Question

When do bases change the colour of methyl orange?

Answer 1

Indicators:

• Indicators are substances that are added during the reaction to indicate the endpoint of the reaction by a colour change or undergo a colour change when exposed to acid or alkali.
• Indicators are basically weak acid or weak base.
• Different indicators are used based on the $\mathrm{pH}$ of the solution.
• Eg: Phenolphathalein and Methyl orange.
• $\mathrm{pH}$$>7$- Base, $\mathrm{pH}<7$- acid and $\mathrm{pH}-7$- Neutral.

Methyl orange:

• Methyl orange is used as an indicator in strong acid weak base titration.
• The colour change depends on the $\mathrm{pH}$ of the solution.
• The methyl orange $\mathrm{pH}$ range is $3.1-4.4$.
• Methyl orange gives red colour in an acidic solution.

Reaction:

• When methyl orange is added to the base solution, the colour changes to yellow.
• The methyl orange changes colour at the pKa of a mid-strength acid
• The colour is yellow because the acid nature of the solution gets decreased as it is a base, so the orange colour changes to fade to yellow.