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Nitrous acid
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<p>Nitrous acid (molecular formula <a href="/facts/Hydrogen/BoYHjl0J">H</a><a href="/facts/Nitrogen/Nf1QpGDz">N</a><a href="/facts/Oxygen/acyn6o8r">O</a>2) is a weak and <a href="/facts/Monoprotic_acid/FySU18n1">monoprotic acid</a> known only in <a href="/facts/Solution_(chemistry)/MGchhMQV">solution</a>, in the gas phase, and in the form of <a href="/facts/Nitrite/qkz2XScK">nitrite</a> (NO−2) salts. It was discovered by <a href="/facts/Carl_Wilhelm_Scheele/kcKBwyXa">Carl Wilhelm Scheele</a>, who called it "<a href="/facts/Phlogiston_theory/6PDveBmv">phlogisticated</a> acid of niter". Nitrous acid is used to make <a href="/facts/Diazonium_salt/5pkN4G5s">diazonium salts</a> from amines. The resulting diazonium salts are reagents in <a href="/facts/Azo_coupling/a8BuMKw2">azo coupling</a> reactions to give <a href="/facts/Azo_dye/5zBT0p8G">azo dyes</a>. </p>