Sea water contains \(\mathrm{Cl}^{-}, \mathrm{Br}^{-}\)and \(\mathrm{I}^{-}\)of \(\mathrm{Na}, \mathrm{K}, \mathrm{Mg}\) and \(\mathrm{Ca}\), but mainly \(\mathrm{NaCl}\). Dried up sea beds contain \(\mathrm{NaCl}\) and carnallite,
\(\mathrm{KCl} . \mathrm{MgCl}_2 \cdot 6 \mathrm{H}_2 \mathrm{O}\). Certain seaweeds contain upto \(0.5 \%\) of iodine as \(\mathrm{NaI}\) and chile saltpetre \(\left(\mathrm{NaNO}_3\right)\) contains upto \(0.2 \%\) of \(\mathrm{NaIO}_3\). Thus, sea is a great source of halogens.