to the textbook, chemical equilibrium is defined as the stage in a chemical reaction when there is no further tendency for the composition of the reaction mixture— the concentrations or partial pressures of the reactants and products— to change. So I believe composition does refer to concentration since the book also states that the equilibrium constant characterizes the composition of a reaction mixture at equilibrium, which refers to the concentrations of the reactants and products that do not change at equilibrium.
I think the book uses the word composition just to encompass both concentrations and partial pressures into its definition of equilibrium. So it's basically correct to say that that the compositions and concentrations in the mixture stay constant at equilibrium. You could also say the partial pressures stay constant at equilibrium if you're dealing with gases.