Physical Properties of Carboxylic Acids
1 · State and Odour 😷
- Aliphatic acids with up to nine carbon atoms are colourless liquids at room temperature and smell pretty unpleasant. :contentReference[oaicite:0]{index=0}
- Bigger (higher-member) acids turn wax-like and almost odour-free because they don’t evaporate much. :contentReference[oaicite:1]{index=1}
2 · Boiling Point 🔥
- Carboxylic acids boil at higher temperatures than aldehydes, ketones, and even alcohols of similar size. Strong hydrogen bonds hold the molecules together, so you need more heat to pull them apart. :contentReference[oaicite:2]{index=2}
- The hydrogen bonding is so tough that it survives in the vapour! Most acid molecules pair up as dimers:
\(2\,\text{RCOOH}\;\longleftrightarrow\;(\text{RCOOH})_2\) 🤝 :contentReference[oaicite:3]{index=3}
3 · Solubility in Water 🌊
- Acids with ≤ 4 carbons happily mix with water because they can form H-bonds with H2O. :contentReference[oaicite:4]{index=4}
- Add more carbons and the hydrocarbon “tail” turns hydrophobic, so solubility drops fast. Long-chain acids are almost water-shy. :contentReference[oaicite:5]{index=5}
- Benzoic acid (the simplest aromatic acid) barely dissolves in cold water. :contentReference[oaicite:6]{index=6}
4 · Solubility in Organic Solvents 🧪
Carboxylic acids dissolve nicely in less-polar solvents such as benzene, ether, alcohol, or chloroform. :contentReference[oaicite:7]{index=7}
High-Yield Points for NEET 🎯
- Hydrogen bonding makes carboxylic acids the high-boiling champs among common functional groups.
- Watch for dimer formation — it explains why acids stay associated even in vapour/aprotic media.
- Solubility trend: miscible (≤ C4) ➡️ decreasing ➡️ almost insoluble (long chains); remember Benzoic acid’s poor water solubility.
- Hydrogen-bonding with water vs. hydrophobic tail tug-of-war is a favourite conceptual question.
- Comparing boiling-point orders (acid > alcohol > ketone ≈ aldehyde) often pops up in match-type and assertion-reason items.
