= Solution
\b[<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.<British Pharmacopoeia (1867)>[Calx Chlorata, printed p. 65]{ref}
An idealized overall <chemical equation> is
$$
\boxed{2\mathrm{Ca(OH)}_2+2\mathrm{Cl}_2\longrightarrow\mathrm{Ca(OCl)}_2+\mathrm{CaCl}_2+2\mathrm{H}_2\mathrm{O}.}
$$
The <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>, $\mathrm{Ca(OCl)}_2$.
Its chief properties follow from the <hypochlorite> chemistry. The aqueous extract is alkaline and oxidizing. The <chemical equilibrium>
$$
\mathrm{OCl}^-+\mathrm{H}^+\rightleftharpoons\mathrm{HOCl}
$$
produces <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>:
$$
\mathrm{OCl}^-+\mathrm{Cl}^-+2\mathrm{H}^+\longrightarrow\mathrm{Cl}_2+\mathrm{H}_2\mathrm{O}.
$$
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.
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