Chemical reaction

Consider the following 
Ca(OH)2 + 2NH4Cl → CaCl2 + 2NH3 + 2H2O 
(Source: <https://chemiday.com/en/reaction/3-1-0-440>)


It appears to be double displacement reaction as CaCl2 is formed, but it’s not as NH4OH is not formed instead NH3 is formed 


Why NH3 is formed instead of NH4OH ? 


Is there intermediate step which transform NH4OH into NH3( I don’t think so but still)




It is postulated that NH4OH as a molecule does not exist in solution. A better way to write might be NH3(aq). As to the evolution of gaseous ammonia, this reaction would only proceed if you heat up (i) a moist paste of NH4Cl and Ca(OH)2 or (ii) strongly heat a solution, otherwise ammonia would remain dissolved in the solution. 


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$\ce{NH4OH <=>NH3 + H2O}$, that is, they are in *equilibrium*. Since the solution is weakly ionized, ([*Kb* is ~1.8 \* 10-5 at 293 K](https://en.wikipedia.org/wiki/Ammonia_solution)), there is little $\ce{NH4OH}$ present at a given instant, so ammonia water is only weakly alkaline.


One might say the answer $\ce{NH3 + H2O}$ *is 99.998% correct*, but *$\ce{NH4OH}$ is still 0.002% correct*, not entirely *wrong*.


See also [this discussion](https://chemistry.stackexchange.com/questions/10230/why-nh4oh-why-not-nh3aq) on what species are present.


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