Boiling point checks ester purity via consistent vaporization

Steps:

• Pure liquids vaporize at a single, sharp temperature, indicating uniformity.
• Impurities lower or broaden the boiling range, showing contamination.
• For food flavorings, consistent boiling ensures safe, unaltered ester composition.
• Measure boiling point experimentally to verify purity before use.

Why A is correct:

• Pure substances boil at a fixed temperature; impurities cause boiling point elevation or depression per colligative properties like Raoult's law.

Why the others are wrong:

• B. Colour: Pure esters are often colorless, but color varies by ester type, not purity.
• C. Smell: Subjective and inconsistent; impurities may not alter scent detectably.
• D. Solubility in water: Inherently low for esters; minor impurities don't significantly affect it.

Final answer: A