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A carboxylic acid is defined as any hydrocarbon with a carboxyl group (–COOH), which looks like a ketone and an alcohol combined. Some examples include acetic acid (found in vinegar), citric acid, acrylic acid, butyric acid, caproic acid, and biotin.

Diagram of a carboxylic acid

In a carboxylic acid, the hydrogen atoms have a slightly positive charge, and the oxygen atoms have a slightly negative charge. This polarity creates an attraction between carboxylic acid molecules that is strong enough to form a hydrogen bond. When two molecules are held together by a hydrogen bond, the compound as a whole is called a dimer.

Carboxyilic acid dimer

This property causes carboxylic acids to have higher boiling points. It is impossible for a gas to form intermolecular hydrogen bonds. Therefore, in order for liquid carboxylic acids to become a gas, the energy added to each molecule has to be enough to overcome the hydrogen bonds between them.

The polarity of carboxylic acids also allows them to be soluble in water. Instead of forming a dimer, the negatively-charged oxygen molecules in the carboxylic acid are attracted to the positively-charged hydrogen molecules in the water, while the oxygen molecules in the water are attracted to the hydrogen molecules in the carboxylic acid. This allows each carboxylic acid molecule to bond to a water molecule, causing the carboxylic acid molecules to be evenly dispersed throughout the solution.

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