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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,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 $17.03/28.97\simeq 0.59$. 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.^[ref]

The aqueous chemical equilibriumexplains 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: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.

Bleaching powder is made by passing chlorine over slaked lime, exposing a large surface of the powder to the gas. The contemporary name “chlorinated lime” is more informative than treating it as pure calcium chloride. The 1867 British Pharmacopoeia (1867) describes this manufacture and a dull-white, partly water-soluble powder with bleaching and disinfecting properties.^[ref]

An idealized overall chemical equation isThe chlorine undergoes disproportionation: one part becomes chloride, with oxidation state $-1$, and another becomes hypochlorite, with oxidation state $+1$. Commercial bleaching powder contains residual calcium hydroxide, water and chlorine-bearing solid phases; the chemical equation is a useful composition balance, not a claim that the powder is one pure molecular substance or simply a fully characterized mechanical mixture. The old formula ${\mathrm{CaOCl}}_2$ is an abbreviated representation, not the formula of pure calcium hypochlorite, ${\mathrm{Ca}(\mathrm{OCl})}_2$.

Its chief properties follow from the hypochlorite chemistry. The aqueous extract is alkaline and oxidizing. The chemical equilibriumproduces hypochlorous acid on lowering the pH. Oxidation of coloured organic compounds can destroy the chromophores responsible for their colour. This is irreversible chemical alteration of a dye, rather than merely washing it away. The historical description that bleaching works through “nascent oxygen” is not a necessary modern mechanism: hypochlorous acid and related reactive chlorine species themselves react with substrates.

With sufficient hydrochloric acid, the hypochlorite and chloride can generate chlorine:This explains acid-induced gas evolution and why acidification is not an innocuous way to use a bleaching material. Exposure, moisture and heating can reduce the stored oxidizing strength. Its useful strength is measured as available chlorine, rather than inferred from the powder's total mass or its smell; inert material and residual lime do not provide the same bleaching capacity.