Steam and liquid water are two different forms of the same chemical (pure) substance: water.
The chemical elements ordered in the periodic table
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.
Estimated distribution of dark matter and dark energy in the universe. Only the fraction of the mass and energy in the universe labeled "atoms" is composed of chemical elements.
Native sulfur crystals. Sulfur occurs naturally as elemental sulfur, in sulfide and sulfate minerals and in hydrogen sulfide.
Periodic table showing the cosmogenic origin of each element in the Big Bang, or in large or small stars. Small stars can produce certain elements up to sulfur, by the alpha process. Supernovae are needed to produce "heavy" elements (those beyond iron and nickel) rapidly by neutron buildup, in the r-process. Certain large stars slowly produce other elements heavier than iron, in the s-process; these may then be blown into space in the off-gassing of planetary nebulae
Potassium ferricyanide is a compound of potassium, iron, carbon and nitrogen; although it contains cyanide anions, it does not release them and is nontoxic.
Abundances of the chemical elements in the Solar System. Hydrogen and helium are most common, from the Big Bang. The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers (the Oddo-Harkins rule), and (2) a general decrease in abundance as elements become heavier. Iron is especially common because it represents the minimum energy nuclide that can be made by fusion of helium in supernovae.
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.
Mendeleev's 1869 periodic table: An experiment on a system of elements. Based on their atomic weights and chemical similarities.
Chemicals in graduated cylinders and beaker.
Dmitri Mendeleev
Henry Moseley

A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species.

- Chemical element

A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) composed of atoms from more than one element held together by chemical bonds.

- Chemical compound

Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., without breaking chemical bonds.

- Chemical substance

Unlike chemical compounds, chemical elements cannot be broken down into simpler substances by any chemical reaction.

- Chemical element

Chemical substances can be simple substances, chemical compounds, or alloys.

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

2 related topics with Alpha

Overall

A thermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake.

Chemical reaction

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A thermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake.
Antoine Lavoisier developed the theory of combustion as a chemical reaction with oxygen.
As seen from the equation CH4 + 2O2 -> CO2 + 2 H2O, a coefficient of 2 must be placed before the oxygen gas on the reactants side and before the water on the products side in order for, as per the law of conservation of mass, the quantity of each element does not change during the reaction
An example of organic reaction: oxidation of ketones to esters with a peroxycarboxylic acid
Isomerization of azobenzene, induced by light (hν) or heat (Δ)
Representation of four basic chemical reactions types: synthesis, decomposition, single replacement and double replacement.
Illustration of a redox reaction
Sodium chloride is formed through the redox reaction of sodium metal and chlorine gas
Ferrocene – an iron atom sandwiched between two C5H5 ligands
Precipitation
In this Paterno–Büchi reaction, a photoexcited carbonyl group is added to an unexcited olefin, yielding an oxetane.
Schematic potential energy diagram showing the effect of a catalyst in an endothermic chemical reaction. The presence of a catalyst opens a different reaction pathway (in red) with a lower activation energy. The final result and the overall thermodynamics are the same.
Solid heterogeneous catalysts are plated on meshes in ceramic catalytic converters in order to maximize their surface area. This exhaust converter is from a Peugeot 106 S2 1100
Mechanism of electrophilic aromatic substitution
E2 elimination
Electrophilic addition of hydrogen bromide
Acid-catalyzed addition-elimination mechanism
The Cope rearrangement of 3-methyl-1,5-hexadiene
Illustration of the induced fit model of enzyme activity
Thermite reaction proceeding in railway welding. Shortly after this, the liquid iron flows into the mould around the rail gap.

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another.

Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur.

In a substitution reaction, a functional group in a particular chemical compound is replaced by another group.

Atomic force microscopy (AFM) image of a PTCDA molecule, in which the five six-carbon rings are visible.

Molecule

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Group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion.

Group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion.

Atomic force microscopy (AFM) image of a PTCDA molecule, in which the five six-carbon rings are visible.
A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings.
AFM image of 1,5,9-trioxo-13-azatriangulene and its chemical structure.
A covalent bond forming H2 (right) where two hydrogen atoms share the two electrons
Sodium and fluorine undergoing a redox reaction to form sodium fluoride. Sodium loses its outer electron to give it a stable electron configuration, and this electron enters the fluorine atom exothermically.
3D (left and center) and 2D (right) representations of the terpenoid molecule atisane
Structure and STM image of a "cyanostar" dendrimer molecule.
Hydrogen can be removed from individual H2TPP molecules by applying excess voltage to the tip of a scanning tunneling microscope (STM, a); this removal alters the current-voltage (I-V) curves of TPP molecules, measured using the same STM tip, from diode like (red curve in b) to resistor like (green curve). Image (c) shows a row of TPP, H2TPP and TPP molecules. While scanning image (d), excess voltage was applied to H2TPP at the black dot, which instantly removed hydrogen, as shown in the bottom part of (d) and in the rescan image (e). Such manipulations can be used in single-molecule electronics.

A molecule may be homonuclear, that is, it consists of atoms of one chemical element, e.g. two atoms in the oxygen molecule (O2); or it may be heteronuclear, a chemical compound composed of more than one element, e.g. water (two hydrogen atoms and one oxygen atom; H2O).

Earlier definitions were less precise, defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties.