Glossary - Dd

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Dalton's law	Dalton's law of partial pressure. The total pressure exerted by a mixture of gases is the sum of the pressures that each gas would exert if it were alone. For example, if dry oxygen gas at 713 torr is saturated with water vapour at 25 torr, the pressure of the wet gas is 738 torr.
Debye	(D) Debye unit. A common non-SI unit of dipole moment, named for Dutch physical chemist Peter Debye. A charge separation equal to one electron charge placed one Ångstrom unit apart has a dipole moment of 4.8 D. In SI units, 1 D = 3.338 × 10^-30 coulomb metres.
Decomposition	A reaction in which a compound is broken down into simpler compounds or elements. Compounds sometimes decompose if heated strongly or if subjected to a strong electric current (electrolysis).
Degenerate	A set of orbitals are said to be degenerate if they all have the same energy. This degeneracy can sometimes be "lifted" by external electric or magnetic fields.
Deliquescent	Deliquescent compounds absorb so much moisture from the air that they dissolve. Examples are calcium chloride and sodium hydroxide.
Denature	A loss of chemical function, usually due to some heat or chemically-induced structural change. For example, heating a protein causes it to lose its three dimensional form and it no longer functions correctly.
Density	(,d) Compare with specific gravity. Mass of a substance per unit volume. Saying "the density of mercury is 13.55 g/cm³ " is the same as saying "the mass of exactly 1 cm³ of mercury is 13.55 g".
Density functional	A model that describes the electronic structure of an atom or molecule by approximating the total energy as a function of electron density.
Dependent variable	A dependent variable changes in response to changes in independent variables. For example, in an experiment where the vapour pressure of a liquid is measured at several different temperatures, temperature is the independent variable and vapour pressure is the dependent variable.
Derived unit	Derived units are units constructed from the system's base units. For example, the SI unit for density is kg/m³, derived from the base units kg and m.
Desalination	Removal of dissolved salts from seawater.
Destructive interference	When the peaks of one wave match the troughs of another, the waves interfere destructively. The amplitudes of the interfering waves cancel to give the resultant wave lower amplitude.
Deuterium	(D, ²H) An isotope of hydrogen that contains one neutron and one proton in its nucleus.
Dextrorotatory	Having the property of rotating plane-polarized light clockwise.
Dialysis	Dialysis is the separation of components in a mixture by passing them across a semipermeable membrane.
Diamagnetism	Diamagnetic materials are very weakly repelled by magnetic fields. The atoms or molecules of diamagnetic materials contain no unpaired spins.
Diamond	A crystalline form of carbon, made of a network of covalent, tetrahedrally bound carbon atoms.
Diastereomer	Stereoisomers which are not mirror images of each other. Diastereomers are chemically similar but distinguishable; they have different melting points and boiling points and they react at different rates.
Diatomic molecule	A molecule that contains only two atoms. All of the noninert gases occur as diatomic molecules; e. g. hydrogen, oxygen, nitrogen, fluorine, and chlorine are H₂, O₂, N₂, F₂, and Cl₂, respectively.
Diazonium salt	A diazonium salt is a compound with general form Ar-NN⁺X^-, where Ar represents a substituted benzene ring and X^- is a halide ion such as chloride. Diazonium salts are unstable and explosive in dry form. They are used to manufacture many different organic compounds, including azo dyes. See also diazotization.
Diazotization	Diazotization is a reaction that converts an -NH₂ group connected to a phenyl ring to a diazonium salt. For example, Diazotization reactions are extremely useful in organic synthesis. The nitrous acid provides NO⁺ which replaces a hydrogen on the -NH₃⁺ group to produce -NH₂NO⁺ and water; a second water is eliminated to produce the -N₂⁺ group.
Dichloromethane	(CH₂Cl₂) Dichloromethane (CH₂Cl₂) is an organic solvent often use to extract organic substances from samples. It is toxic but much less so than chloroform or carbon tetrachloride, which were previously used for this purpose.
Differential thermal analysis	(DTA) A technique that is often used to analyse materials that react or decompose at higher temperatures. The difference in temperature between the sample and an inert reference material is monitored as both are heated in a furnace. Phase transitions and chemical reactions taking place in the sample on heating cause the temperature difference to become larger, at temperatures that are characteristic of the sample.
Diffraction	The ability of a wave to bend around the edges of obstacles or holes. The effect is most noticeable when the obstacle or hole is comparable to the size of the wavelength.
Diffusion	The mixing of two substances caused by random molecular motions. Gases diffuse very quickly; liquids diffuse much more slowly, and solids diffuse at very slow (but often measurable) rates. Molecular collisions make diffusion slower in liquids and solids.
Diffusion rate	The number of randomly moving molecules that pass through a unit area per second. Diffusion rates are fastest when a large concentration difference exists on either side of the unit area. Diffusion rates increase with temperature, and decrease with increasing pressure, molecular weight, and molecular size.
Dilatometer	A device for measuring volume changes.
Dilute	Having a relatively low concentration.
Dilution	Adding solvent to a solution to lower its concentration.
Dipole-dipole interaction	Electrostatic attraction between oppositely charged poles of two or more dipoles.
Dissolved oxygen	The amount of oxygen dissolved in a solvent (usually water). Dissolved oxygen levels are used as a general indicator of water quality.
Displacement	displacement reaction; replacement reaction; replacement A reaction in which a fragment of one reactant is replaced by another reactant (or by a fragment of another reactant). Displacement reactions have the same number of products as reactants, and are described by equations of the form A + BCAB + C (single displacement) or AB + CDAC + BD (double displacement).
Disproportionate	A reaction involving a substance that produces two different forms of the substance, one more oxidized and the other more reduced than the original.
Distillate	The vapour collected and condensed from a distillation.
Distillation	Distillation is a technique for separating components of a mixture on the basis of differing boiling points. The mixture is heated, vaporizing some of the components. The vapour is collected and condensed to isolate the components with the lowest boiling points.
Divalent	Binds to two other things (which may be other atoms, molecules, ions, or electrons). See also divalent anion and divalent cation.
Divalent anion	An ion with a charge of -2.
Divalent cation	An ion with a charge of +2.
DNA	deoxyribonucleic acid A nucleic acid with 2-deoxy-D-ribose as the sugar in its nucleotides. DNA contains encoded genetic information, specifically templates for the synthesis of all of an organism's proteins and enzymes.
Domoic acid	Domoic acid is a toxic amino acid produced by certain species of algae. Domoic acid binds to a receptor that helps nerve cells control the flow of ions across their cell membranes. The receptor no longer works correctly, and the uncontrolled flux of ions damages and eventually kills the nerve cell.
Double displacement	A double displacement or metathesis is a reaction in which two reactants trade fragments: AB + CD = AC + BD Most commonly, the fragments are ions, e. g. AgNO₃(aq) + NaCl(aq) = AgCl(s) + NaNO₃(aq)
Drug	A biologically active compound or mixture used to cure, prevent, or detect disease, to control biological processes, or to alter mental state.
Dry cell	Leclanché cell. A electrolytic cell that uses a moist paste rather than a liquid as an electrolyte. Flashlight batteries are dry cells with a zinc cup for an anode, a carbon rod for a cathode, and a paste made of powdered carbon, NH₄Cl, ZnCl₂, and MnO₂ for an electrolyte.
Ductile	Capable of being drawn into wire. Metals are typically ductile materials.
Dynamic equilibrium	Dynamic equilibrium is established when two opposing processes are occuring at precisely the same rate, so that there is no apparent change in the system over long periods of time.
Dyne	(dyn) The unit of force in the obsolete cgs system of units. A dyne is the force required to accelerate a 1 g mass by 1 cm/s per second.

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