Introduction to Acids and Bases

Key Concepts

• Acids
• Bases
• Salts
• Classification of Acids and Bases
• Methods of preparations of Acids and Bases

Introduction

We use lots of substances like tomatoes, lemon, detergents, soaps, and oranges in our day-to-day life. While lemons and tomatoes taste sour in taste, substances like detergents and soaps feel bitter and slippery. Have you ever thought about what makes these substances taste different? The substances like oranges and lemons taste sour because they contain acids and soaps, detergents feel slippery because they contain a substance known as base. 

There are plenty of acids present in our body. Some of these acids are essential for digestion; some are essential for producing protein and hormones in our bodies. The word acids is derived from a Latin word which means sour. Similarly, bitter substances like baking soda and feels slippery like soaps are called bases. 

Let us learn more about acids, bases, classification, properties, and uses

Acids 

Definition:  Acid is a substance that produces H⁺ Hydrogen (as H₃O⁺ Hydronium) ion when dissolved in water. 

For example, Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃), carbonic acid (H₂CO₃). 

When hydrochloric acid is dissolved in the water, it dissociates into H⁺ ions and chloride ions. 

Similarly, sulphuric acid, when dissolved in water, dissociates into (H⁺ ion) as hydronium ions and sulphate ions. 

Note: In actual practice H⁺ ion never exist independently, it combine with water itself to form hydronium (H₃O⁺) ion.  

Classifications of Acids: 

Acids are classified into five different categories based on different factors as below. 

Based on the source from which they are obtained 

• Organic Acids 
• Mineral Acids 

Organic/ Natural Acids: 

Acids obtained from natural sources are called natural acids or organic Acids. 

Examples: 

• Acetic acid (CH₃COOH) 
• Oxalic acid (C₂H₂O₄) 
• Tartaric acid (C₄H₆O₆) 

Mineral Acids: 

Acids that are prepared from minerals are called Mineral Acids.

Inorganic acids or synthetic acids are also known as Mineral Acids. 

Examples: 

• Sulphuric acid (H₂SO₄) 
• Nitric acid (HNO₃) 
• Carbonic acid (H₂CO₃) 
• Hydrochloric acid (HCl) 
• Phosphoric acids: (H₃PO₄) 

Based on Strength (Ionising capability) of Acid. 

It means the number of hydrogen ions released (ionisation) when acid is dissolved in water. 

Based on strength, acids are divided into two types. 

• Strong Acids
• Weak Acids

Strong Acids: 

The acids that undergo complete dissociation when dissolved in water and give a large number of H⁺ ions are called strong acids. 

Examples: 

• Nitric acid (HNO₃)
• Hydrochloric acid (HCl)
• Sulphuric acid(H₂SO₄) 

Weak Acids:

The acids that dissociated partially when dissolved in water and gave a smaller number of H⁺ ions are called weak acids. 

Examples:

• Hydrogen sulphide(H₂S),
• Acetic acid (CH₃COOH),
• Phosphoric acid (H₃PO₄),
• Carbonic acid (H₂CO₃) 

Based on elements present in the Acids 

Hydracids (Binary acids):

Hydracids are acids that consist of hydrogen and a non-metal element other than oxygen. 

Examples:

• Hydrochloric acid (HCl)
• Hydrogen sulphide (H₂S)
• Hydrobromic acid (HBr)
• Hydroiodic acid (HI) 

Oxyacids (Ternary acids): 

These acids contain hydrogen, non-metal elements, and hydrogen. 

Examples:

• Sulphurous acid (H₂SO₃)
• Chloric acid (HClO₃)
• Perchloric acid (HClO₄)
• Sulphuric acid (H₂SO₄) 

Based on Basicity of Acids 

Basicity is defined as the number of hydronium ions [H⁺ (aq)] released by one acid molecule to complete ionisation. 

Based on basicity, acids are divided into three types. 

Monobasic acid: When one acid molecule on complete ionisation gives one hydronium ion [H⁺ (aq)], the acid is called monobasic acid. 

Examples: 

• HCl
• Hydrogen fluoride acid (HF)
• HBr
• acetic acid (CH₃COOH)
• formic acid (HCOOH)

1. A monobasic acid always ionises in one step in an aqueous solution. 
2. A monobasic acid forms only single salt. 

Dibasic acid:

One molecule of dibasic acid on complete ionisation gives two hydronium ions [H⁺ (aq)]. 

Examples:

• These acids ionise in the two steps. 

Tribasic acid: Here, one molecule of an acid gives three hydronium ions [H⁺ (aq)] on complete ionisation. 

Complete ionisation is taking place in three steps for tribasic acids 

Based on Concentration (water content) of Acid: 

Concentration: It measures the amount of water present in a given sample of acid. 

Based on this, the acids are classified into two types as follows: 

Concentrated acid: 

A sample of an acid that contains only very little or no amount of water. 

Dilute acid: 

For dilute acids, they contain a lot more amount of water than their weight 

General Properties of Acids: 

Taste

• Generally, acids taste sour. 

Effect on Skin

• Strong inorganic acids are corrosive action on the skin, and they can even damage other substances. 

Effect of Indicators

• Indicator: Indicators are the substances used to distinguish between acids and bases. They change their color when dissolved in the acids or bases; that is why they are known as indicators. 
• Effect Of Litmus Paper: Litmus is a natural indicator, and when the blue litmus paper is dipped in the acid solution,  they turn blue litmus paper into the red. 

Effect of Electric Current

• Acids are good conductors of electricity when dissolved in water. This is because, in water, they get dissolved to release Hydrogen (H⁺as Hydronium) ions. These free ions help in the conduction of electricity.  

Uses of Acids 

Oxalic Acid

• Oxalic acid is used for removing food and ink stains. 
• It is also used in wastewater treatment to remove deposits of calcium. 

Sulphuric Acid

• It is very important industrial chemical and used in the preparation of many chemicals.  
• In the car batteries, sulphuric acid is used. 
• It is also used in the preparation of paints, dyes, and fertilizers. 

Acetic Acid

• The common name of acetic acid is vinegar and it used in the food industry to enhance the flavours of food. 
• Apart from that, it is also used to clean the utensil, floors. 
• It also used to remove stain of wooden furniture. 
• It is used as preservatives in many food items like ketchups, pickels, sauces. 

Boric Acid

• Boric acid is used as grain preservatives and used in the manufacture of glasses and adhesives. 

Hydrochloric Acid

• It is responsible for the digestion of the food we eat. 
• It is also used as a bathroom cleaner. 

Carbonic Acid

• In the preparation of soft drinks 

Tartaric Acid

• In the baking powder. 

Citric Acid

• In food prevention 

Ascorbic Acid

• In the treatment of scurvy disease and bone marrow. 

Methods of Preparation of Acids 

Acids can be prepared in many ways; we will study the most important three methods for the preparation of acids. 

Synthetic Method:  

In the synthetic method, acids are prepared by a direct combination of elements, mostly non-metals. 

For example, when hydrogen gas and chlorine react with each other under the action of electricity, the hydrogen chloride gas is absorbed in water to produce hydrochloric acid. 

The chemical reaction can be written as below: 

Another is the formation of hydrogen sulphide; when the non-metals like sulphur and hydrogen gas are boiled, it forms hydrogen sulphide gas. 

By Dissolving Acidic Oxides In Water 

Acidic oxides: some of the oxides of non-metals are dissolved in the water. These oxides are known as acidic oxides. 

When acidic oxides of carbon, i.e., carbon dioxide, react with water, it gives carbonic acid. 

Similarly, sulphur trioxide dissolves in the water forms sulphuric acid. 

The Reaction of an Acid with Salt of Another Acid : 

The principle of this preparation method is that the acid which has a higher boiling point reacts with the salt of an acid having the lower boiling point to produce an acid.  

For example, common salt, i.e., sodium chloride is salt and salt of hydrochloric acid (HCl).   

The boiling point of hydrochloric acid is lesser than sulphuric acid (H₂SO₄), and therefore when sodium chloride(NaCl) reacts with sulphuric acid, it gives the product as hydrochloric acid(HCl). 

Bases 

Definition:  Bases are those substances, when dissolved in water, that give hydroxyl ions (OH^–). 

Generally, bases are bitter in taste and feel soapy to touch. 

Examples:

• Sodium hydroxide (NaOH) or caustic soda 
• Potassium hydroxide (KOH) 
• Calcium hydroxide: Ca(OH)₂ 

When a base like sodium hydroxide dissociates in water, 

 it gives (Na⁺) and hydroxide(OH^–) ions. 

Similarly, calcium hydroxide dissociates in water to give (Ca⁺) and hydroxide(OH^–) ions. 

Alkalis:

Alkalis are the bases that are soluble in water. 

Classification of Bases 

Bases are classified into three categories based on various factors as follows. 

Based on Strength(Ionising Capability): 

• Strong Bases 
• Weak Bases 

Strong Bases: 

The bases which completely dissociates in water and therefore produce a large number of hydroxyl ions (OH^–) are known as strong bases. 

Examples:

• Hydroxide of all reactive metals
• Sodium Hydroxide (NaOH)
• Potassium hydroxide(KOH)
• Aluminium hydroxides (Al(OH)₃) 

Weak Bases: 

The bases that do not completely dissociate in the water and give very few hydroxyl ions are known as weak bases. 

Examples:

• Ammonium hydroxides (Al(OH)₃), 
• Copper hydroxide (Cu(OH)₂),
• Zinc hydroxide (Zn(OH)₂),
• Ferric hydroxide (Fe(OH)₃) 

Based on Concentrations  

Based on the amount of water present in the given base sample, they are classified into two types. 

Concentrated Base: 

A given sample of a base that contains very little or no amount of water is called a concentrated base. 

Dilute base: 

If a given sample contains an amount of water very much than its own weight, then the given base is called the weak base. 

Based on Acidity of Base: 

Acidity: 

Acidity is defined as the number of hydroxyl ions [OH^– ion] provided by one base molecule on complete dissociation in water. 

Types of Bases

1. Moacidic Bases

• Acidity: Dissociates to give one OH^– ion per molecule of base 
• Examples: NaOH, KOH

2. Diacidic Bases

• Acidity: Dissociates to give two OH^– ion per molecule of base 
• Examples: Ca(OH)_2, Mg(OH)

3. Triacidic Bases

• Acidity: Dissociates to give three OH^– ion per molecule of base 
• Examples: Fe(OH)₃, Al(OH)₃

Properties of Bases 

1. Taste: Bases are usually bitter. 
2. Nature: Bases are slippery to feel/soapy in nature. 
3. Action of an Indicator. 

When the red litmus paper dissolved in the solution of base, it turns into blue 

4. Action of Electricity: Bases also conduct electricity when dissolved in water.  

This is because, when bases dissolved in the water, they dissociate into their respective ions (hydroxyl ions) which is helpful to conduct the electricity. 

Methods Of Preparation of Base: 

By the Action of Oxygen on Metals 

When the metals are heated with oxygen or air, they give their respective basic metal oxides. 

The general chemical reaction can be written as: 

Metal + Oxygen    →    Metallic oxide [Basic Oxide] 

For example:

• when the metals like sodium, potassium, calcium burn in the air, they give their corresponding oxides. 

By the Action of Water / Steam on Reactive Metals 

Some reactive metals react with water or stream to give basic hydroxide with the evolution of hydrogen gas. 

Metal     +     Water/Steam     →   Basic hydroxide/oxide+ H₂(g) 

Metals like sodium and potassium react with cold water to form hydroxides of sodium and potassium, respectively, with liberation hydrogen gas. This reaction is very violent. 

Magnesium reacts with steam to give magnesium oxide with the evolution of hydrogen gas. 

When superheated steam is passed over red-hot iron, ferric oxide is formed, and hydrogen gas is evolved. 

By Heating Carbonates(Decomposition) of Some Metals: 

When the carbonates of some metal are heated, they decompose into their respective oxides with the evolution of carbon dioxide gas. 

When calcium carbonate is heated, calcium oxide and carbon dioxide are formed. 

Uses of Bases 

Sodium Hydroxide (NaOH) 

• Used in the manufacture of soaps, detergents. 
• Used in the manufacture of textiles, papers. 

Magnesium Hydroxide(Mg(OH)₂) 

• Used as antacid to relief from acidity. 

Calcium Hydroxide(Ca(OH)₂) 

• Used to neutralize acidity of soil. 
• Used as dry mixes in decorating, painting. 

Salts

Definition:  The ionic compounds formed when acid and base react with each other are called Salts. 

Acid  +  Base  →  Salt +  Water 

1. Salts are neutral in nature. 
2. The salts are formed because of neutralization reaction as both acid and base neutralize each other’s effect. 

Examples:

• Sodium chloride (common salt)
• CaCl_2,
• Barium sulphate(BaSO₄)
• Sodium nitrate(NaNO₃) 

Sodium chloride or common salt is a product of the neutralisation reaction between the hydrochloric acid (acid) and sodium hydroxide (base). 

Neutralization:  

The reaction between a base and an acid to produce salt and water is called Neutralisation. 

For example:

• When sodium hydroxide(NaOH), a base reacts with sulphuric acid (H₂SO₄), it forms salt, i.e., sodium sulphate and water. 

In all neutralization reactions, the product is always water due to the combination of hydrogen ions present in the acid and hydroxyl ions present in the base.  

The formation of water in an acid-base occurs as follows: 

Summary

• Acids are sour in taste. Generally, bases are bitter in taste and soapy to touch.
• Acid is the substance which dissociates in water to give hydronium ions.
• Base is the substance which dissociates in water to give hydroxyl ions.
• Acids are categorized into five types based on their:

1. Sources
2. Elements present
3. Basicity
4. Strength
5. Concentrations

• Bases are categorized into three types based on their:

1. Strength
2. Concentrations
3. Acidity

• An acid and base react with each other to give salts as result of neutralization reaction.
• Acid can be prepared by:

1. Direct combination of elements.
2. By dissolving acidic oxides with water.
3. By the action of acid on salt of another acid

• Whereas bases can be prepared by:

1. Action of oxygen on metals
2. By heating carbonates of some metals.
3. By the Action Of Water/Steam on active metals