Acetic acid contributes two different flavor elements to foods. One is its acidity on the tongue, and the other is its characteristic aroma in the nose, which can intensify to a kind of startling pungency, particularly when the vinegar is heated. The vinegar molecule can exist in two forms: as the intact molecule, and broken into its main portion and a free hydrogen ion. The hydrogen ion gives the main impression of acidity, while only the intact molecule is volatile and can escape from the vinegar or food, travel through the air, and reach the nose. Both the intact and “dissociated” forms coexist side by side, in proportions that are determined by their chemical surroundings. If the food is already acidic— thanks to the presence of tartaric acid in wine vinegar, for example—then less of the acetic acid dissociates, more is intact and volatile, and the vinegar aroma is stronger.