A clear visual guide to Le Chatelier’s Principle, using the colourful equilibrium between chromate(VI) and dichromate(VI) ions to demonstrate how changes in concentration affect the position of equilibrium.

The resource is built around the simple definition:

If a system at equilibrium is disturbed, the position of equilibrium shifts to oppose the disturbance.

DISTURB → SHIFT → OPPOSE

The equilibrium studied is:

2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l)

Students can follow changes in equilibrium through the contrasting colours of yellow chromate(VI) ions and orange dichromate(VI) ions.

The resource demonstrates:

Adding hydrochloric acid increases the concentration of H⁺ ions. The equilibrium shifts to the right to oppose this change, using up some of the added H⁺ and moving the solution colour towards orange.
Adding water shifts the equilibrium in the opposite direction, moving the colour towards yellow.
Clear before-and-after illustrations allow students to connect an experimental observation directly with a shift in equilibrium.
Each change is summarised using the sequence DISTURB → SHIFT → OPPOSE.
The equilibrium equation is presented with charges and state symbols.
Appropriate safety information is included for working with concentrated acid and chromium(VI) compounds.

The resource is particularly useful for showing that an equilibrium shift does not mean complete conversion to products or reactants. The colour moves towards yellow or orange as the equilibrium composition changes.

Suitable for GCSE, IGCSE and A-level Chemistry, particularly for teaching dynamic equilibrium, Le Chatelier’s Principle, concentration changes and transition-metal chemistry.

A QR code links to FranklyChemistry, where students can view the corresponding animated demonstration and explore further chemistry resources.