All oxidations meet criteria 1 and 2, and many meet criterion 3, but this is not always easy to demonstrate. Alternatively, an oxidation can be described as a @T06446@ of an organic substrate that can be rationally dissected into steps or @P04845@. The latter consist in removal of one or several electrons from the substrate followed or preceded by gain or loss of water and/or @H02904@ or hydroxide ions, or by @N04249@ substitution by water or its reverse and/or by an @I03130@ @M03997@. This formal definition allows the original idea of oxidation (combination with oxygen), together with its extension to removal of hydrogen, as well as processes closely akin to this type of @T06446@ transformation (and generally regarded in current usage of the term in organic chemistry to be oxidations and to be effected by 'oxidizing agents') to be descriptively related to definition 1. For example the oxidation of methane to chloromethane may be considered as follows: