Why did the equilibrium move from left to right when acid was added?

The addition of acid means that the system is no longer in equilibrium because the concentration of H⁺ has increased. The equililibrium position moves from left to right to use up some of the H⁺ and CrO₄^2– ions and to produce more CrO₇^2– ions and water.

Why did the equilibrium move from right to left when sodium hydroxide solution was added?

The hydroxide ions react to remove hydrogen ions in the mixture to form water. The concentration of hydrogen ions therefore falls. The equilibrium position moves from right to left to produce more hydrogen ions. The concentration of CrO₄^– ions also increases and the concentration of CrO₇^2– ions falls.

How does this activity build on what has gone before?

Initially, students are asked to carry out activities in which they change the concentration of components of equilibrium mixtures. They might use the iron(II) thiocyanate equilibrium, for example. This allows them to discover the effect on equilibria themselves. In this activity they can be asked to speculate about what will happen before acid and alkali are added and so apply their ideas.