Ionisation of the indicator requires an aqueous medium.
Explanation
Indicators such as litmus are weak acids or bases that change colour in response to the pH of a solution. This colour change is driven by the presence of hydrogen ions ($H^+$) or hydroxide ions ($OH^-$). The manifestation of acidic or basic properties, including the reaction with indicators, is fundamentally dependent on the presence of water (an aqueous medium).
Detailed Analysis:
- Role of Water in Ionisation: In the solid (dry) state, ionic compounds like baking soda ($NaHCO_3$) have their ions held tightly in a crystal lattice. They do not dissociate to release free hydroxide ions ($OH^-$). Without these free ions, the basic property of the substance cannot be exhibited.
- Mechanism of Indicators: Litmus paper works via an ion-exchange mechanism. For the indicator to change colour, it requires a medium that allows for the mobility of ions and the ionisation of the testing substance. In the absence of moisture (both in the baking soda and the litmus paper), no ionisation occurs. Consequently, the chemical reaction required for the colour change does not take place.
- Incorrect Options:
- Baking soda is chemically neutral in its solid state. is incorrect because baking soda is chemically basic, not neutral; it simply appears inactive in the absence of a solvent.
- Baking soda is acidic rather than basic. is incorrect as baking soda is a mild base (alkaline).
- Litmus paper reacts only with strong mineral acids. is incorrect because litmus reacts with both strong and weak acids and bases, provided an aqueous medium is present.
Key Takeaway:
Acids and bases do not dissociate into ions ($H^+$ or $OH^-$) in the dry state. Therefore, they fail to show characteristic properties, such as changing the colour of dry litmus paper, unless water is present to facilitate ionisation.