Acids, Bases Theory And pH Scale


Acids and bases play crucial role in the study of inorganic chemistry. Study of acids and bases helps to understand various chemical reactions and processes. Acids are the substance that donate protons (hydrogen ions) in chemical reactions. Usually, they are sour in taste. Some examples of acids are sulfuric acid, citric acid, acetic acid, etc. Bases are the substance that accepts protons (hydrogen ion) in chemical reactions. Usually, they have a bitter taste and slippery feel. Some examples of bases are sodium hydroxide, sodium bicarbonate, etc. The acids and bases are often described by using the pH scale. pH scale measures the concentration of hydrogen ion in solution. Acids have pH values below 7 and bases have pH values above 7. A pH of 7 is considered neutral.

Scientists have developed various theories to explain the behavior of acids and bases. In this article we will study the Arrhenius theory, Bronsted-Lowry theory and Lewis theory.

Arrhenius theory

Arrhenius for the first time presented the concept of acids and bases. According to Arrhenius theory any substance after dissolving in water produces ions, achieving equilibrium between ionized and unionized molecules.

When a substance is dissolved in water produces hydrogen ions (H+) is known as acid (Arrhenius acid).

HCl = H+ + Cl


When a substance is dissolved in water produces hydroxyl ions (OH) is known as base (Arrhenius base).

NaOH = Na+ + OH

NH4OH = NH4+ + OH

When acid and bases react with each other they form salt and water. This reaction is known as neutralization.

HCl + NaOH = H2O + NaCl

This theory has some limitations as follows,

  • Acids and bases are defined in terms of their aqueous solutions and not in terms of the substance.
  • Arrhenius theory does not explain acidic and basic properties of a substance in non-aqueous solvents.
  • Neutralization of acids and bases in absence of solvent is not explained.
  • Nature of bases which do not contain hydroxide ion is not explained by this theory.

Bronsted-Lowery theory

Bronsted-Lowery theory define acids and bases in terms of proton displacement. Acid is a substance that can donate proton and base is substance that can accept proton. Hence according to Lowry and Bronsted theory acid is a proton donor and base is a proton acceptor.

For acids,

Acid = H+ + Base (conjugate)

This acid could be electrically neutral molecule, negative ion or positive ion as follows,

Electrically neutral molecule, HCl = H+ + Cl

Negative ion, HSO4 = H+ + SO4

Positive ion, C5N5+NH = H+ + C5H5N

Similarly, a base can be an electrically neutral molecule like NH3, a negative ion like OH, Cl.

When acid and base react together, they form another acid and base.

Acid + Base = Acid1 + Base1

H2SO4 + H2O = H3O+ + HSO4

According to Lowry-Bronsted theory water has dual character, as it can accept and donate proton. For example,

Proton donor, H2O + NH3 = NH4+ + OH

Proton acceptor, H2O + HCl = H3O+ + Cl

Lewis theory

Lewis theory defines acids and bases in terms of displacement of lone pair of electrons. Acid is a acceptor of lone pair of electrons and base is a donor of lone pair of electrons.

For example, H+ + :NH3 = NH4

In this reaction, H+ accepts the lone pair of electrons hence it is a Lewis acid and NH3 donates a lone pair of electrons hence it is a Lewis base.

pH scale


pH is defined as the negative logarithm to the base 10 of hydrogen ion concentration [H+].

pH = -log10 [H+]


pOH is defined as the negative logarithm to the base 10 of hydroxide ion concentration [OH].

pOH = -log10 [OH]

The pH scale is used to measure the acidity or basicity of a solution. It ranges from 0 to 14. Solutions having pH less than 7 are acidic and the solutions having pH value more than 7 are basic in nature. Whereas pH 7 is considered as neutral, for example pure water. Let us consider the [H+] in a given aqueous solution.

If solution contains [H+] = 1 mole/litre

pH = -log 1 = 0

If solution contains [H+] = 0.1 mole/lit

pH = -log 0.1 = -(-1.000) = 1

If [H+] in solution is 0.01 mole/lit

pH = -log 0.01 = -(-2) = 2

Hence with the decrease in value of [H+], increases the pH value. Similarly for the hydroxide ions,

If solution contains [OH] = 1 mole/litre

pOH = -log [OH] = -log 1 = 0

since, pH + pOH = 14

pH = 14-0 = 14

If solution contains [OH] = 0.1 mole/lit

pOH = -log 0.1 = -(-1.000) = 1

Since, pH + pOH = 14

pH = 14–1 = 13

Hence with the decrease in value of [OH], decreases the pOH value.


The acidic and alkaline nature of the substance is identified by using pH scale. Understanding the properties and behaviors of acids and bases enhances our knowledge of chemical interactions for the application in various fields such as medicine, agriculture and environmental science. pH plays an important role in various biological system.

Frequently asked questions

What is acid or base?

Acid is a hydrogen ion donor and base is a hydrogen ion acceptor.

What are four types of acid bases?

In titration, acid bases can be classified as strong acid-strong base, weak acid-strong base, strong acid-weak base and weak acid-weak base.

What is the full form of pH?

The full form of pH is concentration of hydrogen ion or potential of hydrogen ion.

What is pH formula?

pH is the negative logarithm of hydrogen ion concentration. Mathematical formula is pH = -log10 [H+].

What is the pH of water?

The pH of pure water is 7.

What is the pH of blood?

pH of blood ranges from 7.35 to 7.45.

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