Chemical composition specifies which chemical components or elements a material contains and in what proportions. The description can use atom ratios, formula units, mole fractions, or mass fractions, depending on the question.

Composition is distinct from spatial structure. Two carbon allotropes can share elemental composition but differ in bonding, while two mixtures containing the same components can differ in their component fractions and physical behavior.

Paraffin wax is a mixture dominated by relatively long-chain saturated hydrocarbons. It usually softens or melts over a range because its molecular components do not all have one identical size and structure.

Chain length, branching, and crystalline packing influence that behavior. A wax specimen is therefore more accurately described by composition and melting characteristics than by assigning it one molecular formula.

A metal is a material characterized by metallic bonding and electronic states that permit substantial electrical conduction. Many metals also conduct heat well and can deform plastically through motion of crystal dislocations. These are typical behaviors rather than a claim that every metal has identical hardness or appearance.

For example, copper is a useful electrical conductor, while mercury is liquid near room temperature. A protective oxide can make a reactive metal resist further corrosion. The word can denote either an elemental chemical classification or a metallic material such as an alloy, so composition and structure should be specified.

A resin is a broad material category covering many natural or synthetic organic substances used as solid or hardenable binding materials. Some are mixtures of small molecules; others contain polymers or reactive precursors.

The term alone therefore gives neither a unique formula nor a complete curing mechanism. Whether a resin hardens by solvent loss, cooling, or formation of new chemical crosslinks must be specified for the particular material.

Gelatin is a mixture of protein-derived chains obtained from collagen through processes that disrupt and partly break down its original structure. It is not one protein molecule with a single fixed sequence and molecular mass.

On cooling a suitable solution, segments can associate into a physical network that traps water. Chain-size distribution, concentration, and thermal history affect the resulting gel, connecting molecular processing with bulk texture.

Diethyl ether is the ether $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$. Its oxygen has lone pairs that can coordinate metal-containing reagents, while its carbon groups provide a relatively nonpolar environment.

This combination explains its role as a solvent in organometallic chemistry. It is a distinct compound from an ester, which contains a carbonyl group adjacent to oxygen, despite some similarity in their names.

Anthracene is an aromatic hydrocarbon with three linearly fused six-membered rings and formula ${\mathrm{C}}_{14}{\mathrm{H}}_{10}$. Its extended pi system gives characteristic electronic absorption and fluorescence.

It differs from phenanthrene, which has the same formula but an angular fused-ring arrangement. Comparing them shows that connectivity, not only the number of carbon atoms, determines a conjugated molecule's energy levels and reactivity.

Cerium magnesium nitrate is a hydrated double nitrate salt, commonly written $\mathrm{C}{\mathrm{e}}_2\mathrm{M}{\mathrm{g}}_3(\mathrm{N}{\mathrm{O}}_3{)}_{12}\cdot 24{\mathrm{H}}_2\mathrm{O}$. The cerium ions give the crystal a low-temperature magnetic response useful in temperature measurement.^[ref]

In a magnetic thermometer, a measured magnetic susceptibility serves as a temperature-dependent signal. A 1968 comparison found a small offset between powdered and single-crystal thermometers, showing that sample form and calibration matter even for the same salt.^[ref]

Corundum is the crystalline alpha form of aluminium oxide, $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$. Its strong bonded structure gives high hardness and chemical durability.

Salts or small amounts of impurity ions can alter its optical properties without changing the basic host lattice. Ruby and sapphire are gem varieties of corundum, demonstrating how composition at the impurity level affects color.

Manganese(II) oxide is $\mathrm{MnO}$, containing manganese predominantly in oxidation state $+2$. Its ideal structure is related to the rock-salt lattice.

It differs from manganese dioxide, $\mathrm{Mn}{\mathrm{O}}_2$, where manganese is formally $+4$. Comparing the two illustrates why the oxidation-state label determines both stoichiometry and redox chemistry.

Aluminium gallium nitride is the semiconductor alloy $\mathrm{A}{\mathrm{l}}_{\mathrm{x}}\mathrm{G}{\mathrm{a}}_{1-\mathrm{x}}\mathrm{N}$. Adjusting the aluminium fraction changes the band gap and lattice parameters.

Interfaces with related nitrides can confine electronic carriers and support ultraviolet optical devices. Composition, crystal orientation, and polarization at interfaces must be considered together when interpreting its behavior.

Aluminium nitride is $\mathrm{AlN}$, a wide-band-gap solid with useful thermal and piezoelectric properties. A common crystal structure is wurtzite.

Its combination of electrical insulation and heat transport supports electronic packaging, while its coupling between strain and electric polarization supports acoustic devices. These properties arise from the bonded lattice rather than from separate aluminium and nitrogen components.

Indium gallium nitride is the semiconductor alloy $\mathrm{I}{\mathrm{n}}_{\mathrm{x}}\mathrm{G}{\mathrm{a}}_{1-\mathrm{x}}\mathrm{N}$. Changing the indium fraction changes its electronic band structure and optical transition energies.

This compositional tuning supports light emission over different wavelength ranges. Strain and phase uniformity also matter, so wavelength is not set by composition independently of the material's structure.

Gallium nitride is $\mathrm{GaN}$, a wide-band-gap semiconductor. Its bonded crystal lattice supports electronic and optical behavior unlike that of either elemental gallium or molecular nitrogen.

It is important in light-emitting devices and power electronics. Crystal quality, defects, and doping influence performance, so elemental composition alone does not determine a device's properties.

Samarium hexaboride is a Kondo insulator whose narrow samarium $4f$-derived states mix with more dispersive conduction states. It motivates effective models of band hybridization between a nearly flat band and a light dispersive band.

Mutarotation is a change in optical rotation as anomers interconvert in a chemical solution. The glucose mutarotation experiment detects redistribution between cyclic forms through monosaccharide ring-chain interconversion.

Alkylation introduces an alkyl group into a molecule. The new group may attach to carbon, nitrogen, oxygen, sulfur, or another suitable atom; the name alone does not specify the mechanism.

For example, an alkoxide can react with a suitable alkyl halide to form an ether by nucleophilic substitution, while Friedel-Crafts alkylation attaches an alkyl group to an aromatic ring through an electrophilic pathway. Competing elimination, multiple substitution, or rearrangement depends on the particular reacting partners.

Chemical reactivity is the tendency of a species to undergo a specified chemical reaction under specified conditions. A meaningful comparison identifies the reacting partner, solvent or phase, temperature, and measured outcome.

Thermodynamic favorability and reaction speed are different: a transformation may lower Gibbs free energy yet proceed slowly because its activation energy is high. A catalyst can change that barrier without changing the equilibrium constant. Reactivity therefore connects molecular electronic structure and geometry with both chemical thermodynamics and chemical kinetics.

A separation process divides a mixture into streams with different compositions by exploiting a difference in physical or chemical behaviour. Distillation uses volatility, filtration uses particle or pore-scale exclusion, and solvent extraction uses partitioning between phases. Separation need not create chemically pure products, and recovery of one component can trade against its purity. A mass balance relates feed and product streams; thermodynamics and transport determine which separations are feasible and how rapidly they proceed.

The study of physical properties and processes in the oceans, including fluid flow, waves and transport of heat.