Museo Galileo
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Substances may be divided into two categories: simple and compound.

Simple substances (or elements) comprise two broad groups: metals and non-metals. The distinction is not clearcut, however. Some simple substances, described as amphoteric, can behave as metals or non-metals in different circumstances. Examples include arsenic, chromium, and manganese.

Even compound substances may be classified into two categories: inorganic, i.e., without carbon; organic, i.e., containing carbon. Inorganic substances comprise three groups: acids, bases, and salts.