= Solution
\b[<Iodine> is a dark, lustrous crystalline <halogen> which gives a violet vapour; historically, an important source was the ash of <iodine>-rich seaweeds.] Elemental <iodine> commonly occurs as diatomic $\mathrm I_2$. It melts at about $114\ ^\circ\mathrm C$ and boils at about $184\ ^\circ\mathrm C$ at ordinary atmospheric <pressure>. It also undergoes <sublimation> below its melting point, depositing crystals again on a cooler surface.<Periodic Table by Royal Society of Chemistry/Iodine — Element information, properties and uses>[Fact box; Appearance; Natural abundance]{ref}
<Iodine> is sparingly soluble in <water>, giving a brownish solution, and dissolves more freely in <ethanol> and in an aqueous <iodide> solution. In the latter case the <chemical equilibrium>
$$
\mathrm I_2+\mathrm I^-\rightleftharpoons\mathrm I_3^-
$$
forms <triiodide>. <Iodine> with <starch>, under suitable conditions, gives a sensitive dark-blue colour; this is a useful recognition test, but oxidants that liberate <iodine> from added <iodide> must be distinguished from <iodine> initially present. Reaction with reactive <metals> gives <iodides>; in such compounds the <iodine> usually has <oxidation state> $-1$.
For the historical seaweed route, collect and burn <iodine>-accumulating seaweeds to produce <kelp ash>, then extract the soluble salts with <water>. Crystallization removes much of the common salt content, leaving an iodide-enriched mother liquor. Oxidation releases <iodine>, which can be distilled or sublimed and collected on a cool surface. One representative net <chemical equation> for liberation from <iodide> is
$$
\boxed{2\mathrm I^-+\mathrm{MnO}_2+4\mathrm H^+\longrightarrow\mathrm I_2+\mathrm{Mn}^{2+}+2\mathrm H_2\mathrm O.}
$$
The <manganese dioxide> is the <oxidizing agent>; the seaweed does not contain a supply of ready-made violet <iodine> crystals. This describes the chemical principle of a historical extraction, rather than prescribing one unique factory recipe.
<Iodine> also occurs in natural brines and in <iodate>-bearing deposits associated with nitrate ores. Modern industrial supply relies heavily on brines and iodate ores; it should not be equated with the seaweed source emphasized in nineteenth-century teaching.<Periodic Table by Royal Society of Chemistry/Iodine — Element information, properties and uses>[Natural abundance, iodide in seawater and modern brine/iodate sources]{ref}
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