The equilibrium between chromate (yellow) and dichromate (orange) ions shifts with the addition of acid or base. Acidic conditions favor dichromate, while basic conditions favor chromate.

1. Dissolve potassium chromate (K₂CrO₄) in water to make a yellow solution.
2. Dissolve potassium dichromate (K₂Cr₂O₇) in water to make an orange solution.
3. Place chromate solution into two flasks (1 and 2), and dichromate solution into two flasks (3 and 4).
4. Add hydrochloric acid (HCl) to one chromate and one dichromate flask. The solutions turn orange as dichromate ions dominate.
5. Add sodium hydroxide (NaOH) to the other chromate and dichromate flasks. The solutions turn yellow as chromate ions dominate.
6. Discuss how the observed color change demonstrates Le Chatelier’s principle.

Metal Complexes 7. Chromate - Dichromate equilibrium - FranklyChemistry:

Chromate Dichromate Ion Equilibrium - LeChatelier's Principle Lab Part 2 - North Carolina School of Science and Mathematics:

📄 CHROMATE/DICHROMATE EQUILIBRIUM - scilearn.sydney.edu.au: https://scilearn.sydney.edu.au/fychemistry/demonstrations/files/9.2.pdf

• Use smaller test tubes instead of flasks for a classroom-scale version.
• Try diluting the solutions to emphasize the difference between yellow and orange more clearly.

• Wear safety goggles, gloves, and a lab coat.
• Handle chromium(VI) compounds with extreme care - they are toxic and carcinogenic.
• Avoid skin contact and inhalation of dusts or vapors.
• Collect and dispose of all waste solutions in labeled hazardous waste containers.
• Do not pour chromium solutions down the drain.

• What color corresponds to chromate ions and what color corresponds to dichromate ions? (Chromate = yellow, Dichromate = orange.)
• How does adding acid affect the equilibrium? (Shifts to the right, forming dichromate.)
• How does adding base affect the equilibrium? (Shifts to the left, forming chromate.)
• Which principle explains this shift in equilibrium? (Le Chatelier’s principle - the system responds to minimize the change.)