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GENERAL
Acids contain hydrogen which can be wholly or partly replaced by metals (or by ions with analogous properties, e.g., the ammonium ion (NH4+)) as a result salts are formed. Acids react with bases to form salts, and with alcohols to form esters. In the liquid state or in solution, they are electrolytes which produce hydrogen at the cathode. When one or more molecules of water are eliminated from those acids containing oxygen, anhydrides are obtained. Most oxides of non‑metals are anhydrides.
This sub‑Chapter covers inorganic oxygen compounds of non‑metals (anhydrides and other), and also inorganic acids, the anode radical of which is a non‑metal.
On the other hand it excludes anhydrides and acids formed, respectively, by metal oxides or hydroxides; these generally fall in sub‑Chapter IV (e.g., metal oxides, hydroxides and peroxides, such as acids or anhydrides of chromium, molybdenum, tungsten and vanadium). In certain cases, however, they fall elsewhere, e.g., in heading 28.43 (compounds of precious metals), heading 28.44 or 28.45 (compounds of radioactive elements and isotopes) or heading 28.46 (compounds of rare‑earth metals, of scandium or yttrium).
Oxygen compounds of hydrogen are also excluded and are classified under heading 22.01 (water), heading 28.45 (heavy water), heading 28.47 (hydrogen peroxide) or heading 28.53 (distilled and conductivity water and water of similar purity, including water treated with ion‑exchange media).
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