Carboxylic acids are organic compounds that contain a carboxyl functional group. Common examples include acetic acid in vinegar and citric acid in fruit. Acids with longer carbon chains often have unpleasant smells, like butyric acid which causes the smell of rancid butter and is also responsible for the odor of sweaty socks. Carboxylic acids can form salts through reactions with metals or metal carbonates that produce hydrogen, water, or carbon dioxide as byproducts.