Acid, Base and Salts

ACID, BASE AND
SALTS

What are acids and alkalis?

Definition:
An acid or an alkali refers to a chemical substance that
when ionised in water releases hydrogen ions, H+ (acid)
or hydroxide ions, OH− (alkali).

The hydrogen ion reacts with
water to form hydroxonium ion.

H+ + H2O -> H3O+

Basicity of acids

The basicity of an acid refers to the
amount of H+ ions that can be
produced per molecule of acid
when ionised in water.

For example, hydrochloric acid is a monoprotic acid as it
releases ONE hydrogen ion per molecule of HCl

→H2O + HCl H3O+ + Cl-

What is a base? Base

A base is a chemical substance that
can react with acids to form salt
and water. For example:

→Ca(OH)2 + 2HNO3 Ca(NO3)2 + 2H2O

Calcium hydroxide (base) + nitric acid (acid) -> calcium nitrate (salt) + water

Alkali

Bases that are soluble in
water are known as alkali.
One of the most frequently used
alkali is sodium hydroxide, NaOH

Chemical substances only show acid or alkali
properties in the presence of water

Without the presence
of water to ionise
acids and alkalis, the
hydrogen ions and
hydroxide ions cannot
be produced

pH

The pH scale

The pH scale ranges from 0-
14 and is used to indicate

the acidity or alkalinity of an
aqueous solution.

Acid: pH<7 What is pH?
Neutral: pH=7
Alkaline: pH>7 pH is a logarithmic measure of
the concentration of hydrogen
Measuring the
ions in an aqueous solution.

pH of a solution

given its pOH

The pH of a substance can
be measured using the

formulae stated. The sum
of pH+pOH is 14. Thus,the
following relationship can

be deduced: