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AllChemicals.info - Chemical glossary Category: Sciences > Chemical Date & country: 10/12/2007, UK Words: 611

Absolute Entropy (of a substance)
The increase in the entropy of a substance as it goes from a perfectly ordered crystalline form at 0 °K (where its entropy is zero) to the temperature in question.

Absolute Zero
The zero point on the absolute temperature scale, -273.15°C or 0 K, theoretically, the temperature at which molecular motion ceases. The concept of an absolute zero temperature was first deduced from experiments with gases. When a fixed volume of gas is cooled, its pressure decreases with its temperature.Absolute zero can't be reached through exper…

Absorption Spectrum
Spectrum associated with absorption of electromagnetic radiation by atoms (or other species) resulting from transitions from lower to higher energy states. An absorption spectrum is the inverse of an emission spectrum.

Accuracy
How closely a measured value agrees with the correct value.

Acetic Acid
CA₃COOH, clear, colorless liquid, pungent odor. Boiling point 140C, flash point 54C (closed cup), autoignition temperature 38OC. A 99.7% solution is used in the S.S.E. laboratory for junction depth measurements. Acetic acid is also present in the metal etch solution used for Aluminum etch procedure.

Acid
A substance that produces H+(aq) ions in aqueous solution. Strong acids ionize completely or almost completely in dilute aqueous solution. Weak acids ionize only slightly. Acids taste sour, turn litmus red, gives a solution with a pH of less than 7 when dissolved in water and often react with some metals to produce hydrogen gas.

Acid Anhydride
Compound produced by dehydration of a carbonic acid. General formula is R--C--O--C--R. Chemical compound that reacts with water to form an acid and are usually oxides of nonmetallic elements. Anhydrides are generally more reactive than their corresponding acids because they are able to react with water to form their corresponding acid.

Acidic Salt
A salt containing an ionizable hydrogen atom, does not necessarily produce acidic solutions.

Actinides
Elements 89 to 103 (between lawrencium and actinium) on the periodic table. Only the first four have been found in nature in appreciable amounts. The remainder have been produced synthetically.

Activation Energy
Amount of energy that must be absorbed by reactants in their ground states to reach the transition state so that a reaction can occur. In other words, activation energy is the minimum energy required for a chemical reaction to occur.

Active Metal
Metal with low ionization energy that loses electrons readily to form cations.

Activity Series
A listing of metals (and hydrogen) in order of decreasing activity.

Actual Yield
Amount of a specified pure product actually obtained from a given reaction. Compare with Theoretical Yield.

Acyl Group
Compound derived from a carbonic acid by replacing the --OH group with a halogen (X), usually --Cl, general formula is O R--C--X.

Addition Reaction
A reaction in which two atoms or groups of atoms are added to a molecule, one on each side of a double or triple bond. Types of addition reaction include electrophilic, nucleophilic (polar) and free radical addition (non-polar).

Adhesive Forces
Forces of attraction between a liquid and another surface.

Adsorption
Adhesion of a species onto the surfaces of particles

Alcohol
Hydrocarbon derivative containing an --OH group attached to a carbon atom not in an aromatic ring. Alcohols are a class of organic compounds containing the hydroxyl group, OH, attached to a carbon atom.Alcohols are neither acid nor alkaline. They are characterized by many common reactions, the most important of which is the reaction with acids form…

Aldehyde
Compound in which an alkyl or aryl group and a hydrogen atom are attached to a carbonyl group and a hydrogen atom are attached to a carbonyl group, general formula, O-R-C-H

Alkali Metals
Metals of Group IA (Na, K, Rb).

Alkaline Battery
A dry cell in which the electrolyte contains KOH.

Alkaline Earth Metals
Group IIA metals

Alkenes (Olefins)
Unsaturated hydrocarbons that contain one or more carbon-carbon double bonds.

Alkyl Group
A group of atoms derived from an alkane by the removal of one hydrogen atom.

Alkylbenzene
A compound containing an alkyl group bonded to a benzene ring.

Alkynes
Unsaturated hydrocarbons that contain one or more carbon-carbon triple bonds.

Allotropes
Different forms of the same element in the same physical state.

Alloying
Mixing of metal with other substances (usually other metals) to modify its properties.

Alpha (a) Particle
Helium ion with 2⁺ charge, an assembly of two protons and two neutrons.

Alpha Particles
A helium nucleus.

Alums
Hydrated sulfates of the general formula M+M3+(SO4)2.12H2).

Amide
Compound containing the O-C-N group.Compound that can be considered a derivative of ammonia in which one or more hydrogens are replaced by a alkyl or aryl groups.

Amine
Derivatives of ammonia in which one or more hydrogen atoms have been replaced by organic groups.

Amine Complexes
Complex species that contain ammonia molecules bonded to metal ions.

Amino Acid
Compound containing both an amino and a carboxylic acid group.The --NH2 group.

Amorphous Solid
A noncrystalline solid with no well-defined ordered structure.

Ampere
Unit of electrical current, one ampere equals one coulomb per second.

Amphiprotism
Ability of a substance to exhibit amphiprotism by accepting donated protons.

Amphoterism
The ability to react with both acids and bases.Ability of substance to act as either an acid or a base.

Anion
A negative ion, an atom or goup of atoms that has gained one or more electrons.

Anode
In a cathode ray tube, the positive electrode. Electrode at which oxidation occurs.

Antibonding Orbital
A molecular orbital higher in energy than any of the atomic orbitals from which it is derived, lends instability to a molecule or ion when populated with electrons, denoted with a star (*) superscript or symbol.

Aromatic Hydrocarbons
Benzene and its derivatives.

Artificial Transmutation
An artificially induced nuclear reaction caused by the bombardment of a nucleus with subatomic particiles or small nucei.

Aryl Group
Group of atoms remaining after a hydrogen atom is removed from the aromatic system.

Associated Ions
Short-lived species formed by the collision of dissolved ions of opposite charges.

Atmosphere
A unit of pressure, the pressure that will support a column of mercury 760 mm high at 0 °C.

Atom
The smallest particle of an element.

Atomic Mass Unit (amu)
One twelfth of a mass of an atom of the carbon-12 isotope, a unit used for stating atomic and formula weights, also called dalton.

Atomic Number
Integral number of protons in the nucleus, defines the identity of element.

Atomic Orbital
Region or volume in space in which the probability of finding electrons is highest.

Atomic Radius
Radius of an atom.

Atomic Weight
Weighted average of the masses of the constituent isotopes of an element, The relative masses of atoms of different elements.

Aufbau ('building up') Principle
Describes the order in which electrons fill orbitals in atoms.

Autoionization
An ionization reaction between identical molecules.

Avogadro's Law
At the same temperature and pressure, equal volumes of all gases contain the same number of molecules.

Avogadro's Number
The number (6.022x10^23) of atoms, molecules or particles found in exactly 1 mole of substance.

Background Radiation
Ratiation extraneous to an experiment. Usually the low-level natural radiation form cosmic rays and trace radioactive substances present in our environment.

Band
A series of very closely spaced, nearly continuous molecular orbitals that belong to the crystal as a whole.

Band of Stability
Band containing nonradioactive nuclides in a plot of number of neutrons versus atomic number.

Band Theory of Metals
Theory that accounts for the bonding and properties of metallic solids.

Barometer
A device for measuring pressure.

Base
A substance that produces OH (aq) ions in aqueous solution. Strong soluable bases are soluble in water and are completely dissociated. Weak bases ionize only slightly.

Basic Anhydride
The oxide of a metal that reacts with water to form a base.

Basic Salt
A salt containing an ionizable OH group.

Beta Particle
Electron emitted from the nucleus when a neuton decays to a proton and an electron.

Binary Acid
A binary compound in which H is bonded to one or more of the more electronegative nonmetals.

Binary Compound
A compound consisting of two elements, may be ionic or covalent.

Biodegradability
The ability of a substance to be broken down into simpler substances by bacteria.

Boiling Point
The temperature at which the vapor pressure of a liquid is equal to the applied pressure, also the condensation point

Boiling Point Elevation
The increase in the boiling point of a solvent caused by the dissolution of a nonvolatile solute.

Bomb Calorimeter
A device used to measure the heat transfer between system and surroundings at constant volume.

Bond Energy
The amount of energy necessary to break one mole of bonds of a given kind (in gas phase).The amount of energy necessary to break one mole of bonds in a substance, dissociating the sustance in the gaseous state into atoms of its elements in the gaseous state.

Bond Order
Half the numbers of electrons in bonding orbitals minus half the number of electrons in antibonding orbitals.

Bonding Orbital
A molecular orbit lower in energy than any of the atomic orbitals from which it is derived, lends stability to a molecule or ion when populated with electron

Bonding Pair
Pair of electrons involved in a covalent bond.

Born-Haber Cycle
A series of reactions (and accompanying enthalpy changes) which, when summed, represents the hypothetical one-step reaction by which elements in their standard states are converted into crystals of ionic compounds (and the accompanying enthalpy changes.)

Boron Hydrides
Binary compounds of boron and hydrogen.

Boyle's Law
At constant temperature the volume occupied by a definite mass of a gas is inversely proportional to the applied pressure.

Breeder Reactor
A nuclear reactor that produces more fissionable nuclear fuel than it consumes.

Bronsted-Lowry Acid
A proton donor.

Bronsted-Lowry Base
A proton acceptor

Buffer Solution
Solution that resists change in pH, contains either a weak acid and a soluble ionic salt of the acid or a weak base and a soluble ionic salt of the base.

Buret
A piece of volumetric glassware, usually graduated in 0.1-mL intervals, that is used to deliver solutions to be used in titrations in a quantitative (dropwise) manner.

Calorie
The amount of heat required to raise the temperature of one gram of water from 14.5°C to 15.5°C. 1 calorie = 4.184 joules.

Calorimeter
A device used to measure the heat transfer between system and surroundings.

Canal Ray
Stream of positively charged particles (cations) that moves toward the negative electrode in cathode ray tubes, observed to pass through canals in the negative electrode.

Capillary
A tube having a very small inside diameter.

Capillary Action
The drawing of a liquid up the inside of a small-bore tube when adhesive forces exceed cohesive forces, or the depression of the surface of the liquid when cohesive forces exceed the adhesive forces.

Carbanion
An organic ion carrying a negative charge on a carbon atom.

Carbonium ion
An orgainic ion carrying a positive charge on a carbon atom.

Carcinogen
A substance capable of causing or producing cancer in mammals.

Catalyst
A substance that speeds up a chemical reaction without being consumed itself in the reaction.A substance that alters (usually increases) the rate at which a reaction occurs.

Catenation
Bonding of atoms of the same element into chains or rings.The bonding together of atoms of the same element to form chains.The ability of an element to bond to itself.

Cathode
Electrode at which reduction occurs in a cathode ray tube, the negative electrode.

Cathode Ray Tube
Closed glass tube containing a gas under low pressure, with electrodes near the ends and a luminescent screen at the end near the positive electrode, produces cathode rays when high voltage is applied.

Cathodic Protection
Protection of a metal (making ir a cathode) against corrosion by attaching it to a sacrifical anode of a more easily oxidized metal.

Cation
A positive ion, an atom or group of atoms that has lost one or more electrons.

Cell Potential
Potential difference, Ecell, between oxidation and reduction half-cells under nonstandard conditions.

Central Atom
An atom in a molecule or polyatomic ion that is bonded to more than one other atom.