Science / Chemistry Glossary

Ab Initio: A calculation or prediction that is based purely on theory rather than on experimental data. Accurate ab initio predictions are an important test of a theory. (Lat., 'from first principles').

Abrasive: A very hard, brittle, heat-resistant substance that is used to grind the edges or rough surfaces of an object. boron carbide, diamond, and corundum are abrasives.

Absolute Error: Absolute uncertainty. Compare with relative error. The uncertainty in a measurement, expressed with appropriate units. For example, if three replicate weights for an object are 1.00 g, 1.05 . . . View Full Definition

Absolute Temperature: Temperature measured on a scale that sets absolute zero as zero. In the SI system, the kelvin scale is used to measure absolute temperature.

Absolute Zero: (0 K) The temperature at which the volume of an ideal gas becomes zero; a theoretical coldest temperature that can be approached but never reached. Absolute zero is zero on the Kelvin scale, . . . View Full Definition

Absorbance: (A, D, E) optical density; extinction; decadic absorbance. A measure of the amount of light absorbed by a sample. The absorbance (A) equals minus the base-10 log of the transmittance.

Absorption: Compare with adsorption and sorption. 1. Penetration of molecules into the bulk of a solid or liquid, forming either a solution or compound. Absorption can be a chemical process (a strong so . . . View Full Definition

Absorption Spectroscopy: A technique for determining the concentration and structure of a substance by measuring the amound of electromagnetic radiation the sample absorbs at various wavelengths.

Absorption Spectrum: A plot that shows how much radiation a substance absorbs at different wavelengths. Absorption spectra are unique for each element and compound and they are often used as chemical 'fingerprin . . . View Full Definition

Absorptivity: (a) extinction coefficient; absorption cross section; decadic absorptivity. Compare with molar absorptivity and absorbance. The absorbance of a solution per unit of path length and per unit . . . View Full Definition

Accelerator: 1. A substance that makes vulcanization of rubber occur more quickly or at a lower temperature. 2. A substance that makes crosslinking in a polymer occur more quickly or at a lower temperatu . . . View Full Definition

Accuracy: Compare with precision and trueness. Accuracy is the correctness of a single measurement. The accuracy of a measurement is assessed by comparing the measurement with the true or accepted val . . . View Full Definition

Acetate: (CH3COO-, C2H3O2-) acetate ion. 1. An ion formed by removing the acidic hydrogen of acetic acid, HC2H3O2. 2. A compound derived by replacing the acidic hydrogen in acetic acid. 3. A fiber ma . . . View Full Definition

Acetic Acid: (CH3COOH, HC2H3O2) ethanoic acid; vinegar acid; methanecarboxylic acid. A simple organic acid that gives vinegar its characteristic odor and flavor. Glacial acetic acid is pure acetic acid.

Acid: ([Lat. Acidus, sour]) Compare with base. 1. A compound which releases hydrogen ions (H+) in solution (Arrhenius). 2. A compound containing detachable hydrogen ions (Bronsted-Lowry). 3. A com . . . View Full Definition

Acid Anhydride: Nonmetallic oxides or organic compounds that react with water to form acids. For example, SO2, CO2, P2O5, and SO3 are the acid anhydrides of sulfurous, carbonic, phosphoric, and sulfuric aci . . . View Full Definition

Acid Dissociation Constant: (Ka) acid ionization constant. Compare with base hydrolysis constant. The equilibrium constant for the dissociation of an acid into a hydrogen ion and an anion. For example, the acid dissoci . . . View Full Definition

Acid Error: A systematic error that occurs when glass pH electrodes are used in strongly acidic solutions. Glass electrodes give pH readings that are consistently too high in these solutions.

Acid Halide: Acid chloride; acyl halide; acyl chloride. Compounds containing a carbonyl group bound to a halogen atom.

Acid-Base Indicator: A weak acid that has acid and base forms with sharply different colors. Changes in pH around the acid's pKa are 'indicated' by color changes.

Acidic Solution: A solution in which the hydrogen ion activity is higher than that of the hydroxide ion, when the solvent is water.

Acidulant: A substance added to food or beverages to lower pH and to impart a tart, acid taste. Phosphoric acid is an acidulant added to cola drinks.

Actinide: Elements 89-102 are called actinides. Electrons added during the Aufbau construction of actinide atoms go into the 5f subshell. Actinides are unstable and undergo radioactive decay. The most . . . View Full Definition

Activated Charcoal: Activated carbon; active carbon. A porous form of carbon that acts as a powerful adsorbent, used to decolorize liquids, recover solvents, and remove toxins from water and air.

Activated Complex: Transition state. An intermediate structure formed in the conversion of reactants to products. The activated complex is the structure at the maximum energy point along the reaction path; the . . . View Full Definition

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