Acids and bases are divided into three groups. An acid generates \({{\rm{H}}^ + }\) in solution, while a base generates \({\rm{O}}{{\rm{H}}^ – }\), according to the Arrhenius definition. In \(1883\), Svante Arrhenius came up with this theory. Later, two more complex and general theories of the nature of acids and bases were then proposed. These are the Bronsted-Lowry and the Lewis definitions of acids and bases. Let’s explore more about the nature of acids and bases in this article.

Nature of Acids and Bases: Overview

Acids and bases are common solutions that can be found all around the world. Except for water, almost every liquid we come across in our daily lives has acidic and basic qualities. They have fundamentally distinct characteristics and can neutralise to create \({{\rm{H}}_2}{\rm{O}}\). The following table compares their properties:

Acids	Bases
They taste sour.	They taste bitter.
Acids are colourless when placed in phenolphthalein indicator.	Bases are pink when placed in phenolphthalein indicator.
They turn blue litmus paper to red.	They turn red litmus paper to blue.
They have a \({\rm{pH}} < 7\).	They have a \({\rm{pH}} > 7\).
Common examples: Lemon, orange, hydrochloric acid.	Common examples: Toothpaste, limewater, soap.
Most acids are corrosive in nature, but all acids are not corrosive.	Most bases are also corrosive in nature, but all bases are not corrosive.
Acids are soluble in water.	All bases are not soluble in water.

Arrhenius Concept of Acids and Bases

Arrhenius defined acids and bases as follows:

• An acid is a substance that produces hydrogen ions when dissolved in water.
• A base is a substance that produces hydroxyl ions when dissolved in water.

Examples for acids: \({\rm{HCl}},{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4},{\rm{HN}}{{\rm{O}}_3}\)
\({\rm{HCl}} + {{\rm{H}}_2}{\rm{O}} \leftrightarrow {{\rm{H}}_3}{{\rm{O}}^ + } + {\rm{C}}{{\rm{l}}^ – }\)

Examples for bases: \({\rm{NaOH}},{\rm{KOH}}\), etc.
\({\rm{NaOH}} + {{\rm{H}}_2}{\rm{O}} \leftrightarrow {\rm{N}}{{\rm{a}}^ + } + {\rm{O}}{{\rm{H}}^ – }\)

Limitations of Arrhenius Theory of Acids and Bases

1. It only recognises acid and base dissociation in an aqueous media.
2. It limits acids and bases to just hydrogen-containing molecules and hydroxide-containing compounds.

According to the Arrhenius concept, carbon dioxide, sulphur dioxide, etc., are not regarded as acids, and \({\rm{N}}{{\rm{H}}_3},{\rm{CaO}},{\rm{MgO}}\), etc. are not regarded as bases.

Bronsted–Lowry’s Concept of Acids and Bases

Bronsted and Lowry suggested a solvent-independent definition of acids and bases. Acids and bases are defined as follows according to this concept:

A species (a molecule, a cation, or an anion) capable of giving one or more protons to any other substance known as a Bronsted-Lowry acid is known as an acid.

A base is a species (a molecule, a cation, or an anion) capable of receiving one or more protons from an acid; the Bronsted-Lowry base is one such base. As a result, a proton donor is an acid, and a proton acceptor is a base.

Examples for acids: \({\rm{HCl}},{{\rm{H}}_2}{\rm{O}},{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4},{\rm{HN}}{{\rm{O}}_3}\)
\({{\rm{H}}_2}{\rm{O}} \to {{\rm{H}}^ + } + {\rm{O}}{{\rm{H}}^ – }\)

Examples for bases: \({\rm{N}}{{\rm{H}}_3},{\rm{O}}{{\rm{H}}^ – },{\rm{C}}{{\rm{N}}^ – }\)
\({\rm{N}}{{\rm{H}}_3} + {{\rm{H}}^ + } \to {\rm{N}}{{\rm{H}}_4}^ + \)

Limitations of Bronsted-Lowry’s Concept

The acidic character of carbon dioxide, sulphur dioxide, and sulphur trioxide, as well as the basic nature of calcium oxide, magnesium oxide, and barium oxide, cannot be explained by Bronsted-idea Lowry’s of acids and bases.

Conjugate Acids and Bases

A conjugate acid-base pair is a pair of Bronsted acids and bases that vary by one proton. A weak conjugate base has a strong acid, while a strong base has a weak conjugate acid.

Lewis Theory of Acids and Bases

G.N. Lewis presented a more comprehensive theory of acids and bases in \(1923\). This hypothesis claims that,

“A substance that can accept an electron pair to form a coordinate covalent bond with the donor” is called acid.

Examples: \({{\rm{H}}^ + },{\rm{B}}{{\rm{F}}_3},{\rm{SnC}}{{\rm{l}}_4}\), etc.

A base is defined as “a substance that may donate a lone pair of electrons to create a coordinate covalent bond with the acceptor.”

Examples: \({{\rm{H}}_2}{\rm{O}},{\rm{N}}{{\rm{H}}_3}\) etc.

Study Acids and Bases Concept

Limitation of Lewis Theory

1. The strength of acids and bases cannot be explained by this hypothesis.
2. Acids such as hydrochloric acid and sulphuric acid react with bases like sodium hydroxide and potassium hydroxide, but they do not form coordinate covalent bonds.
3. Coordinate covalent bond formation is not present in all acid-base reactions.

Strong and Weak Acids and Bases

Strong acids are molecular compounds that effectively ionise to completion in an aqueous solution, dissociating into \({{\rm{H}}^ + }\) ions and an extra anion; strong acids are uncommon. All other acids are “weak acids,” meaning they only partially ionise in water. Acids and bases that dissociate completely are said to be strong acids and bases, e.g.:

\({\rm{HBr}} \to {{\rm{H}}^ + } + {\rm{B}}{{\rm{r}}^ – }\)
\({\rm{C}}{{\rm{H}}_3}{{\rm{O}}^ – } + {{\rm{H}}_2}{\rm{O}} \to {\rm{C}}{{\rm{H}}_3}{\rm{OH}} + {\rm{O}}{{\rm{H}}^ – }\)

Weak acids like acetic acid \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{COOH}}} \right)\) and weak bases like ammonia \(\left( {{\rm{N}}{{\rm{H}}_3}} \right)\) on the other hand, they dissociate only minimally in water – typically a few per cent depending on concentration and exist largely as undissociated molecules.

Strong acids: \({\rm{HCl}},{\rm{HN}}{{\rm{O}}_3}\)
Weak acids: \({\rm{HCN}},{\rm{HF}},{\rm{HCOOH}}\)

Strong acids, such as \({\rm{HCl}}\), dissociate to form spectator ions, such as \({\rm{C}}{{\rm{l}}^ – }\) which serve as conjugate bases, whereas weak acids form weak conjugate bases. This is shown for acetic acid and its conjugate base, the acetate anion, in the diagram below. Acetate is a weak base \(\left( {{{\rm{K}}_{\rm{b}}} = {\rm{Kw}}/{\rm{Ka}} = 5.6 \times {{10}^{ – 10}}} \right)\) while acetic acid is a weak acid \(\left( {{{\rm{K}}_{\rm{a}}} = 1.8 \times {{10}^{ – 5}}} \right)\).

Strong and weak bases are categorised similarly to acids based on the degree of ionisation. In an aqueous solution, strong bases dissociate completely. There aren’t as many strong bases. Weak bases are molecular compounds with incomplete ionisation.

Strong Bases: \({\rm{NaOH}},{\rm{KOH}}\)
Weak bases: \({\rm{N}}{{\rm{H}}_3},{{\rm{C}}_5}{{\rm{H}}_5}\;{\rm{N}}\)

How to Define Acids and Bases?

Three sets of hypotheses have been used to define acids and bases in chemistry. One is the Arrhenius definition, which states that acids are compounds that ionise (break apart) in an aqueous solution to produce hydrogen \(\left( {{{\rm{H}}^ + }} \right)\) ions, whereas bases make hydroxide \(\left( {{\rm{O}}{{\rm{H}}^ – }} \right)\) ions. The Bronsted-Lowry concept, on the other hand, defines acids as compounds that donate protons \(\left( {{{\rm{H}}^ + }} \right)\) and bases as substances that take protons. According to the Lewis theory of acids and bases, acids are electron-pair acceptors, while bases are electron-pair donors. Physical and chemical facts can be used to define acids and bases.

Neutralisation

Acids and bases have the unique capacity to neutralise the qualities of each other. The \({{\rm{H}}^ + }\) ions from the acid and the \({\rm{O}}{{\rm{H}}^ – }\) ions from the base react to form water in an acid-base (or neutralisation) process \(\left( {{{\rm{H}}_2}{\rm{O}}} \right)\). An ionic substance known as salt is another product of a neutralisation reaction. As a result, an acid-base reaction takes the following general form:

Acid-Base Indicators

An indicator is defined as a substance that gives a visible sign, usually by a colour change, of the presence of an acid or a base in a solution. Indicators show different characteristics like colour and odour in acidic and basic mediums. Ostwald developed a theory of acid-base indicators that explains the colour change with the change in nature of the solution. These indicators may be natural or synthetic.

Natural Indicators

The natural substances that show different characteristics like colour/odour in the acidic and basic medium are called natural indicators. For example, litmus, turmeric, red cabbage, china rose, etc., are common natural indicators widely used to show the acidic or basic nature of substances. Litmus solution, a purple dye extracted from lichens, is the most commonly used indicator in the laboratory. Some natural indicators with their colour in different media are listed below.

S. No.	Indicator	Neutral Medium	Acidic Medium	Basic Medium
1	Litmus	Purple	Red	Blue
2	Red Cabbage Leaves	Red	Red	Green
3	Turmeric	Yellow	Yellow	Red (Reddish Brown)

Synthetic Indicators

The synthetic indicators are the artificial compounds manufactured in industries or prepared in the labo­ratory. The most common synthetic indicators are methyl orange and phenolphthalein. The neutral colour of methyl orange is ‘orange’, whereas the neutral colour of phenolphthalein is ‘colourless’.

The colours shown by phenolphthalein and methyl orange in different media are listed below.

S. No.	Indicators	Colour in Neutral Medium	Colour in Acidic Medium	Colour in Basic Medium
1	Phenolphthalein	Colourless	Colourless	Pink
2	Methyl Orange	Orange	Red	Yellow

Indicators giving Different Odours in Acidic and Basic Medium

Some substances give a type of odour in acidic medium and different odours in a basic medium. These differences help to detect whether the given substance is acidic or basic. Such indicators are also called olfactory indicators.

The term’ olfactory’ refers to ‘sense of smell’. Thus, olfactory indicators can be defined as “those substances whose smell or odour changes differently in acidic medium or basic medium.”

S.No.	Indicator	Neutral Medium	Acidic Medium	Basic Medium
1	Onion juice	Characteristic Smell	Characteristic Smell	The smell cannot be detected
2	Vanilla essence	Characteristic Smell	Characteristic Smell	The smell cannot be detected
3	Clove oil	Characteristic Smell	Characteristic Smell	The smell cannot be detected

Summary

The acids and bases have been used for a long and are as old as chemistry. Different views regarding their definitions have been put forward from time to time. Boyle, in \(1680\) proposed that acid is sour in taste and turns blue litmus red, reacts with active metals to liberate hydrogen and neutralises bases to form salts. Roulle, in \(1774\) proposed that a base is bitter in taste, neutralises acids, possesses soapy touch and turns red litmus blue.

Leibig, in \(1838\) proposed that acids are compounds that contains one or more replaceable hydrogen atoms. Many workers gave modern concepts of defining acids and bases from time to time. The article taught us about different theories of acids and bases, strong and weak acids and bases, neutralisation, and acid-base indicators.

FAQs

Q.1. What is the nature of the acid?
Ans: Acids are acidic in nature and sour in taste. Arrhenius defined acids as the substances which dissociate in water to give hydrogen ions \(\left( {{{\rm{H}}^ + }} \right)\).

Q.2. How to identify acids and bases?
Ans: Litmus paper may be used to distinguish between acids and bases. When a drop of solution is put to litmus paper, it turns red, indicating that it is an acid. If the litmus paper turns blue, it’s most likely a base.

Q.3. What are the five properties of acids and bases?
Ans: Properties of Acids:
1. Acids are sour.
2. They are soluble in water.
3. All acidic solutions conduct electricity.
4. They turn blue litmus red.
5. They are corrosive.
Properties of Bases:
1. Bases in aqueous solutions can conduct electricity due to the presence of ions.
2. They change the colour of red litmus paper to blue.
3. On heating, bases react with some active metals, such as zinc to create salt and hydrogen gas.
4. Acids and bases react to produce salt and water. The neutralisation reaction is the name given to this process. Non-metallic oxides being acidic in nature, react with bases to form salt and water.

Q.4. What are basic in nature?
Ans: When a substance is soapy and bitter in taste, and when it converts red litmus blue, it is basic in nature. This is a fundamental aspect of substance.

Q.5. What would be the colour of litmus in a solution of sodium carbonate?
Ans: The solution of sodium carbonate is basic in nature. Hence, the colour of litmus in a solution of sodium carbonate would be blue.

Q.6. How is the concentration of hydroxide ions \(\left( {{\rm{O}}{{\rm{H}}^ – }} \right)\) affected when excess base is dissolved in a solution of sodium hydroxide?
Ans: A solution of sodium hydroxide already contains hydroxide \(\left( {{\rm{O}}{{\rm{H}}^ – }} \right)\) ions. The addition of excess base further produces more \({{\rm{O}}{{\rm{H}}^ – }}\) ions thus, the concentration of hydroxide ions increases.

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