Sides of the pink phenolphthalein agar will become colourless when placed in vinegar.
As hydrogen ion present in vinegar enters the agar cube, the colour of the agar changes.
We can see the colour change up to the distance/depth where the H+ ion diffused into the agar
Phenolphthalene is a pH indicator. It is pink in colour in alkaline pH and colourless in acidic pH. We know vinegar is acetic acid. Therefore when the pH changes from alkaline to acidic, pink colour of the agar will change and it will become colourless up to the depth where the H+ ion diffused into.
If the H+ ion diffuse fully upto the centre, the whole agar block will become colourless
This is an experiment used to the measure the time taken and distance travelled by the diffusing H+
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