Qualitative Tests for Carboxylic Acids

Carboxylic acids are identified by their acidity. They are weak acids, but strong enough to react with carbonates and release carbon dioxide.Method: add aqueous sodium carbonate (or sodium hydrogencarbonate, or solid carbonate) to the sample.• Positive result: effervescence (fizzing).
• Bubble the gas through limewater: it turns milky (cloudy), confirming CO₂.With ethanoic acid:2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂The salt formed is sodium ethanoate. The ionic equation is the same for any carboxylic acid:2H⁺ + CO₃²⁻ → H₂O + CO₂Why this test is useful: phenols and alcohols are too weakly acidic to react with carbonates, so only the carboxylic acid fizzes. This makes the carbonate test a way to distinguish a carboxylic acid from a phenol.Worked example: how many moles of CO₂ form when 0.020 mol of propanoic acid reacts with excess sodium carbonate?
From the equation, 2 mol acid gives 1 mol CO₂.
So 0.020 mol acid gives 0.020 ÷ 2 = 0.010 mol CO₂.The same reaction happens with metal carbonates such as CaCO₃, and carboxylic acids also react with reactive metals (giving H₂) and with bases.