Okay, let's keep going! The next class is ketones, which are similar to aldehydes, but they have two carbon atoms attached to the carbonyl group. One of the simplest ketones is acetone (CH3-CO-CH3). After ketones, the next class is carboxylic acids, which are molecules that contain a carboxyl group, which is a carbon atom double-bonded to an oxygen atom and also bonded to a hydroxyl group. One of the simplest carboxylic acids is acetic acid (CH3-COOH).