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Anise oil is a volatile oil obtained from anise material. Its aromatic fraction differs from the complete chemical composition of the seed-like fruit.

Basic oxygen steelmaking refines molten pig iron into steel by blowing high-purity oxygen into the bath. Oxidation lowers the carbon content and removes or redistributes other constituents. The reactions release heat, helping melt added recycled steel. The basic oxygen furnace is the vessel in which the operation takes place.^[ref]

The word basic refers to the chemical character of the lining and slag, not to the simplicity of the process. A basic slag helps capture acidic oxides formed during refining. A useful process description must therefore track both the metallic product and the nonmetallic reaction products; a low final carbon content alone does not specify a steel's full chemical composition.

The Bessemer process converts molten pig iron toward steel by blowing air through the liquid. Oxidation of constituents, especially silicon and carbon, releases heat within the bath. This combination of mixing, chemical reaction and internal heating made rapid bulk conversion possible without supplying all the required heat from an external fire.^[ref]

The product depends on the feed chemical composition, the extent of oxidation and subsequent adjustment of the carbon content. Removing carbon alone does not guarantee useful steel: other constituents also affect its properties. This illustrates why a successful process in metallurgy must control both its desired reaction and the fate of unwanted constituents. Henry Bessemer's patent describes the central idea of generating heat by reaction inside the molten material.

Broom is a common name for several woody flowering plants, including sources of historical medicinal material. A vernacular name alone does not fix one species or chemical composition.

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.

A chemoreceptor is a sensory cell or receptor system responding to chemical composition. In respiratory regulation, carbon-dioxide-related chemical signals contribute to feedback controlling ventilation. Blood transport and neural response introduce delays, so a lumped ventilation model may respond to past rather than present carbon-dioxide partial pressure.

A complex medium is a culture medium containing ingredients whose complete molecular composition is not specified. Common examples include yeast extract and peptone. A recipe can state the mass of such an ingredient precisely while leaving the amounts of its individual molecular constituents incompletely known.^[ref]

That distinction can be expressed quantitatively. In a constructed example, two hypothetical extracts are each present at $10\mathrm{g}{\mathrm{L}}^{-1}$. Suppose a constituent X has mass fraction $0.01$ in the first extract and $0.02$ in the second. Its mass concentration is then respectivelyEqual masses of the whole mixture therefore permit a twofold difference in X. These fractions are invented to illustrate the information missing from an ingredient-level specification; they are not measurements of commercial extracts.

A defined medium makes individual chemical inputs easier to track. A complex medium can instead provide many growth-supporting constituents together, which can be useful when an organism's requirements are incompletely characterized.^[ref] Neither classification by itself establishes the outcome of a bacterial culture experiment.

For an observed change in growth, the useful questions include which ingredient was varied, what was measured about its chemical composition, and whether other conditions were matched. The word complex describes incomplete compositional specification; it does not mean that every measured property of the preparation is unknown.

A defined medium is a culture medium whose individual chemical constituents and specified amounts are known. This makes chemical composition an explicit experimental variable. It differs from a complex medium, which contains ingredients with an incompletely specified molecular composition. A defined medium can help investigate which nutrients an organism requires under stated conditions.^[ref]

Known composition permits a quantitative inventory. Consider an illustrative chemical solution containing glucose at $10\mathrm{mmol}{\mathrm{L}}^{-1}$ in a volume of $0.250\mathrm{L}$. Its amount of substance isIf dilution increases the final volume to $1.000\mathrm{L}$ without consuming glucose, its final molar concentration is $2.5\mathrm{mmol}{\mathrm{L}}^{-1}$. Because each glucose molecule contains six carbon atoms, that is $15\mathrm{mmol}{\mathrm{L}}^{-1}$ of carbon atoms supplied by this ingredient. Molecular concentration and elemental inventory answer related but different questions.

This calculation is an accounting illustration, rather than a complete culture medium formulation. It predicts neither how much biomass will form nor whether other required nutrients are present. A known starting inventory makes such questions testable; it does not supply their biological answers.

Composition and physical presentation are separate classifications. A liquid preparation and an agar-solidified preparation can be compared by their physical state, while chemically defined and complex describe what is specified about their ingredients.^[ref]

Joseph Priestley's historical name for the unusually combustion-supporting air now associated with oxygen. The name expressed a phlogiston theory interpretation, rather than the modern account of chemical composition.

Diffraction contrast is image-intensity variation caused by spatially varying electron diffraction conditions. JEOL documents a silicon semiconductor-interconnect specimen at $200\mathrm{kV}$: its differently oriented crystals are imaged in bright-field transmission electron microscopy and same-area dark-field transmission electron microscopy, alongside an electron diffraction pattern. Selecting the marked diffracted beam makes the corresponding crystal bright after it was dark. The preparation route, thickness and exposure are not supplied. The changed objective aperture selection provides a comparison within one specimen; the readout supports orientation-dependent contrast rather than a map of surface height or chemical composition.^[ref]

This comparison is useful precisely because the material has not been replaced between the two images. It does not prove that thickness and chemical composition never influence transmission electron microscopy contrast. Instead, it supplies a concrete counterexample to assigning either quantity uniquely from brightness. A proposed extension is to repeat the first aperture setting after the second: recovery of the original contrast tests whether specimen motion or irradiation changed the comparison.

Edgar Bain was an American materials scientist who studied transformations in steel. His 1924 paper The Nature of Martensite by Edgar Bain examined how the structure of martensite relates to its parent austenite, connecting the mechanical behaviour of hardened steel with changes in crystal structure.^[ref]

The Bain path expresses a geometric connection between face-centered cubic and body-centred cubic structures through a tetragonal deformation. It helps make a structural transformation concrete: changing the relative lengths along different axes can map one lattice geometry to another. Such a geometric path by itself does not specify the complete chemical kinetics or every observed feature of a transformation in real steel; those also depend on chemical composition and microstructural constraints.

An electric arc furnace supplies heat through an electric arc between electrodes and the charge or molten bath. In steelmaking, it melts and refines material that can include recycled steel and direct reduced iron. It provides a different heating route from the internal oxidation heat central to basic oxygen steelmaking.^[ref]

The electrical input does not by itself determine the product's chemical composition. The charge, additives, slag and refining reactions must also be controlled. Similarly, the route's carbon dioxide emissions depend on how its electricity and metallic feed were produced; melting scrap and reducing fresh iron ore are different system boundaries.

Extractive metallurgy develops processes for separating useful metals from ores and refining them to a required purity. A process may combine physical concentration, high-temperature reactions, dissolution and electrochemical recovery. The appropriate route depends on the mineral's chemical composition, the metal's reactivity and the fate of unwanted constituents.

For iron extraction, an iron oxide must lose oxygen. In a simplified reaction, $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+3CO\to 2Fe+3C{\mathrm{O}}_2$, carbon monoxide is the reducing agent. Separating the resulting iron from slag and controlling remaining impurities are further tasks. Extraction is therefore a sequence of reaction and separation steps, rather than just melting the original rock.^[ref]

Fingering convection is a form of double-diffusive convection in which a rapidly diffusing stabilizing contribution to buoyancy competes with a more slowly diffusing destabilizing contribution. The combined mass density stratification can resist an ordinary vertical displacement, yet unequal diffusion permits narrow rising and sinking structures to grow.

A displaced parcel initially carries both contributions. If the restoring anomaly diffuses away faster than the destabilizing anomaly, the remaining buoyancy reinforces its motion. In the common thermal-compositional case, thermal diffusivity exceeds solutal diffusivity; the initial stability of the combined mass density profile is therefore insufficient to decide stability once diffusion acts.^[ref]

In seawater, salt fingering realizes this competition with temperature and salinity: the thermal contribution stabilizes while salt-rich material above fresher material supplies the destabilizing contribution. Its laboratory examples make the different transport rates visible.^[ref]

In a star, chemical composition can play the slower role. A larger mean molecular weight tends to increase mass density at fixed pressure and temperature, so a molecular-weight inversion can oppose the thermal restoring effect. Rapid thermal transport can then permit fingering even when the total local stratification is stable.^[ref]

The density ratio measures the competition, and the salt-finger instability criterion shows why there is also an upper limit to the stabilizing contribution: if it is too strong, unequal diffusion no longer produces growing modes. The elevator mode gives a particularly simple local pattern with vertical motion and no height dependence.

Henry Bessemer was an engineer associated with the Bessemer process, a method for converting molten pig iron toward steel using an air blast. His 1856 patent describes introducing air into molten iron so that reactions within the bath generate heat while changing its chemical composition.^[ref]

The important design insight is the coupling between chemical reaction, mixing and heating: the unwanted constituents become part of the source of energy for processing the material. The Bessemer process consequently illustrates how understanding a material's chemistry can change the scale and speed of manufacture. Obtaining a desired steel still requires control of the starting chemical composition and the final carbon content.

Identity and portraits: National Portrait Gallery, sitter biography and dated portraits.

Mandrake is a plant of the genus Mandragora and the root material historically derived from it. Botanical identity and chemical composition distinguish it from unrelated plants sharing the common name.

Metallurgy studies how to obtain metals from their source materials and control their chemical composition, microstructure and useful properties. Extractive metallurgy deals with separating and refining metals; physical metallurgy connects their internal structure to properties such as mechanical strength, ductility and electrical conductivity.

Steelmaking illustrates the connection between chemistry and materials science: removing oxygen from an iron oxide produces iron, controlling its carbon content produces different steels, and subsequent heat treatment changes the arrangement of crystal structures and hence the mechanical properties. Two samples with similar overall chemical composition can therefore behave differently because their microstructures differ.^[ref][ref]

A nanoparticle is a particle whose external dimensions lie on the nanoscale. Its behaviour can depend strongly on its size, shape and surface chemistry, even when its overall chemical chemical composition is fixed.

For a sphere of radius $r$, the ratio of surface area to volume isHalving $r$ therefore doubles the surface area exposed per unit volume. At fixed total volume, making many smaller particles exposes more material to the surrounding medium. This helps explain why surface reactions and attached molecules can have a large effect on nanoparticles.^[ref]

The size of an individual nanoparticle also differs from the size of an particle aggregate containing many particles. Reports of particle size consequently need to identify what was measured. Gold nanoparticles provide a visible example: their interaction with light can differ markedly from bulk gold.

A nanostructure has structural features on the nanoscale. The relevant dimension may be the thickness of a thin film, the diameter of a nanowire, or the overall size of a nanoparticle. A material can therefore be macroscopically large while containing nanostructures internally.

The geometry matters because it changes the relative importance of surfaces, the distances over which charge carriers move, and sometimes the allowed quantum states. A thin film and a nanoparticle made from the same chemical material need not have the same properties: their quantum confinement dimensions and exposed surface areas differ. Characterizing a nanostructure requires specifying which dimensions are small, its shape and its chemical chemical composition, rather than reporting a single size alone.^[ref]

Nanotechnology studies and controls matter whose important dimensions are on the nanoscale, conventionally about $1$–$100$ nanometres. A nanometre is $10^{-9}$ metres. The scientific interest comes from changes in behaviour at these dimensions, including the increasing role of surfaces and, in suitable systems, quantum confinement.^[ref]

For example, subdividing a fixed volume into smaller nanoparticles increases the exposed surface area, changing how much material can participate in a surface reaction. Nanostructures can also alter electrical conductivity, interaction with light and mechanical properties by controlling geometry rather than changing only chemical chemical composition. Useful nanotechnology therefore combines fabrication with measurements of structure and properties, such as electron microscopy or atomic force microscopy.^[ref]