Chemistry – Acids, bases and salts - The characteristic properties of acids and bases | e-Consult

An alkali solution causes a change in colour in an indicator because the hydroxide ions (OH^-) in the alkali react with the acidic components of the indicator molecule. This reaction typically involves the removal of a proton (H⁺) from the indicator molecule by the hydroxide ion, leading to a change in the molecule's structure and, consequently, a change in the wavelength of light it absorbs. This change in absorption results in a different colour being observed.

Here's how different alkalis would affect the colour of the indicators:

• Litmus: Any alkali solution (e.g., NaOH, KOH, Na₂CO₃) will turn red litmus paper blue. The hydroxide ions neutralize the acidic components of the litmus dye.
• Thymolphthalein: A strong alkali solution (e.g., NaOH, KOH) will turn thymolphthalein from colourless to pink/purple. The hydroxide ions remove the acidic proton from the thymolphthalein molecule.
• Methyl Orange: A strong alkali solution (e.g., NaOH, KOH) will turn methyl orange from yellow to orange. The hydroxide ions remove the acidic proton from the methyl orange molecule.

The strength of the alkali will affect the extent of the colour change. Stronger alkalis will cause a more pronounced colour change because they produce a higher concentration of hydroxide ions.