= Chemistry
{wiki}
Chemistry connects the composition and structure of <chemical substances> to their properties and transformations. Its central surprise is that rearranging the same <chemical atoms> can produce a different substance: <molecular hydrogen> and <molecular oxygen> can become <water> without changing which <chemical elements> are present. <Chemical bonds> explain the connections; <chemical reactions> describe the transformations; <stoichiometry> makes quantitative predictions from the bookkeeping.
This gives a useful route from an invisible world to measurable quantities. A <molecule> has a definite composition, such as two <hydrogen> <chemical atoms> and one <oxygen> <chemical atom> in <water>. A sample contains an enormous number of such <molecules>. Counting them through <amount of substance>, measured in <moles>, lets the same small-number ratios describe a macroscopic sample. The beauty is that the bookkeeping survives this huge change of scale.
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