CAPT STRAND II: Chemical Structures and Properties
D 12. Explain the chemical composition of acids and bases, and explain the change in pH
in neutralization reactions.
Definitions:
1. acid: an acid is any compound that produces hydrogen ions (H+1) when the compound is
dissolved in water. The (aq) after the compound formula means that the compound is dissolved
in water or is in aqueous solution.
Examples: hydrogen chloride (HCl) dissolves in water to become hydrochloric acid: HCl(aq);
hydrogen nitrate dissolves in water to become nitric acid: HNO3(aq). Other common acids
include sulfuric acid: H2SO4(aq); carbonic acid: H2CO3(aq); acetic acid: HC2H3O2(aq); and
phosphoric acid: H3PO4(aq).
In water, each of the acids forms H+1(aq) and the anion (the negative ion) in aqueous solution:
Example: HCl(aq) → H+1(aq) + Cl-1(aq)
2. base: a base is any compound that produces hydroxide ions (OH-1) when the compound is
dissolved in water. Most compounds that are bases contain hydroxide ions: sodium hydroxide,
NaOH; potassium hydroxide, KOH. Ammonia gas, NH3(g), does not contain hydroxide ions in
its chemical formula but produces hydroxide ions when ammonia gas is bubbled into water:
NH3(g) + H2O(l) →
NH4+1(aq) + OH-1(aq)
Ammonia gas doesn’t dissolve in water but rather REACTS with water to form the ammonium
ion (NH4+1) and the hydroxide ion (OH-1). Because hydroxide ions are formed in aqueous
solution, ammonia gas is a base.
3. pH: a number used to indicate the hydrogen ion concentration, or acidity, of an aqueous
solution; the pH value is the negative exponent of the hydrogen ion concentration written in
scientific notation: a solution having a hydrogen ion concentration of 1 x 10-4 would have a pH
of 4.
A pH of 7 is considered to be neutral: the number of H+1 equals the number of OH-1 in solution.
If the number of H+1 is greater than the number of OH-1 in solution, the pH of the solution will be
less than 7 and the solution is said to be acidic. If the number of H+1 is less than the number of
OH-1 in solution, the pH of the solution will be greater than 7 and the solution is said to be basic
(or alkaline).
4. neutralization: a chemical reaction in which an acid reacts with a base to produce a salt
(contains the positive ion of the base and the negative ion of the acid) dissolved in water;
HCl(aq) + KOH(aq) →
KCl(aq) + H2O(l)
(acid)
(salt)
(base)
(water)