Acids, Bases and Salts: Definition, Types and Discussion - JEE Important Topic

People have been using curd, tamarind, lemon juice, salt, sugar, etc. as flavouring agents, preservatives, etc. from ancient times. Nowadays, we are using baking soda, bleaching powders, cleaning solutions, etc. in our daily lives, which has made our lives easier. They are generally acids, bases and salts. They are found abundantly in nature. Let's discuss the difference between acid, base, and salt. They are electrolytes, as in aqueous solution they conduct electricity.

As most of the acid tastes sour, the term acid is taken from the Latin term ‘acidus’, which means sour. An acid is a substance which generally gives hydrogen ions (H+) when dissolved in water. They also turn blue litmus paper red. Most of the acids react with some metals liberating hydrogen gas. Naturally occurring acid examples are citric acid in lemon and orange, tartaric acid in tamarind, lactic acid in curd, etc.

A base is a substance which mostly furnishes OH- ions when dissolved in water. Red litmus paper turns blue when kept in a basic medium. Bases are generally bitter in taste and slippery in nature. Commonly used bases are sodium hydroxide in washing soap, calcium hydroxide in white wash, magnesium hydroxide present in the antacid to remove acidity from the stomach, etc.

When the correct proportion of acid and base are reacted together, a neutralisation reaction happens and salt is formed. Some examples of salt are sodium chloride (common salt), potassium chloride, sodium nitrate, etc.

Definitions of Acids and Bases

Let us define acid and base based on various theories.

1. Arrhenius Concept of Acids and Bases

Acids and bases can be defined based on Arrhenius theory. According to this theory, acids are substances that dissociate in water to give hydrogen ions, while bases are substances that produce hydroxyl ions when dissolved in water. 

Consider the ionisation of an acid for example HCl

$\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})$

The generated H+ is very reactive and it combines with oxygen atoms in the water molecule to form hydronium ion H3O+. 

Consider the ionisation of a base say NaOH

$\mathrm{NaOH}(\mathrm{aq}) \rightarrow \mathrm{Na}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})$

The hydroxyl ion also exists in the hydrated form in the aqueous solution. Arrhenius' concept of acid and base has a limited use as it only applies to aqueous solutions. 

2. The Brønsted-Lowry Acids and Bases

According to Brønsted-Lowry theory, an acid is a substance that donates a hydrogen ion H+ and a base is a substance which accepts a hydrogen ion, H+. This means we can say that acids are proton donors and bases are proton acceptors. 

$\begin{align} &\mathrm{NH}_{3}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{NH}^{4+}(\mathrm{aq})+ \mathrm{OH}^{-}(\mathrm{aq}) \end{align}$

Ammonia accepts the proton and acts like a Brønsted-Lowry base, and water molecules donate protons and act like a Brønsted-Lowry acid.

In the reverse reaction, NH4+ ions act as proton donors; hence it is called conjugate acid; OH− ions act as proton acceptors, hence called a conjugate base. The acid-base pair that differs only by one proton is called a conjugate acid-base pair. Hence, here OH−  is called the conjugate base of an acid H2O and NH4 + is called conjugate acid of the base NH3. Also we can see that if Brønsted acid is a strong acid then its conjugate base is a weak base and vice versa.

3. Lewis Acids and Bases 

According to G.N. Lewis, an acid is a substance that accepts a pair of electrons and base is a substance that donates a pair of electrons.

Consider the reaction of electron deficient species Zn2+ ion  with NH3. 

$\mathrm{Zn}^{2+}+4\mathrm{NH}_{3} \rightarrow\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}\right]^{+2}$

Here, the Lewis acid is the zinc ion as it accepts the electron pair and the Lewis base is ammonia as it donates the electron pair.

Ionisation of Acids and Bases

According to the Arrhenius concept, perchloric acid (HClO4), hydrochloric acid (HCl), etc. are strong acids as they can completely dissociate and produce H3O+ ions in the aqueous medium. Similarly, bases like lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), etc. can almost completely dissociate into ions in an aqueous medium giving hydroxyl ions. It means that strong acid dissociates completely in water and gives weak conjugate base. For example HCl on dissociation gives Cl- ions which is a weak conjugate base. Similarly a very strong base on ionisation in aqueous media would give a very weak conjugate acid.  Weak acids ionise partially in aqueous solutions and give a strong conjugate base. Acetic acid, hydrofluoric acid are examples of weak acid.

Some substances like water are unique as it can act both as an acid and a base. Consider the following reaction

$\mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})$

The ionic product of water $Kw = [H^+ ][OH^–]$

The dissociation of water produces equal numbers of H+ and OH– ions and the concentration of H+ is  1.0 $\times$ 10–7 M at 298 K. 

The pH of a solution is defined as the negative logarithm to base 10 of the activity hydrogen ion.

$pH = - \log {\dfrac{ [H^+ ]}{mol~L^{-1}}}$

An acidic solution has pH < 7, basic solution has pH > 7  and a neutral solution has pH = 7.

Ionisation Constant for Weak Acid and Weak Base

Consider a weak acid HX which is partially ionised in the aqueous solution. 

$\mathrm{HX}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})+\mathrm{X}^{-}(\mathrm{aq})$ 

The equilibrium constant for acid ionisation is called the acid ionisation constant Ka. It  measures the strength of an acid. Ka is defined by 

$K_a=\dfrac{C\alpha^2}{(1-\alpha)}$

where 

C = Initial concentration of the undissociated acid (M)

$\alpha=$ Extent to which HX is ionised into ions.

At a given temperature T, the larger the value of Ka, the stronger is the acid. Ka is a dimensionless quantity.

The ionisation of a base MOH can be represented by equation:

$\mathrm{MOH}(\mathrm{aq}) \rightleftharpoons \mathrm{M}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})$

The equilibrium constant for base ionisation is called base ionisation constant and is represented by Kb. It is given by

$K_b=\dfrac{C\alpha^2}{(1-\alpha)}$

where 

C = Initial concentration of the undissociated base (M)

$\alpha=$ Extent up to which base is ionised into ions.

Weak bases have a very small  Kb value.

Types of Acids and Bases

Let us see how many types of acids and bases there are. There are strong acids and bases and weak acids and bases as mentioned earlier. The other types are inorganic and organic acids.

• Inorganic acids are also known as mineral acids. They are acids containing inorganic substances. E.g., Sulphuric acid, Hydrochloric acid, Nitric acid, etc.

• Organic acids are acids obtained from organic materials and contain carbon. E.g., Acetic acid, formic acid, oxalic acid, uric acid, etc.

Another classification is based on basicity of acid and acidity of bases.

The number of hydronium ions liberated by one molecule of an acid on complete ionisation is called its basicity.

• Monobasic acids: In this, one molecule of an acid on complete ionisation produces one hydronium ion H+(aq).

Eg. HNO3,HCl

• Polybasic acids: Some of the acids have more than one ionisable proton per molecule of the acid. They are called polybasic or polyprotic acids. For example, oxalic acid, sulphuric acid and phosphoric acid.

The number of hydroxyl ions generated per molecule of base on complete ionisation is called acidity of base.

• Monoacidic bases: If one molecule of a base on complete ionisation in water produces one hydroxyl ion then it is a monoacidic base. Example: LiOH, NaOH

• Polyacidic bases: When one molecule of an alkali produces two or more hydroxyl ions on complete dissociation in water, then it is a polyacidic base.

Example: Mg(OH)2, Ca(OH)2, Al(OH)3

Salts

Salts are the product of the neutralisation reaction between acids and bases. They are ionic compounds. A lot of heat is produced during this reaction. Hence, it is an exothermic process. Some examples are

$\begin{align} &\text { Acid }+\text { Base } \rightarrow \text { Salt }+\text { Water } \\ &\mathrm{HNO}_{3}+\mathrm{KOH} \rightarrow \mathrm{KNO}_{3}+\mathrm{H}_{2} \mathrm{O} \\ &\mathrm{Ca}(\mathrm{OH})_2+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{CaSO}_{4}+2\mathrm{H}_{2} \mathrm{O} \end{align}$

Classification of Salts

• Normal salts: A normal salt is formed when complete neutralisation of a strong acid by a strong  base occurs. For example, NaCl, KCl, NaNO2.

• Acidic salts: Now let us find out what acidic salt is. Most of the acidic salts contain some replaceable hydrogen atoms. Salts that are formed by the neutralisation of a strong acid by a weak base are acidic in nature.

Which salt is acidic in nature? Some examples of acidic salts are sodium hydrogen sulphate (NaHSO4), Sodium dihydrogen phosphate etc.

• Basic salts: Salts that are produced by neutralisation of a weak acid by a strong base will always produce salt solutions that are basic in nature.For example, Mg(OH)2, CaCO3 etc.

Nature of Salts

• Sodium chloride (NaCl) (common salt or table salt) is the most essential part of our diet. Sea water is the major source of sodium chloride. Let us see whether common salt is an acid or a base. It is the product of the reaction between solutions of sodium hydroxide and hydrochloric acid. As both are strong bases, the acid neutralisation reaction will be complete, hence the salt formed will be neutral.

• An ionic bond is strong in the salts as the oppositely charged ions are attracted towards each other with an electrostatic force of attraction.

• Most of the salts when dissolved in water are good conductors of electricity as they ionise, and hence they are good electrolytes.

• Most of them are crystalline solids.

• Salts of potassium, ammonium and sodium ions are soluble in water. 

• Salts like Lead sulphate, barium sulphate, silver chloride etc. are insoluble in water.

Conclusion

Acids, bases, and salts are electrolytes, and they conduct electricity in their aqueous solutions. Salts also conduct electricity in molten state. Arrhenius theory suggests that acids produce hydrogen ions and bases produce hydroxyl ions in their aqueous solutions. According to the Brönsted-Lowry theory, an acid is a proton donor and a base is a proton acceptor. Lewis defined an acid as an electron pair acceptor and a base as an electron pair donor. Neutralisation of an acid with a base forms the corresponding salt, which is an ionic solid. Most of the salts undergo hydrolysis in aqueous solution. Salts can be acidic, basic, or neutral in nature. In this article, examples, definitions, and types of acids, bases and salts have been discussed.