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 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 + NH 3 → NH Cl - ACIDBASE

NH Cl - → HCl + NH 3 ACIDBASE 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 + H 2 O → H 3 O + + Cl - NH 3 + H 2 O → NH OH - Substances that can act as both an acid and base are called amphoteric.

H 3 PO 4 + H 2 O → H 2 PO H 3 O + H 2 PO H 2 O → HPO H 3 O + HPO H 2 O → PO H 3 O + Acids that can give off more than one H + (protons) are called polyprotic. H 3 PO 4 is a triprotic acid, and H 2 SO 4 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.

Strong Acid Weak Acid Weak Base 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 + NaCl WaterSalt

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.