2CrO
42-(aq, yellow) + 2H
3O
+(aq)
Cr
2O
72-(aq, orange) + 3H
2O(l)
In the above equation, if CrO42- is formed, yellow colour is observed.
If Cr2O72- is formed, orange colour is observed.
When HCl is added to the mixture,
HCl ionizes with water to form H3O + ion
So with the addition of HCl, the concentration of reactents increases.
According to Le-Chatlier's principle, if external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset and the system reaches a new equilibrium position.
So with the increase in concentration of reactents, the equilibrium begins to shift to the right i.e towards products.
This causes a increases in formation of products which are orange in colour.
So on addition of HCl, the student will observe a colour change from yellow to orange.