The title says it all.
I had this question when helping someone with their high-school chemistry homework. What are the reasons that someone would use an (from my point of view) obfuscated notation?
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1$\begingroup$ So that you can send the not-so-bright students out for a quest to search for solid $\ce{NH4OH}$, if they need a base but both $\ce{KOH}$ and $\ce{NaOH}$ would contaminate their sample for further cation testing. $\endgroup$– JanFeb 2 '16 at 23:04
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$\begingroup$ Related: Ammonium Hydroxide Name $\endgroup$– user7951Nov 16 '19 at 16:10
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The lone pair at the nitrogen is very well suited to form hydrogen bonds. This bond is especially strong, since ammonia has a quite high proton affinity, it is a medium strong base.
Now consider the autoprotolysis reactions in pure water and (liquid) ammonia: \begin{align} \ce{2H2O &<=> H3O+ + OH-}\tag1\\ \ce{2NH3 &<=> NH4+ + NH2-}\tag2 \end{align}
Mixing these systems, the reaction of ammonia with water will yield \begin{align} && \ce{2H2O + 2NH3 &<=> H3O+ + OH- + NH4+ + NH2-}\tag3 \\ & (~-~)& \ce{H3O+ + NH2- &-> H2O + NH3}\\\hline && \ce{H2O + NH3 &<=> NH4+ + OH-}\tag4\label{eq:ammonia+water} \end{align} \eqref{eq:ammonia+water} is incorporated into a huge and strong network of hydrogen bonds. In this case the equilibrium will be very much pushed to the right side, meaning, that almost all ammonia molecules will be protonated.
Therefore it is more appropriate to write: \begin{align} \ce{NH4OH (aq) &<=> NH4+ (aq) + OH- (aq)}\tag{4a} \end{align}
However, it is always advisable to look at the conditions of the whole reaction system.
Note that $\ce{NH4OH}$ is a pretty outdated notation, if you would want to consider one water entity explicitly, $\ce{NH3.H2O}$ would be more appropriate. See the answer by Maurice for some more information: Can acid-base reactions occur in a non-aqueous medium?
answered Apr 25 '14 at 5:02
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$\ce{NH4OH}$ is a terrible notation which makes no chemical sense, as there is no such a chemical species and therefore not a dissociation of a mythical species, but deprotonation of water what is behind the basicity.
But I guess textbooks like to use it for "didactical" purposes (means it is easier to tell the kids that $\ce{NaOH}$, $\ce{KOH}$, $\ce{NH4OH}$ are bases then tell the chemistry behind it).
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2$\begingroup$ Well that statement might contain some truth, the part of
no chemical senseis brutally overestimated. While $\ce{NH4OH}$ might not completely appropriate, there are crystal structures of $\ce{NH3.H2O}$. $\endgroup$– Martin - マーチン ♦May 29 '14 at 8:41 -
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2$\begingroup$ @Martin First of all, the notation $\ce{NH3.H2O}$ codes something completely different than $\ce{NH4OH}$, second, there is no a molecular species like $\ce{NH4OH}$ in water, therefore it is totally misleading notation. Why should one use a notation that has no merit, only to hide the chemistry and confuse kids? $\endgroup$– GregOct 11 '14 at 14:43
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$\begingroup$ Simple: like every chemical situation we derive, it's a model. I mean, why do we teach kids how to use lewis structures when MO theory is far more precise? It's an instructive oversimplification. $\endgroup$ Feb 2 '16 at 6:27
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1$\begingroup$ @BreakingBioinformatics Because explaining MO theory and tell people to calculate them in head is far more difficult and then count till eight; while telling people that ammonia is not NH4OH takes about 2 minutes, and an excellent didactic choice to introduce acid-base theory at high school level. Also, if NH4OH is a proper choice of notation, I am really curious how do you handle organic amines, amino acids etc in a consistent way. $\endgroup$– GregMar 6 '17 at 5:16