Acids and Bases

Basic concepts of Acids and Bases

All the compounds known so far can be classified into acids, bases, and salts based on their chemical properties. Initially, they were classified into acids and bases only on the basis of taste and effect on litmus paper. Substances that have a sour taste and turn blue litmus paper red are called acids and gin which have a bitter taste and turn red litmus paper blue.
But the concept could not apply to all substances because neither all the bases are bitter nor all the acids are sour.

Arrhenius Theory of Acids and Bases

The Arrhenius acid-base theory was proposed by Swedish Svante Arrhenius in 1887 AD. On the basis of his Ionic Theory Arrhenius has defined acids and bases as following-

acids and bases


According to Arrhenius the substances that provide only hydrogen ions or protons in their aqueous solution after dissociation are called acids. So it is clear that all acids have hydrogen ions. The acids like HNO3, HCl, etc. give one proton on dissociation, called monoprotic acids. The acids like H2SO4, H3PO4, etc. which have more than one hydrogen atom and give more than 1 H+ ion on dissociation, called polyprotic acids. Acidic solutions have a pH less than 7, with lower pH values corresponding to increasing acidity.

Types of Acids

On the basis of the amount of ionization, acids can be divided into these groups-

1. Strong Acids

The strength of an acid refers to the ease with which the acid loses a proton. The acids that are completely ionized in their aqueous solution are called strong acids. Generally, all mineral acids are strong acids.
Examples- H2SO4, HCl, HNO3, etc.
H2SO4 (aq) ⇌ H+ + SO4-2

2. Weak acid

Those acids which are partially ionized in water are called weak acids or a weak acid is one that doesn’t ionize fully when it is dissolved in water. Generally, acids obtained from organic materials are weak acids.
examples- HCOOH, CH3COOH, HCN, etc.
HCN ⇌ H+ + CN

The Base or Ash

According to Arrhenius, The chemical substances that give hydroxide ions after ionization in water are called base bases. In other words, a base increases the concentration of OH ions in an aqueous solution. So it is clear that all bases have hydroxide ions. Basic solutions have a pH of more than 7, with higher pH values corresponding to increasing acidity.

Types of Bases

Based on the amount of iron, the bases can be divided into the following two parts –

1. Strong Base

The bases that completely ionized in their aqueous solution are called strong bases. These compounds ionize in water to yield one or more hydroxide ions (OH) per molecule of the base.
Examples – KOH, NaOH, Ba (OH) 2, etc.

2. Weak Bases

The bases that not ionized completely or partially ionized in their aqueous solutions are called weak bases. So that the resulting aqueous solution contains only a small proportion of hydroxide ions
Examples- AL (OH) 3 NH4OH etc.

acids and bases

Basicity of Acids

The basicity of an acid is the number of hydrogen ions, which can be produced by one molecule of the acid. Like the basicity of HCl is 1 and H2SO4 is 2.

The Acidity of Bases

The acidity of a base is the number of hydroxide ions, which can be produced by one molecule of the base. As the acidity of NaOH is 1 and Ba (OH) 2 is 2.

Limitation of Arrhenius Theory

The Arrhenius acid-base theory can not apply to all acids and bases because some acids and bases exhibit acidity and basicity, even when the hydrogen and hydroxide ions are absent. Like acids AlCl3, CuSO4, CO2, SO2.  and bases Na2CO3, NH3, etc.

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