[BaO]

today my teacher showed us how an indicator works by an experiment : we had 2 tubes . tubeA contained acetic acid, tube B contained NaOH
(i dont remember exactly how much  their concentrations were , but i guess [NaOH] was smaller.)

then, we added some drops of phenolphthalein to each tube ,tube A turned into a colour just like milk's. tube B turned pink . when i poured NaOH (from tube B) into tube A , the colour in A still remained the same .
And this is my question: why the colour didnt change? as i know , acid + Base -> salt+ water, the indicator still works in salt and water?

[jubba]

Phenolphthalein changes colour at around a ph of 8. So depending on how much and what concentration of NaOH you add all the acetic acid may not be neutralised and ph may still be less than 8.
If you look at a ph curve you'll see that the ph changes are quite sudden and so you will not usually observe any graduall changes in colour.