Final answer:
The use of acid or base in an extraction process is applied to change the water solubility of an unknown compound, a key factor in liquid-liquid extraction techniques.
Step-by-step explanation:
In the context of methods of extraction, the use of either acid or base to treat an unknown mixture is typically applied in order to change the water solubility of an unknown compound. This is fundamental in liquid-liquid extractions, where the goal is the selective partitioning of a solute between two immiscible phases. By altering the pH of the aqueous solution with an acid or a base, certain compounds can be converted from a non-ionized form to an ionized form (or vice versa), which significantly affects their solubility in water.
For instance, when dealing with organic compounds, many of which have acidic or basic functionalities, adding an acid to a basic compound will often protonate the base, making it more water-soluble. Conversely, adding a base to an acidic compound deprotonates the acid, again making it more water-soluble. This principle is commonly exploited in acid-base extraction techniques to separate compounds from a mixture based on their differing solubilities in an aqueous environment versus an organic solvent.
Therefore, the correct answer to the question is that the use of acid or base in an extraction process is applied in order to change the water solubility of an unknown compound.