Acids Bases and Salts - Definition, Types, Concept, FAQs

Acids

The word acid is derived from the word acidus meaning sour to taste. Acidic chemical compounds are known to give a sour or acidic taste when allowed to dissolve in water. An acid can remain energetically favourable even after loss of a hydrogen ion from its compound. Acids turn blue litmus red. The reason for this output is that the pigment in the litmus paper reacts with the H⁺ ions, resulting in chemical changes where the chemical bonds are tuned to reflect light of longer wavelength, making it appear red to our eyes. The reaction between the pigment in the litmus paper and the H⁺ ions of an acid leads to the absorption of blue to green wavelengths. Acids mainly oxidise other chemical compounds or simply change the colour of the substance. Acids can be either organic or inorganic, the former containing a carboxyl group, a hydroxyl group and hydrogen atoms, and the latter containing a metal ion.

Bases

Bases are identified by their slippery texture and a bitter taste. Bases that dissolve in water are defined as alkalis. Bases react with acids to produce a salt and water, and this reaction is called neutralisation. Bases turn red litmus blue. The reason behind this colour change is explained as follows: The OH^- ions or hydroxyl ions react differently with the pigment present in the litmus paper. It absorbs green and red wavelengths on reacting with the hydroxyl ion, reflecting short wavelengths to appear blue in colour. Bases are capable of changing the colour of the indicators. Phenolphthalein turns pink in the presence of a base.

Salts

A salt is an ionic compound formed when the replaceable hydrogen ion (H⁺) of an acid is wholly or partially replaced by a metal ion or an ammonium ion (NH₄⁺). In general, salts are produced by the neutralisation reaction between an acid and a base.

$\mathrm{HCl}+\mathrm{NaOH} \rightarrow \mathrm{NaCl}+\mathrm{H}_2 \mathrm{O}$

Here, sodium chloride (NaCl) is the salt formed.

Characteristics of Salts

• Salts are generally ionic compounds.
• They consist of cations (positive ions) and anions (negative ions).
• Most salts are crystalline solids at room temperature.
• Many salts are soluble in water and conduct electricity in a molten or aqueous state.

Types of Salts

Normal Salts – Formed by complete replacement of acidic hydrogen.

Examples: $\mathrm{NaCl}, \mathrm{K}_2 \mathrm{SO}_4, \mathrm{NaNO}_3$

Acid Salts – Formed by partial replacement of acidic hydrogen.

Examples: $\mathrm{NaHSO}_4, \mathrm{NaHCO}_3, \mathrm{KH}_2 \mathrm{PO}_4$

Basic Salts – Contain hydroxyl groups due to incomplete neutralisation of a base.

Examples: $\mathrm{Pb}(\mathrm{OH}) \mathrm{Cl}, \mathrm{Mg}(\mathrm{OH}) \mathrm{Cl}$

Double Salts – Contain two simple salts crystallised together in a definite ratio.

Examples: Potash Alum, Mohr's Salt

Complex Salts – Contain complex ions.

Examples: Potassium Ferrocyanide, Potassium Ferricyanide

Arrhenius Concept of Acids and Bases

The Arrhenius theory, proposed by Svante Arrhenius in 1887, defines acids and bases based on the ions they produce in aqueous solution.

Arrhenius Acid
An Arrhenius acid is a substance that produces $\mathbf{H}^{+}$ions when dissolved in water.

Examples:

$\begin{gathered}
\mathrm{HCl} \rightarrow \mathrm{H}^{+}+\mathrm{Cl}^{-} \\
\mathrm{HNO}_3 \rightarrow \mathrm{H}^{+}+\mathrm{NO}_3^{-}
\end{gathered}$

Arrhenius Base
An Arrhenius base is a substance that produces $\mathbf{O H}^{-}$ions when dissolved in water.

Examples:

$\begin{aligned}
\mathrm{NaOH} & \rightarrow \mathrm{Na}^{+}+\mathrm{OH}^{-} \\
\mathrm{KOH} & \rightarrow \mathrm{~K}^{+}+\mathrm{OH}^{-}
\end{aligned}$

Neutralization Reaction
According to Arrhenius theory, an acid reacts with a base to form a salt and water.

$\mathrm{HCl}+\mathrm{NaOH} \rightarrow \mathrm{NaCl}+\mathrm{H}_2 \mathrm{O}$

Bronsted-Lowry Concept of Acids and Bases

The Brønsted-Lowry theory, proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923, defines acids and bases in terms of proton $\left(\mathrm{H}^{+}\right)$transfer.

Brønsted-Lowry Acid
A Brønsted-Lowry acid is a substance that donates a proton $\left(\mathrm{H}^{+}\right)$to another substance.
Example:

$\mathrm{HCl}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{Cl}^{-}$

Here, $\mathbf{H C l}$ donates a proton to water and acts as an acid.

Brønsted-Lowry Base
A Brønsted-Lowry base is a substance that accepts a proton $\left(\mathrm{H}^{+}\right)$from another substance.
Example:

$\mathrm{NH}_3+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{OH}^{-}$

Here, $\mathbf{N H}_{\mathbf{3}}$ accepts a proton from water and acts as a base.
Conjugate Acid-Base Pairs
When an acid donates a proton, it forms its conjugate base. When a base accepts a proton, it forms its conjugate acid.

$\mathrm{HCl}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{Cl}^{-}$

• $\mathrm{HCl} \rightarrow$ Acid
• $\mathrm{Cl}^{-} \rightarrow$ Conjugate Base
• $\mathrm{H}_2 \mathrm{O} \rightarrow$ Base
• $\mathrm{H}_3 \mathrm{O}^{+} \rightarrow$ Conjugate Acid

Lewis Concept of Acids and Bases

The Lewis theory, proposed by Gilbert N. Lewis in 1923, defines acids and bases in terms of electron pair transfer.

Lewis Acid
A Lewis acid is a species that accepts an electron pair from another species.
Examples:

• $\mathrm{BF}_3$
• $\mathrm{AlCl}_3$
• $\mathrm{H}^{+}$
• $\mathrm{Fe}^{3+}$

These species have vacant orbitals and can accept an electron pair.

Lewis Base
A Lewis base is a species that donates an electron pair to another species.

Examples:

• $\mathrm{NH}_3$
• $\mathrm{H}_2 \mathrm{O}$
• $\mathrm{OH}^{-}$
• $\mathrm{Cl}^{-}$

These species possess one or more lone pairs of electrons that can be donated.
Example of Lewis Acid-Base Reaction

$\mathrm{BF}_3+: \mathrm{NH}_3 \rightarrow \mathrm{~F}_3 \mathrm{~B} \leftarrow \mathrm{NH}_3$

• $\mathrm{BF}_3$ accepts an electron pair → Lewis Acid
• $\mathrm{NH}_3$ donates an electron pair → Lewis Base

A coordinate covalent bond is formed between $\mathrm{BF}_3$ and $\mathrm{NH}_3$.

Also Read:

• NCERT solutions for Class 11 Chemistry Chapter 1 Some basic concepts of Chemistry
• NCERT Exemplar Class 11 Chemistry Solutions Chapter 1 Some basic concepts of Chemistry
• NCERT notes Class 11 Chemistry Chapter 1 Some basic concepts of Chemistry

Is Salt Acidic or Basic?

Salts can be acidic, basic or neutral.

NaCl and KCl are neutral salts because these salts are formed out of a strong acid and a strong base, which do not hydrolyse. The pH remains constant, i.e. neutral at 7. The cation doesn’t alter the H⁺ ion concentration, and the anion does not attract the H⁺ ion. Hence, the salt remains neutral. Acidic salts are formed as a product of a neutralisation reaction between a strong acid and a weak base.

E.g. – ammonium chloride NH₄Cl

Basic salts are a result of a reaction between Weak acids and a strong base. E.g. – sodium acetate (NaAc).

Difference Between Acids, Bases and Salts

Also check-

Some Solved Examples

Question 1: According to Brosted $\mathrm{H}_2 \mathrm{O}$ behaves like

1) Bronsted Acid

2) Bronsted Base

3) (correct) Amphoteric

4) None of these

Solution:

As we learned from
Dual nature of $\mathrm{H}_2 \mathrm{O}$ -
Water ( $\mathrm{H}_2 \mathrm{O}$ ) can play the role of an acid as well as a base.
- wherein

$
\mathrm{HCl}+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{Cl}^{-}
$

$\mathrm{H}_2 \mathrm{O}$ act as base

$
\mathrm{H}_2 \mathrm{O}+\mathrm{NH}_3 \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{O} \overline{\mathrm{H}}
$

$\mathrm{H}_2 \mathrm{O}$ act as acid
With strong acid $\mathrm{H}_2 \mathrm{O}$ will release $\mathrm{H}^{+}$and strong base $\mathrm{H}_2 \mathrm{O}$ will gain $\mathrm{H}^{+}$
Hence, the answer is option (3).

Question 2: The conjugate base of H₂O will be :

1) (correct) OH^-

2) H₃O⁺

3) O^2-

4) None of these

Solution:

As we learned from

Conjugate acid-base pair -

The acid-base pair that differs only by one proton is called a conjugate acid-base pair.

Bronsted acid

$-\mathrm{H}^{+} \rightarrow$ Conjugate base

$\mathrm{H}_2 \mathrm{O}-\mathrm{H}^{+} \rightarrow \mathrm{OH}^{-}$

Hence, the answer is option (1).

Question 3: Correct pair of Lewis acid and Lewis base, respectively.

1) NH₃ , BF₃

2) NH₃, H₂O

3) BF₃ , BCl₃

4) (correct) BF₃ , NH₃

Solution:

As we learned

Lewis acids and bases -

Lewis defined an acid as a species that accepts an electron pair and a base that donates an electron.

In Lewis acids, many acids do not have protons.

$
\begin{aligned}
& \text { e.g. } \mathrm{BF}_3 \\
& \mathrm{BF}_3+\mathrm{NH}_3 \rightarrow B F_3: \mathrm{NH}_3
\end{aligned}
$
Hence, the answer is option (4).

Question 4: An aqueous solution of a salt turns blue litmus red. The salt is most likely:
(A) NaCl
(B) $\mathrm{CH}_3 \mathrm{COONa}$
(C) $\mathrm{NH}_4 \mathrm{Cl}$
(D) $\mathrm{Na}_2 \mathrm{CO}_3$

Solution:

A salt solution that turns blue litmus red is acidic.

• NaCl: Neutral (strong acid + strong base)
• CH₃COONa: Basic (weak acid + strong base)
• NH₄Cl: Acidic (strong acid + weak base)
• Na₂CO₃: Basic (weak acid + strong base)

Hence, NH₄Cl produces an acidic solution in water.

Hence, the correct answer is (C)

Practice More Questions With The Link Given Below