Why is ammonia written as NH3·H2O and not NH3·2H2O?

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When studying the solubility of ammonia, a classic question arises: according to textbook knowledge, the solubility of ammonia is 1:700 (1 L of water can dissolve 700 L of ammonia). At room temperature, with a molar volume of around 24 L/mol, 1 L of water (approximately 55.6 mol) can dissolve 700 L of ammonia (approximately 29.2 mol), resulting in a molar ratio of ammonia to water of about 1:2. As we all know, ammonia molecules (NH3) are bonded to water molecules (H2O) through hydrogen bonds; theoretically, each ammonia molecule can form up to four hydrogen bonds (and even six in solid ammonia). The main component of ammonia water is ammonium hydroxide (NH3·H2O), and it is generally believed that ammonia gas and water molecules combine through hydrogen bonds, which raises some questions. A molar ratio of 1:2 means that one NH3 molecule corresponds to two H2O molecules. Since hydrogen bonds allow an NH3 molecule to bind with one H2O molecule, why can’t the other half of the water molecules form hydrogen bonds as well, resulting in NH3·2H2O? First, we need to know that NH3·H2O is not a \"molecule with a fixed ratio.\" The NH3·H2O representation we see in textbooks is actually a simplified form; it does not mean that one ammonia molecule can only bind to one water molecule, forming a fixed 1:1 molecular structure. Many students mistakenly assume that NH3 and H2O combine in a strict 1:1 ratio, just like crystalline water, to form stable \"ammonium monohydrate molecules.\" In reality, however, the actual structure of ammonia solution is much more complex – after ammonia dissolves in water, the vast majority of NH3 does not exist in the fixed form of \"NH3·H2O.\" The reason why NH3·H2O is used in textbooks is primarily to facilitate the representation of the weak basicity of ammonia water and its ionization equilibrium. The true structure of ammonia gas in ammonia solution is \"netted around\" by water molecules. The essence of ammonia dissolving in water is that a large number of water molecules use hydrogen bonds to \"solvate and enclose\" the NH3 molecules. This type of bonding has two key characteristics: first, hydrogen bonds are \"dynamic and non-stoichiometric\"; they are not as stable as chemical bonds, as they keep breaking and forming again, and there is no strict \"one-to-one\" binding rule. Around one NH3 molecule, up to 3–6 water molecules can be bound through hydrogen bonds, while a single water molecule can also be connected to multiple NH3 molecules or other water molecules, forming a network of hydrogen bonds. Second, hydrogen bonds do not have \"exclusivity\". The hydrogen bond formed by a water molecule does not belong to just one NH3 molecule; it can participate in the formation of multiple hydrogen bonds at the same time. In other words, there is no mandatory rule that requires “one NH3 to exclusively occupy one H2O”; multiple NH3 molecules can share the hydrogen bond network formed by water molecules. In other words, the hydrogen bonds in ammonia are “networked and interwoven,” rather than “one-to-one paired,” so it is not possible for one NH3 molecule to bind to two H2O molecules to form NH3·2H2O. Why does the textbook have to use NH3·H2O? Since NH3·H2O does not have a true fixed structure, why do textbooks and references still use this notation? In fact, this is a \"conventional simplification\" in chemistry, and there are mainly two reasons for it: one is to facilitate the representation of ionization equilibrium. Ammonia water is a weak base that does not ionize completely; its ionization process requires a concise expression to describe it, and NH3·H2O meets this requirement perfectly. The well-known ionization equation: NH3·H2O⇌NH4＋＋OH－, uses NH3·H2O as an intermediate product, which allows for a clear representation of the process by which NH3 gradually ionizes to produce OH－ upon dissolving in water; this does not affect subsequent equilibrium calculations. Second is the continuation of historical traditions. When early scientists studied ammonia, they mistakenly believed it was composed of \"ammonia monohydrate molecules with a fixed composition.\" It was only through more in-depth structural studies that its true network of hydrogen bonds was discovered. Nevertheless, the simplified notation NH3·H2O has been widely used, and it has been retained for the sake of ease in teaching and communication. In fact, in some university textbooks, the ionization of ammonia water is written in the form of ammonia gas + ionization of water: