You
have learnt in your earlier classes about three types of
substances–acids, bases and salts. They are vital to
many life processes and are valuable to industry. Let us
do a quick revision about them.
Acids
An acid is defined as a substance that furnishes hydrogen
ions (H+) in its solution. Actually, the hydrogen
ion, H+ does not exist in the aqueous solution as such.
Instead, it attaches itself to a water molecule to form
the hydronium ion (H3O+). It is customary, however,
to simplify equations by using the symbol for the hydrogen
ion (H+).
The strongest acids are the mineral or inorganic acids.
These include sulphuric acid, nitric acid, and hydrochloric
acid. More important to life are hundreds of weaker organic
acids. These include acetic acid (in vinegar), citric acid
(in lemons), lactic acid (in sour milk), and the amino acids
(in proteins).
Acids have sour taste and turn blue litmus red. They react
with metals (which are more reactive than hydrogen) to liberate
hydrogen.
Zn
( s ) + H2SO4 ( aq ) —> ZnSO4
( aq ) + H2
Bases
Bases are the substances which furnish hydroxyl ions OH–
in their solutions. The hydroxides of metals are the compounds
that have the hydroxyl group. They are called bases. Hydroxides
of alkali metals–lithium, sodium, potassium, rubidium,
and caesium have the special name of alkalies. A basic solution
is also called an alkaline solution. Bases have bitter taste
and turn red litmus blue.
Taste
of acids and bases
Although
you will find mention of taste of acids being sour
and that of bases being bitter in books, never attempt
to taste them yourself. Many of them can cause serious
damage if swallowed or even on their contact with
tongue.
|
Salts
A
salt is a substance produced by the reaction of an acid
with a base. It consists of the cation (positive ion) of
a base and the anion (negative ion) of an acid. The reaction
between an acid and a base is called a neutralization reaction.
In solution or in the molten state,most salts are completely
dissociated into cation and anion and are good conductors
of electricity.
| 2NaOH(s) |
+ |
H2SO4(l) |
—> |
Na2SO4(aq) |
+ |
2H2O(l) |
| sodium
hydroxide |
|
sulphuric
acid |
|
sodium
sulphate |
|
|
Another
typical acidbase reaction is between calcium hydroxide
and phosphoric acid to produce calcium phosphate and water:
| 3Ca(OH)2(s) |
+ |
2H3PO4(l) |
—> |
Ca3(PO4)2(aq) |
+ |
6H2O(l) |
| calcium
hydroxide |
|
phosphoric
acid |
|
calcium
phosphate |
|
|
