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An indicator shows whether an substance is a base or an acid but during a neutralisation reaction the colour change occurs due to the solution be neutral (i.e pH is 7). But since the indicators only show colour change in an acidic or basic condition doesn't this mean the solution is either acidic or basic?

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closed as unclear what you're asking by Mithoron, a-cyclohexane-molecule, A.K., Tyberius, Nuclear Chemist Oct 1 '18 at 15:15

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    $\begingroup$ Different indicators change colour at different pHs, not necessarily at neutral pH $\endgroup$ – Waylander Sep 30 '18 at 20:04
  • $\begingroup$ And change for different reasons. but as an example see: Phenolphthalein $\endgroup$ – A.K. Sep 30 '18 at 22:35
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You asked two different questions.

(1) Why does a colour change in neutralisation occur?

In general for acid-base indicators, the indicator molecule either gives up or acquires a $\ce{H^+}$ which changes the bonding levels in the molecule. Electrons traveling between the bonding energies thus have different energies which show up as different colors for the two different molecular configurations.

(2) But since the indicators only show colour change in an acidic or basic condition doesn't this mean the solution is either acidic or basic?

Essentially yes, but it is foremost a significant figures problem. So if 50 ml of the acid is 0.02 molar and the equivalence point is at pH = 7.00, the different in moles of base (used to neutralize the acid) between pH = 7.00 and say pH = 8.2 for the phenolphthalein endpoint is inconsequential.

Also not all "equivalence points" are at pH 7.00 The equivalence point for the titration of an acid with a base is the pH at which you have added as many moles of base as there are moles of acid. Since the pKa's of acids vary widely, so do the pH's of the equivalence points.

  • For a strong acid (ie assuming complete ionization and no protonation of the conjugate base) then the "equivalence point" would be pH = 7.00.

  • For a weak acid, some of the conjugate base is protonated (ie $\ce{A- + H2O -> HA + OH-}$ ) so $\ce{pH_{eq} > 7.00}$. Thus the $pK_{eq}$ would be the same pH as if the sodium salt of the conjugate base ($\ce{Na+A-}$) was dissolved in the final volume of the solution.

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  • $\begingroup$ So a colour change does not neccesarily refer to a neutral solution being formed. $\endgroup$ – Manny Sep 30 '18 at 20:55
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    $\begingroup$ Correct. The pH of the color change depends on the indicator. Link to table of acid-base indicators. Note the various transition pHs. $\endgroup$ – MaxW Sep 30 '18 at 20:58
  • $\begingroup$ I'll add that the easiest transitions to detect visually are for indicators that go from colorless to colored. $\endgroup$ – MaxW Sep 30 '18 at 22:11

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