= Solution
\b[<Ammonia> is the volatile <nitrogen>–<hydrogen> <chemical compound> $\mathrm{NH}_3$, obtained historically from ammoniacal salts and from the decomposition of nitrogen-containing organic matter.] Sources include the decay of animal and vegetable material, the conversion of <urea> in stale urine, and the destructive distillation of <coal>, bones or horn. In gasworks, much of the <Ammonia> was captured in the aqueous <ammoniacal liquor> during cooling and washing of crude <coal gas>. These sources do not mean that nitrogen-free constituents of <coal> spontaneously produce <nitrogen>.
An <ammonium> salt releases <Ammonia> when heated with a suitable <chemical base>. For <ammonium chloride> and <slaked lime>,
$$
\boxed{2\mathrm{NH}_4\mathrm{Cl}+\mathrm{Ca(OH)}_2\longrightarrow2\mathrm{NH}_3+\mathrm{CaCl}_2+2\mathrm H_2\mathrm O.}
$$
The ionic principle is $\mathrm{NH}_4^++\mathrm{OH}^-\to\mathrm{NH}_3+\mathrm H_2\mathrm O$. A generating vessel supplies a drying stage and then an inverted dry collecting vessel in which the lighter <Ammonia> displaces <air>. It is unsuitable for ordinary collection over <water> because it dissolves so readily. <Quicklime> can serve as a drying agent; a strongly acidic drying material would instead consume the <Ammonia>.
<Ammonia> is a colourless <gas> with a penetrating, irritating odour. Its approximate <relative density> against <air> is $17.03/28.97\simeq0.59$. It is readily liquefied under suitable cooling or <pressure>, and its dissolution in <water> releases <heat>. It is toxic and corrosive at sufficient exposure; direct smelling is not an analytical requirement.<Ammonia in the NIOSH Pocket Guide>[Physical Description, Molecular Weight, Solubility and Symptoms]{ref}
The aqueous <chemical equilibrium>
$$
\boxed{\mathrm{NH}_3+\mathrm H_2\mathrm O\rightleftharpoons\mathrm{NH}_4^++\mathrm{OH}^-}
$$
explains the alkaline reaction and the change of moist red <litmus> to blue. The historical name “ammonium hydroxide” for the solution should not imply that every dissolved molecule has become a discrete molecule of $\mathrm{NH}_4\mathrm{OH}$. With <hydrochloric acid>, <Ammonia> forms <ammonium chloride>:
$$
\mathrm{NH}_3+\mathrm{HCl}\longrightarrow\mathrm{NH}_4\mathrm{Cl}.
$$
The familiar white cloud where the gases meet consists of fine solid salt particles, rather than liquid <water> alone. In solution, <Ammonia> also acts as a <ligand>; an excess can produce a deep-blue copper–ammine <coordination complex> from suitable <copper(II)> solutions. Although <Ammonia> does not readily sustain an ordinary flame in <air>, it is not intrinsically incapable of <combustion>; sufficiently favourable mixtures burn.
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