Science / Chemistry Glossary

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

Activation Energy: (Ea) The minimum energy required to convert reactants into products; the difference between the energies of the activated complex and the reactants.

Active Site: A pocket or crevice on an enzyme molecule that fits reactant molecules like a hand in a glove. The active site lowers the activation energy for reaction.

Activity: (a) An effective concentration used in thermodynamic calculations in place of the actual concentration to allow equations developed for ideal solutions to be used to treat real solutions.

Activity Coefficient: (gamma) The ratio of activity to concentration; a = gamma c where a, gamma, and c are the activity, activity coefficient, and concentrations, respectively. Activity coefficients are usually . . . View Full Definition

Addition Compound: An addition compound contains two or more simpler compounds that can be packed in a definite ratio into a crystal. A dot is used to separate the compounds in the formula. For example, ZnSO4· . . . View Full Definition

Adhesion: (cohesion) Attraction between different substances on either side of a phase boundary.

Adiabat: A line on an indicator diagram that represents an adiabatic process.

Adiabatic: A process that neither absorbs nor releases energy into the surroundings. For example, a chemical reaction taking place in a closed thermos bottle can be considered adiabatic. Very fast proc . . . View Full Definition

Adiabatic Ionization Energy: The lowest energy required to remove an electron from an atom, ion, or molecule in the gas phase. The adiabatic ionization energy is the difference between the ground state energy of the ion . . . View Full Definition

Adsorb: To collect molecules of a substance on a surface.

Adsorbent: A substance that collects molecules of another substance on its surface. For example, gases that make water taste bad are strongly adsorbed on activated charcoal granules in water filters.

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