What makes an acid an acid or a base a base can vary depending on definition being used. The first definition was created by Svante Arrhenius in 1883. Arrhenius said compounds that will ionize to give off H+ ions are acids and compounds that will ionize to give off OH- ions are bases.

HCl → H+ + Cl-

NaOH → Na+ + OH-

ACID

BASE

The Bronsted-Lowry definition for a base is that it is an H+ acceptor. The definition of an acid is still the same as for Arrhenius - an acid gives off H+

HCl + NH3 → NH4+ + Cl-

ACID BASE

NH4+ + Cl- → HCl + NH3

ACID BASE

The equation backwards shows chlorine accepting hyrdrogen., making it a base. In the other equation HCL gave off hydrogen, making it an acid. Therefore we call the pair a conjugate acid-base pair.

HCl + H2O → H3O+ + Cl-

NH3 + H2O → NH4+ + OH-

Substances that can act as both an acid and base are called amphoteric.

H3PO4 + H2O → H2PO4- + H3O+

H2PO4- + H2O → HPO4

-2 + H3O+

HPO4-2 + H2O → PO4

-3 + H3O+

Acids that can give off more than one H+ (protons) are called polyprotic. H3PO4 is a triprotic acid, and H2SO4 would be a diprotic acid.

A concentrated acid has a lot of acid dissolved in water, and a dilute acid has only a little

Strong acids ionize completely, and weak ones only ionize a little.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Strong Acid WeakAcid

WeakBase

Strong Base

To determine the pH of a substance requires an indicator. Indicators are substances that turn colors at certain pH’s.

When an acid and base are added together in appropriate amounts, the resulting pH is near 7 - neutral. This is why acids and bases are said to “neutralize” each other. In a neutralization reaction the acid and base (according to the Arrhenius definition) react to form water and a salt.

HCl + NaOH → HOH + NaClWater Salt

Neutralizing an acid and base to determine an unknown amount of H+ or OH- is called titration. In a titration a small amount of indicator is added that will change color when the neutralization is complete.