Laboratory tests for carboxyl (-COOH) group

Three distinct laboratory tests confirm the presence of a -COOH group in an unknown compound. (a) LITMUS TEST (valid specifically for water-SOLUBLE substances): an aqueous solution of a -COOH-containing compound turns BLUE litmus RED, indicating an acidic functional group is present -- though it should be noted that water-soluble PHENOLS also turn blue litmus red, so this test alone cannot, by itself, distinguish a genuine carboxylic acid from a phenol. (b) SODIUM BICARBONATE TEST: adding sodium bicarbonate (NaHCO3) to a -COOH-containing compound gives a brisk EFFERVESCENCE of carbon dioxide gas; even a water-INsoluble acid will dissolve into aqueous solution here, as its sodium salt, and will then re-precipitate back out as the free acid on subsequent acidification with concentrated HCl: R-COOH + NaHCO3(aq) [losing CO2 and H2O] gives R-COONa(aq), and this with HCl gives R-COOH(down-arrow, re-precipitated) + NaCl(aq). Unlike the litmus test above, phenol does NOT evolve any CO2 gas at all when treated with sodium bicarbonate -- so this particular test genuinely CAN distinguish a true carboxylic acid from a phenol, where the litmus test could not. (c) ESTER TEST: one drop of concentrated sulfuric acid is added to a mixture of the -COOH-containing compound with about 1 mL of ethanol, and the mixture is warmed for around five minutes in a hot-water …