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Home :Glossary

Glossary: Acids and bases

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-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.

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 dissociation constant for acetic acid is the equilibrium constant for HC2H3O2(aq) doublearrow H+(aq) + C2H3O2-(aq), which is Ka = [H+][C2H3O2-]/[HC2H3O2].

Having a pH* greater than 7.

amphiprotic solvent. Compare with aprotic solvent*.
Solvents that exhibit both acidic and basic properties; amphiprotic solvents undergo autoprotolysis*. Examples are water, ammonia, and ethanol.

amphoteric. ampholyte.
A substance that can act as either an acid or a base in a reaction. For example, aluminum hydroxide can neutralize mineral acids ( Al(OH)3 + 3 HCl = AlCl3 + 3 H2O ) or strong bases ( Al(OH)3 + 3 NaOH = Na3AlO3 + 3 H2O).

aprotic solvent. Compare with amphiprotic solvent*.
A solvent that does not act as an acid or as a base; aprotic solvents don't undergo autoprotolysis*. Examples are pentane, pet ether, and toluene.

aqua regia.
A mixture of nitric and hydrochloric acids, usually 1:3 or 1:4 parts HNO3 to HCl, used to dissolve gold.

autoprotolysis. autoionization; autoionization constant; autoprotolysis constant.
Transfer of a hydrogen ion between molecules of the same substance, e. g. the autoprotolysis of methanol (2 CH3OH = CH3OH2+ + CH3O-). Autoprotolysis of water into hydronium* ions and hydroxide ions* results in equilibrium concentrations that satisfy Kw = [H3O+][OH-], where the autoprotolysis constant Kw is equal to 1.01 × 10-14 at 25°C.

base hydrolysis constant. (Kb) base ionization constant; basic hydrolysis constant. Compare with acid dissociation constant*.
The equilibrium constant* for the hydrolysis* reaction associated with a base. For example, Kb for ammonia is the equilibrium constant for NH3(aq) + H2O(ell) doublearrow NH4+(aq) + OH-(aq), or Kb = [NH4+][OH-]/[NH3].

Brösted acid. Compare with acid*.
A material that gives up hydrogen ions in a chemical reaction.

Brösted base. Compare with base*.
A material that accepts hydrogen ions in a chemical reaction.

buffer. pH buffer; buffer solution.
A solution that can maintain its pH* value with little change when acids or bases are added to it. Buffer solutions are usually prepared as mixtures of a weak acid with its own salt or mixtures of salts of weak acids. For example, a 50:50 mixture of 1 M acetic acid and 1 M sodium acetate buffers pH around 4.7.

hydronium ion. (H3O+) hydronium.
The H3O+ ion, formed by capture of a hydrogen ion by a water molecule. A strong covalent bond* is formed between the hydrogen ion and water oxygen; all hydrogen ions in aqueous solution are bound inside hydronium ions.

ionic dissociation. ionize; ionization.
When ionic substances dissolve, their ions are surrounded by solvent molecules and separated from each other. This phenomena is also called ionization.

Kw. Kw.
Symbol for the autoprotolysis constant* for water, equal to 1.01 × 10-14 at 25°C.

A mixture of pigments extracted from certain lichens that turns blue in basic solution and red in acidic solution.

litmus paper. litmus test.
Paper impregnated with litmus*, usually cut in narrow strips. Dipping red litmus paper into a basic solution turns it blue; dipping blue litmus paper into an acidic solution turns it red.

1. having no net electrical charge. Atoms are electrically neutral; ions are not. 2. A solution containing equal concentrations of H+ and OH-.

neutralization reaction. neutralization; acid-base reaction.
A chemical change in which one compound aquires H+ from another. The compound that receives the hydrogen ion is the base; the compound that surrenders it is an acid.

nitric acid. (HNO3) aqua fortis.
A corrosive liquid with a sharp odor that acts as a strong acid* when dissolved in water. Nitric acid is used to synthesize ammonium nitrate for fertilizers, and is also used in the manufacture of explosives, dyes, and pharmaceuticals. Salts of nitric acid are called nitrates*.

pH is a measure of effective concentration of hydrogen ions in a solution. It is approximately related to the molarity of H+ by pH = - log [H+]

An organic compound used as an acid-base indicator. The compound is colorless in acidic solution and pink in basic solution (with the transition occuring around pH 8.3). Phenolpthalein was used for many years as a laxative in very low concentrations- high concentrations are toxic!

proton donor. acid. Compare with base*.
Because a free H+ ion is technically a bare proton, acids* are sometimes referred to as "proton donors" because they release hydrogen ions in solution. The term "proton donor" is misleading, since in aqueous solution, the hydrogen ion is never a bare proton- it's covalently bound* to a water molecule as an H3O+ ion. Further, acids don't "donate" protons; they yield them to bases with a stronger affinity for them.

strong acid. Compare with weak acid*.
A strong acid is an acid* that completely dissociates into hydrogen ions and anions in solution. Strong acids are strong electrolytes*. There are only six common strong acids: HCl (hydrochloric acid), HBr (hydrobromic acid), HI (hydroiodic acid), H2SO4 (sulfuric acid), HClO4 (perchloric acid), and HNO3 (nitric acid).

strong base.
A strong base is an base* that completely dissociates into ions in solution. Strong bases are strong electrolytes*. The most common strong bases are alkali metal and alkaline earth metal hydroxides.

weak acid. Compare with strong acid*.
An acid* that only partially dissociates into hydrogen ions and anions in solution. Weak acids are weak electrolytes*. Recognize weak acids by learning the six common strong acids*; any acid that doesn't appear on the list of strong acids is usually a weak acid.

weak base. Compare with strong base*.
A base* that only partially dissociates into ions in solution. Weak bases are weak electrolytes*. Ammonia is an example of a weak base; the reaction NH3(aq) + H2O(l) = NH4_+(aq) + OH-(aq) is reversible.

General Chemistry Online! Acids and bases

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/acidbase/glossary.shtml