A<unk>  is a material with a definite<unk>  composition. It is a concept that became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure<unk>  compounds such as basic copper carbonate.  He deduced that: "All samples of a compound have the same composition; that is, all samples have the same proportions, by mass, of the elements present in the compound".  This is now known as the law of constant composition. Later with the advancement of methods for<unk>  synthesis particularly in the realm of organic chemistry; the discovery of many more<unk>  elements and new techniques in the realm of  analytical chemistry used for isolation and purification of elements and compounds from<unk> s that led to the establishment of modern chemistry, the concept was defined as is found in most chemistry textbooks. However, there are some controversies regarding this definition mainly because the large number of<unk> s reported in chemistry literature need to be indexed.
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.   A pure<unk>  cannot be separated into other<unk> s by a process that does not involve any<unk>  reaction and is rarely found in nature. Some typical<unk> s can be diamond, gold, salt (sodium chloride) and sugar (sucrose). Generally,<unk> s exist as solid, liquid, or gas, and may change between these phases of matter with changes in temperature or pressure.
Chemical substance
Biological pigment
Biomolecule
Organic compound
Metal
Nutrient
Microorganism
Food additive