When ethanoic acid and ethanol are heated with a little concentrated sulfuric acid as a catalyst, ethyl ethanoate (ethyl acetate) and water form. A reaction like this, in which a carboxylic acid and an alcohol join and lose a molecule of water, is called esterification.
The reaction is reversible, so not all of the acid and the alcohol change into the ester. When the mixture is poured into water, ethyl ethanoate, which does not dissolve much in water and is less dense than water, floats to the top and gives off a fruity smell.
Many esters have pleasant fruity smells, and they give fruits and flowers part of their fragrance. They are used as artificial flavourings and in perfumes.
Ethyl ethanoate dissolves many substances, so it is used as a solvent in glues and nail polish remover.
When an ester is heated with water and a little acid, it is split into a carboxylic acid and an alcohol. This is called hydrolysis, and it is the reverse of esterification.
When an ester is heated with sodium hydroxide solution, it is hydrolysed completely, giving the sodium salt of the carboxylic acid and an alcohol. Hydrolysis by a base like this is called saponification.
\mathrm{CH_3COOC_2H_5} + \mathrm{NaOH} \longrightarrow \mathrm{CH_3COONa} + \mathrm{C_2H_5OH}
Fats and oils are esters formed from glycerol, which has three hydroxy groups, and three molecules of long-chain carboxylic acids called fatty acids.
Common fatty acids include stearic acid, \mathrm{C_{17}H_{35}COOH}, which has only single bonds between its carbon atoms, and oleic acid, \mathrm{C_{17}H_{33}COOH}, which has one double bond.
Fats and oils are an important source of energy for living things, giving about 9 kcal (38 kJ) per gram.
Fats and oils that are solid at room temperature, such as butter and lard, are called fats, while those that are liquid, such as olive oil and soybean oil, are called oils.
Saturated fatty acids have straight chains that pack closely together, so fats rich in them have high melting points. In unsaturated fatty acids, the chain is bent at each double bond, so the molecules cannot pack closely, and the melting point is lower.
When hydrogen is added to the double bonds of an oil with a nickel catalyst, the oil becomes solid. This is called hardened oil, and it is used to make margarine.