Acids and Bases: formulas & examples

Dr. Walt Volland revised July 5, 2010 all rights reserved


Definition of an acid - The Arrhenius model

Acids are compounds that can donate H+ ions in water solutions.The role of water is essential in this definition. Pure HF is not considered to be an acid. It must be dissolved in water to act like an acid. There are many different acids, but they all have something in common. They all have hydrogen attached to a nonmetal from either group 6A or group 7A. This last idea makes it easier to identify acids.

A key idea is that an acidic hydrogen atom must be bonded to one of these atoms
F,  Cl,  Br,  I,  O  or  S

Binary acids

Binary acids are acids that have hydrogen combined with an atom form either group 7A or 6A. The examples of binary acids are

HF, HCl, HBr, HI, H2S and H2O.

It may be difficult to picture water as an acid but under certain conditions the water molecule breaks up to form H+ ions and OH- ions. This is one of nature's tricks. Pure water is not typically considered very acidic, because an OH- ions is released along with every H+ ion.
Ternary acids
Ternary acids contain three elements. The oxygen atoms act as a bridge between the cental nonmetal atom and the hydrogen atoms. These types of acids have hydrogen attached to oxygen. The oxygen is conected to a central atom. Examples of common ternary acids are:
HNO3, HNO2, H2SO4, H2 SO3, H3PO4, H3PO3, H2 CO3
You should see a common characteristic in these acids. They all have a nonmetal element as a central atom. They all have oxygens that are bridges between the hydrogen and the central atom. The reason these substances are acids is that the connection or bond between H and O is weak in these compounds. The bond breaks with the formation of a H+ ion and a polyatomic anion.

nitric acid






nitrate ion

HNO3    +      H2O ------->  H1+ (aq) +   NO31- (aq)
The (aq) means the ion is dissolved in water. The ion is in an aqueous mixture.

The oxy acids or ternary acids have their origins in oxides of the nonmetals like CO2(gas). The strange thing about this is that these nonmetal oxides form acids when they mix with water. These compounds are the villains that cause the acid rain problem.

Polyprotic acids

Acids with more than one acidic proton/hydrogen can release all of the protons. This means H2SO4 has two acidic hydrogens that can be released form each formula unit. Common polyprotic acids are as follows:

sulfurous acid, H2SO3, sulfuric acid, H2SO4, carbonic acid, H2CO3, phosphoris acid, H3PO4, phoshorus acid, H3PO3

sulfuric acid


two protons

sulfate ion





2 H+ (aq)



The (aq) means the particle is dissolved in water. The particle is in an aqueous mixture.


Non-acidic hydrogens

There are many compounds that contain hydrogen atoms. These hydrogens are so tightly bonded to a non-metal atom that the H-atoms are not able to break away.This explains why compounds like butane are not acids even though there are many hydrogen atoms in the formula. In butane the hydrogen atoms are attached to carbon (a non-metal) which is in group 4A not the required 6A or 7A. The hydrogen atoms are strongly bonded to a nonmetal atom that doesn't attract electrons enough to stabilize a negative charge.



Definition of a base

Bases are compounds that can readily donate OH-1 ions in water solutions. The role of water is essential in this definition. Pure solids like NaOH must be dissolved in water to act like a base. There are many different bases, but they have something in common. They all have an OH1- and a metal. This last idea makes it easier to identify bases.

Bases have a hydroxide ion combined with a metal. The examples of bases are:

LiOH, KOH, NaOH, CsOH, RbOH and H2O.
Mg(OH)2, Ca(OH)2, Ba(OH)2, Sr(OH)2
Al(OH)3, Sn(OH)2, Pb(OH)2, Fe(OH)2, Fe(OH)3



magnesium hydroxide


magnesium ion

hydroxide ion





Mg 2+ (aq)


2 OH1- (aq)

The (aq) means the ion is dissolved in water. The ions are in an aqueous mixture. The water does not get balanced
Exercise: Which of the following is an acid? click here for answer