Ball-and-stick model of citric acid.
枸櫞酸的球棍模型

Ball-and-stick model of propane. If one hydrogen atom of each of its three carbon atoms is replaced by a carboxyl group, and the remaining hydrogen atom of its second carbon atom (in the middle) is replaced by a hydroxyl group, the result is citric acid.
丙烷的球模型。如果其三個碳原子中的每個氫原子被羧基取代，其第二碳原子的剩餘氫原子（中間）被羥基取代，結果即是枸櫞酸。