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From their early experience of indicators in the spiral curriculum, some students may hold the notion that indicators are red in acidic solutions and green in neutral solutions. Students can find choosing an indicator is tricky and they need to understand that a particular indicator undergoes a colour change at a specific pH range.

Students also have to be aware or colour mixing, a bit like mixing paint, for example, red plus yellow makes orange. They may struggle to grasp that both species (HIn and In^‒) are present in the mixture and exist in equilibrium with one another and that one species predominates under acidic/alkaline conditions.

What colour is methyl orange at the end point?

At the end point it is orange. This is due to a mixture of the yellow and red species.

Under what circumstances might you tell your class that the end point for methyl orange is red and when would you tell them it is orange?

At pre-16 level, your students may be relieved to see a colour change at all and it may be prudent to accept red as there might typically be a bit of overshoot. Post-16 classes studying more advanced concepts will be capable of understanding that the end point is orange and why this is the case.