Acids are substances that have the following properties
in aqueous solution.
They have a sour taste.
Change the colour of litmus, from blue to red.
React with active metals (such as iron, tin and zinc) to
liberate hydrogen gas.
React with bases to form salt (ionic compounds) and
Their aqueous solutions conduct an electric current
because they contain ions, they are electrolytes.
Similarly bases are defined as substances that have the
following properties, when dissolved in water.
They have bitter taste.
Feel soapy and slippery on the skin.
Turn colour of litmus paper from red to blue.
React with acids to form salt and water.
They give aqueous solutions which conduct an
electricity, they are electrolytes.
Arrhenius proposed that an acid is a substance that
yields hydrogen (H+) ions in water solution,
while base produces hydroxide (OH) ions.
The Lewis theory defines a base, as any substance that
has one or more pair of electrons for bond formation. A
Lewis base is an electron-pair donor, while Lewis acid
is electron-pair acceptor.
Strong acid is one that ionizes in dilute solution
almost completely and weak acid is one that ionizes only
slightly. Thus a solution of strong acid has relatively
high concentration of (H+) ions and a
solution of weak acid has a relatively low concentration
of (H+) ions. Similarly strong base is one
that ionizes in dilute solution almost completely and
weak base is one which ionizes only slightly. Thus a
solution of strong base has a relatively high
concentration of (OH) ions and a solution of weak base
has a relatively low concentration of (OH) ions.
Acids may be classified in terms of number of proton per
molecule of acid that can given up in a reaction. Acids
HClO4, HI, HBr, HCL, HNO3 and HCN
that contain one ionizable hydrogen atom in a molecule
of acid are called monoprotic acids. Di-protic acids
contain two ionizable hydrogen atoms in one molecule of
the acid, ionization of such acids occurs in two steps.
Tri-protic acids, such as phosphoric acid, ionizes in
three steps. Mono protic, Di-protic and tri-protic acids
are commonly called mono basic, dibasic and tribasic.
Similarly, bases that produce one (OH) ion per molecule
are called mono-acidic bases that produces two (OH) ions
per molecule are called di-acidic bases and that produce
three (OH) ions per molecule are called tri-acidic
When acids react with bases, salt and water are formed,
and is known as neutralization. Many of these reactions
can be denoted by a single net-ionic equation.
H+ + OH-
Salts are formed by any positive ion except hydrogen,
combined with any negative ion, except hydroxide ion.
Salts are ionic substances which are completely
dissociated in water solution and are known as
The acidity of an aqueous solution is expressed by its
pH, which is defined as the negative logarthim of the
hydrogen ions concentration. (in moles per litre). The
neutral solution has pH=7, acidic solution has a pH is
less then 7 and basic solution has pH is more then 7
Nowadays pH of a solution is determined by pH-meter.
Most salts are strong electrolytes and dissociates
completely in to ions in aqueous solutions. The reaction
of these ions, with water lead to the acidic or basic
solution, called hydrolysis, the solution of strong
acids and weak bases are acidic and the solutions of
weak acids and strong bases are basic in nature, but the
solutions of strong acids and strong bases are neutral
because of the formation of equal concentration of
strong acid and strong base.
Water is amphoteric because it is both an acid and a
base. It undergoes self ionization to give small
quantities of hydrogen ion (H+) and hydroxide
Standard solution is that whose concentration is known.
The concentration of solution is usually expressed as
molarity (M), which is defined as the number of moles of
solute dissolved per litre of solution. A solution of
exact concentration can often be determined by
titration. Titration is a chemical process by which we
can determine the concentration of unknown (acid)
solution by reacting it with a solution of known
concentration of (base) solution.