Ball-and-stick model of citric acid. 枸橼酸的球棍模型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。如果其三个碳原子中的每个氢原子被羧基取代，其第二碳原子的剩馀氢原子（中间）被羟基取代，结果即是枸橼酸。
