Le Chatelier’s Principle in Colour: Equilibrium Experiments That Speak for Themselves
Some chemistry experiments need a lot of explanation. This one doesn’t. When A-Level students investigate equilibrium using cobalt chloride, the chemistry literally changes colour in front of their eyes.
The Experiment
We use the equilibrium between two forms of cobalt chloride:
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The pink hexaaqua complex dominates in cold, dilute solutions.
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The blue tetrachlorocobaltate dominates when the solution is heated or concentrated with chloride ions.
In practice:
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A test tube of cobalt chloride solution is placed in cold water → it turns pink.
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The same tube in hot water shifts to blue.
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Adding hydrochloric acid pushes the equilibrium even further towards blue.
No lengthy explanation needed — the colours show the equilibrium shift.
Linking to Le Chatelier’s Principle
Le Chatelier’s Principle states: If a system at equilibrium is disturbed, it will shift to oppose the change.
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Heat added (endothermic direction): The equilibrium shifts to favour the blue complex.
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Heat removed (exothermic direction): The equilibrium shifts to favour the pink complex.
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More chloride ions added: The equilibrium shifts right, producing more of the blue complex.
The colour changes give an immediate, visual confirmation of the principle.
Why Students Remember This One
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It’s dramatic — the tube can go from pale pink to deep blue in seconds.
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It’s clear — no graphs needed to “prove” the shift.
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It’s extendable — students can design their own tests, like diluting or concentrating, to predict and check the outcome.
Teaching Tip
Ask students to predict first: What will happen if I cool this? What if I add more chloride? Then run the experiment and let the colour answer. The simplicity means the principle lodges in memory.
✅ Sometimes the best chemistry demonstrations are the ones that don’t need words. With cobalt chloride, Le Chatelier’s Principle speaks for itself — in pink and blue.
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