Steam and liquid water are two different forms of the same<unk>  (pure)<unk> : water. A<unk>  is a form of matter having constant<unk>  composition and characteristic properties. Some references add that<unk>  cannot be separated into its constituent elements by physical separation methods, i.e., without breaking<unk>  bonds.<unk> s can be simple<unk> s,<unk>  compounds, or alloys.<unk>  elements may or may not be included in the definition, depending on expert viewpoint.<unk> s are often called 'pure' to set them apart from mixtures. A common example of a<unk>  is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory. Other<unk> s commonly encountered in pure form are diamond (carbon), gold, table salt (sodium chloride) and refined sugar (sucrose). However, in practice, no<unk>  is entirely pure, and<unk>  purity is specified according to the intended use of the<unk> .<unk> s exist as solids, liquids, gases, or plasma, and may change between these phases of matter with changes in temperature or pressure and time.<unk> s may be combined or converted to others by means of<unk>  reactions.
Chemical substance
Bioactive compound
Biological agent
Volatile organic compound
Drug 
Drug
Biomaterial
Immunogen