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

Acids are substances that release hydrogen ions (H⁺) when dissolved in water. This excess of H⁺ ions is responsible for their characteristic properties. The effect of acids on different indicators is as follows:

• Litmus: Litmus paper is a natural indicator derived from lichens. In a neutral solution, litmus paper is blue. In an acidic solution, litmus paper turns red. This occurs because the H⁺ ions in the acid react with the hydroxide ions (OH^-) present in the litmus dye, altering its colour.
• Thymolphthalein: Thymolphthalein is an artificial pH indicator that is colorless in acidic solutions (pH 8.3), it turns blue. The colour change is due to the deprotonation of the thymolphthalein molecule, which is favoured by the presence of hydroxide ions in alkaline conditions. Acids, by neutralizing the hydroxide ions, prevent the colour change.
• Methyl Orange: Methyl orange is another artificial pH indicator. It is red in acidic solutions (pH yellow in alkaline solutions (pH > 6.2). The colour change is due to the protonation of the methyl orange molecule in acidic conditions, leading to a different electronic structure and thus a different absorption of light. As the pH increases, the concentration of H⁺ decreases, and the molecule deprotonates, resulting in the yellow colour.

In summary, acids increase the concentration of H⁺ ions, which causes a shift in the equilibrium of the indicator molecules, leading to a change in colour.