TrafficLightsThis is a Redox reaction. A variant on the classic ‘Blue Bottle’.

The mixture starts off green. Gently shaking it produces one oxidised form with a different colour – red, while vigorous shaking will then turn this into a different oxidised form again, with yet another colour still -yellow this time.
As the mixture is allowed to stand, the dye reverts back to its reduced forms with the accompanying colours again becoming visible.

The indicator can be oxidised many times by shaking.

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Traffic Lights – Risk Assessment