Aliphatic carboxylic acids up to nine carbon atoms are colourless liquids with unpleasant odours, while higher acids are wax-like solids with low volatility, resulting in practically odourless characteristics.
How do the physical states and odours of aliphatic carboxylic acids vary with increasing carbon atoms, and what accounts for the change in volatility?
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The physical states and odors of aliphatic carboxylic acids change with increasing carbon atoms. Short-chain acids (1-4 carbons) are usually liquid with pungent odors (e.g., formic acid smells like ants). Medium-chain acids (5-10 carbons) are often oily liquids with a fruity or rancid odor. Long-chain acids (11+ carbons) are solid at room temperature with weak or no distinct odors. The increase in carbon atoms contributes to higher molecular weight and stronger intermolecular forces, leading to increased boiling points and decreased volatility. This trend in physical properties reflects the influence of molecular size on the behavior of carboxylic acids.