Understand: equilibrium and Le Châtelier’s principle

A lab group prepares an equilibrium reaction by acidifying a solution of chromate ions $\text{(}\mathrm{CrO}{\phantom{A}}_4{\phantom{A}}^{2-}\text{)}$‍ in a beaker. The resulting products are the dichromate ion $\text{(}\mathrm{Cr}{\phantom{A}}_2\mathrm{O}{\phantom{A}}_7{\phantom{A}}^{2-}\text{)}$‍ and water.

After equilibrium is reached, the teacher asks the members of the lab group to explain what’s happening in the flask right now. Two student answers are shown below.

Student A: The reaction is still going on, but the rates of the forward reaction and the reverse reaction are now equal. In other words, dichromate is still being produced, but at the same rate that it's also being broken back down to chromate.

Student B: At the beginning of the reaction, the chromate and hydrogen ions began producing dichromate and water. As the reaction moved towards equilibrium, the concentration of the reactants decreased and the concentration of the products increased. Now that the reaction is at equilibrium, these concentrations are constant.