Teacher resources and professional development across the curriculum

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Glossary by Alphabet

A compound that has a proton or protons that can dissociate in water; also, when one molecule has a proton or protons that dissociate more readily than those of another (i.e., it has a higher Ka), the first is said to be the more acidic molecule.
Acid dissociation constant
A form of the equilibrium constant for the dissociation of an acidic molecule into a proton and its conjugate base. It is abbreviated "Ka." The acid dissociation serves as a measure of how acidic the molecule is; the larger the value of Ka, the more acidic the molecule.
Acid dissociation equation
The chemical equation for the separation of an acidic molecule into a proton and its conjugate base. The general form can be written:

HA + H2O equilibrium arrows H3O+ + A-

where HA is the acidic molecule, H3O+ is the stable form of the proton, and A- is the conjugate base of the acidic molecule.
Acidic solutions
Solutions containing a higher concentration of hydronium ion (H3O+) than that found in pure water (i.e., having a pH below 7); also, when one solution has a greater concentration of hydronium than another, it is said to be the more acidic solution.
Activated complex
A chemical species midway between reactant molecules and product molecules.
Activation energy
The amount of energy that the reactants need to reach the activated complex.
Activity series
A list of various chemical species arranged in order of reactivity. It is usually used to describe the ability of various metals to displace hydrogen from water or from acids.
Any two or more forms of the same element with different arrangements of atoms in their structures. For example, dioxygen (O2) and ozone (O3) are allotropes.
A substance that contains more than one (usually metallic) element and has metallic properties, such as strength, conductivity, ductility, and malleability.
Alpha particle
A product of nuclear decay that is two protons and two neutrons, which form a particle with a structure identical to that of a helium nucleus with a charge of +2.
Amorphous solid
A solid whose atoms or molecules are not arranged in regular, repeating patterns.
A negatively charged atom or molecule.
In an electrochemical cell, the electrode at which the oxidation half-reaction occurs.
The description of a modified metal surface through reaction with another chemical component, forcing the metal surface to undergo oxidation (i.e., act as an anode).
A substance made of antiparticles, such as positrons and antiprotons, which have the same mass but opposite charge as their matter counterparts. When a particle and its antiparticle collide, both are annihilated and energy is released.
Aqueous solution
A solution where the solvent (primary component) is water.
Arrhenius acid
Any molecule that can dissociate in an aqueous solution to produce a proton (H+).
Arrhenius base
Any molecule that can dissociate in an aqueous solution to produce a hydroxide ion (OH-).
Atmospheric pressure
The force exerted by the weight of the atmosphere on a given unit area. Atmospheric pressure on Earth at sea level is 14.7 pounds per square inch (psi), 760 torr or 1 atm.

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