An oil painting of a chemist (Ana Kansky, painted by Henrika Šantel in 1932)
A diagram representing at the microscopic level the differences between homogeneous mixtures, heterogeneous mixtures, compounds, and elements
Laboratory, Institute of Biochemistry, University of Cologne in Germany.
Solutions of substances in reagent bottles, including ammonium hydroxide and nitric acid, illuminated in different colors
A diagram of an atom based on the Bohr model
Standard form of the periodic table of chemical elements. The colors represent different categories of elements
Carbon dioxide (CO2), an example of a chemical compound
A ball-and-stick representation of the caffeine molecule (C8H10N4O2).
A 2-D structural formula of a benzene molecule (C6H6)
Diagram showing relationships among the phases and the terms used to describe phase changes.
An animation of the process of ionic bonding between sodium (Na) and chlorine (Cl) to form sodium chloride, or common table salt. Ionic bonding involves one atom taking valence electrons from another (as opposed to sharing, which occurs in covalent bonding)
In the methane molecule (CH4), the carbon atom shares a pair of valence electrons with each of the four hydrogen atoms. Thus, the octet rule is satisfied for C-atom (it has eight electrons in its valence shell) and the duet rule is satisfied for the H-atoms (they have two electrons in their valence shells).
Emission spectrum of iron
During chemical reactions, bonds between atoms break and form, resulting in different substances with different properties. In a blast furnace, iron oxide, a compound, reacts with carbon monoxide to form iron, one of the chemical elements, and carbon dioxide.
The crystal lattice structure of potassium chloride (KCl), a salt which is formed due to the attraction of K+ cations and Cl− anions. Note how the overall charge of the ionic compound is zero.
Hydrogen bromide exists in the gas phase as a diatomic molecule
Democritus' atomist philosophy was later adopted by Epicurus (341–270 BCE).
15th-century artistic impression of Jābir ibn Hayyān (Geber), a Perso-Arab alchemist and pioneer in organic chemistry.
Antoine-Laurent de Lavoisier is considered the "Father of Modern Chemistry".
In his periodic table, Dmitri Mendeleev predicted the existence of 7 new elements, and placed all 60 elements known at the time in their correct places.
Top: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed. 
Bottom: Observed results: a small portion of the particles were deflected, indicating a small, concentrated charge.

In chemistry, a mixture is a material made up of two or more different chemical substances which are not chemically bonded.

- Mixture

Matter can be a pure chemical substance or a mixture of substances.

- Chemistry
An oil painting of a chemist (Ana Kansky, painted by Henrika Šantel in 1932)

2 related topics

Alpha

Making a saline water solution by dissolving table salt (NaCl) in water. The salt is the solute and the water the solvent.

Solution (chemistry)

Making a saline water solution by dissolving table salt (NaCl) in water. The salt is the solute and the water the solvent.
Water is a good solvent because the molecules are polar and capable of forming hydrogen bonds (1).

In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances.

Steam and liquid water are two different forms of the same chemical (pure) substance: water.

Chemical substance

Form of matter having constant chemical composition and characteristic properties.

Form of matter having constant chemical composition and characteristic properties.

Steam and liquid water are two different forms of the same chemical (pure) substance: water.
Colors of a single chemical (Nile red) in different solvents, under visible and UV light, showing how the chemical interacts dynamically with its solvent environment.
Native sulfur crystals. Sulfur occurs naturally as elemental sulfur, in sulfide and sulfate minerals and in hydrogen sulfide.
Potassium ferricyanide is a compound of potassium, iron, carbon and nitrogen; although it contains cyanide anions, it does not release them and is nontoxic.
Cranberry glass, while appearing homogeneous, is a mixture consisting of glass and gold colloidal particles of about 40nm in diameter, giving it a red color.
Chemicals in graduated cylinders and beaker.

Chemical substances are often called 'pure' to set them apart from mixtures.

Later with the advancement of methods for chemical synthesis particularly in the realm of organic chemistry; the discovery of many more chemical elements and new techniques in the realm of analytical chemistry used for isolation and purification of elements and compounds from chemicals that led to the establishment of modern chemistry, the concept was defined as is found in most chemistry textbooks.