Introduction to Acids, Bases and Salts

Classification of matter

On the basis of
a) composition –  elements, compounds and mixtures
b) state – solids, liquids and gases
c) solubility – suspensions, colloids and solutions

Types of mixtures – homogeneous and heterogeneous

Types of compounds – covalent and ionic

What Is an Acid and a Base?

Ionisable and non-ionisable compounds

An ionisable compound when dissolved in water or in its molten state, dissociates into ions almost entirely. Example: NaCl, HCl, KOH, etc.
A non-ionisable compound does not dissociate into ions when dissolved in water or in its molten state. Example: glucose, acetone, etc.

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Arrhenius theory of acids and bases

Arrhenius acid – when dissolved in water, dissociates to give H⁺ (aq) or H₃O⁺ ion.
Arrhenius base – when dissolved in water, dissociates to give OH⁻ ion.

Examples

Acids 

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

Bases

• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)
• Calcium hydroxide (Ca(OH)₂)

Bronsted Lowry theory

A Bronsted acid is an H⁺ (aq) ion donor.
A Bronsted base is an H⁺ (aq) ion acceptor.

Example

In the reaction: HCl (aq) +NH₃ (aq) →NH⁺₄(aq) +Cl⁻ (aq)
HCl – Bronsted acid and Cl⁻ : its conjugate acid
NH₃ – Bronsted base and NH⁺₄ : its conjugate acid

Physical test

Given are two possible physical tests to identify an acid or a base.

a. Taste

An acid tastes sour whereas a base tastes bitter.
The method of taste is not advised as an acid or a base could be contaminated or corrosive.

b. Effect on indicators by acids and bases

An indicator is a chemical substance which shows a change in its physical properties, mainly colour or odour when brought in contact with an acid or a base.
Below mentioned are commonly used indicators and the different colours they exhibit:

a) Litmus
In a neutral solution – purple
In acidic solution – red
In basic solution – blue

Litmus is also available as strips of paper in two variants – red litmus and blue litmus.
An acid turns a moist blue litmus paper to red.
A base turns a moist red litmus paper to blue.

b) Methyl orange
In a neutral solution – orange
In acidic solution – red
In basic solution – yellow

c) Phenolphthalein
In a neutral solution – colourless
In acidic solution – remains colourless
In basic solution – pink

Acid-Base Reactions

Reactions of acids and bases

a) Reaction of acids and bases with metals

Acid + active metal →  salt + hydrogen + heat

2HCl+Mg→MgCl₂+H₂ (↑)

Base + metal → salt + hydrogen + heat

2NaOH+Zn→Na₂ZnO₂+H₂ (↑)

A more reactive metal displaces the less reactive metal from its base.

2Na+Mg (OH) ₂→2NaOH+Mg

b) Reaction of acids with metal carbonates and bicarbonates

Acid + metal carbonate or bicarbonate →  salt + water + carbon dioxide.

2HCl+CaCO₃→CaCl₂+H₂O+CO₂
H₂SO₄+Mg (HCO₃)₂→MgSO₄+2H₂O+2CO₂

Effervescence indicates liberation of CO₂ gas.

c) Neutralisation reaction

1. Reaction of metal oxides and hydroxides with acids
Metal oxides or metal hydroxides are basic in nature.
Acid + base → salt + water + heat

H₂SO₄+MgO→MgSO₄+H₂O
2HCl+Mg (OH) ₂→MgCl₂+2H₂O

2. Reaction of non-metal oxides with bases

Non-metal oxides are acidic in nature
Base + Non-metal oxide →  salt + water + heat

2NaOH+CO₂→Na₂CO₃+H₂O

Water

Acids and bases in water

When added to water, acids and bases dissociate into their respective ions and help in conducting electricity.

Difference between a base and an alkali

Base-

• Bases undergo neutralisation reaction with acids.
• They are comprised of metal oxides, metal hydroxides, metal carbonates and metal bicarbonates.
• Most of them are insoluble in water.

Alkali –

• An alkali is an aqueous solution of a base, (mainly metallic hydroxides).
• It dissolves in water and dissociates to give OH⁻ ion.
• All alkalis are bases, but not all bases are alkalis.

Hydronium ion

Hydronium ion is formed when a hydrogen ion accepts a lone pair of electrons from the oxygen atom of a water molecule, forming a coordinate covalent bond.

Dilution

Dilution is the process of reducing the concentration of a solution by adding more solvent (usually water) to it.
It is a highly exothermic process.
To dilute acid, the acid must be added to water and not the other way round.

Strength of acids and bases

Strong acid or base: When all molecules of a given amount of an acid or a base dissociate completely in water to furnish their respective ions, H⁺(aq) for acid and OH⁻(aq) for base).
Weak acid or base: When only a few of the molecules of a given amount of an acid or a base dissociate in water to furnish their respective ions, H⁺(aq) for acid and OH⁻(aq) for base).

Dilute acid: contains less number of H⁺(aq) ions per unit volume.
Concentrated acid: contains more number of H⁺(aq) ions per unit volume.

Universal indicator

A universal indicator has a pH range from 0 to 14 that indicates the acidity or alkalinity of a solution.
A neutral solution has pH=7

pH

pH=−log₁₀[H⁺]
In pure water, [H⁺]=[OH⁻]=10⁻⁷ mol/L. Hence, the pH of pure water is 7.
The pH scale ranges from 0 to 14.
If pH < 7 – acidic solution
If pH > 7- basic solution

  pH scale

Importance of pH in everyday life

1. pH sensitivity of plants and animals

Plants and animals are sensitive to pH. Crucial life processes such as digestion of food, functions of enzymes and hormones happen at a certain pH value.

2. pH of a soil

The pH of a soil optimal for the growth of plants or crops is 6.5 to 7.0.

3. pH in the digestive system

The process of digestion happens at a specific pH in our stomach which is 1.5 – 4.
The pH of the interaction of enzymes, while food is being digested, is influenced by HCl in our stomach.

4. pH in tooth decay

Tooth decay happens when the teeth are exposed to an acidic environment of pH 5.5 and below.

5. pH of self-defence by animals and plants

Acidic substances are used by animals and plants as a self-defence mechanism. For example, bee and plants like nettle secrete a highly acidic substance for self-defence. These secreted acidic substances have a specific pH.

Manufacture of Acids and Bases

Manufacture of acids and bases

a) Non-metal oxide + water → acid

SO₂(g)+H₂O(l)→H₂SO₃(aq)
SO₃(g)+H₂O(l)→H₂SO₄(aq)
4NO₂(g)+2H₂O(l)+O₂(g)→4HNO₃(aq)

Non-metal oxides are thus referred to as acid anhydrides.

b) Hydrogen + halogen → acid

H₂(g)+Cl₂(g)→2HCl(g)
HCl(g)+H₂O(l)→HCl(aq)

c) Metallic salt + conc. sulphuric acid → salt + more volatile acid
2NaCl(aq)+H₂SO₄(aq)→Na₂SO₄(aq)+2HCl(aq)
2KNO₃(aq)+H₂SO₄(aq)→K₂SO₄(aq)+2HNO₃(aq)

d) Metal + oxygen → metallic oxide (base)
4Na(s)+O₂(g)→2Na₂O(s)
2Mg(s)+O₂(g)→2MgO(s)

e) Metal + water → base or alkali + hydrogen
Zn(s) + H₂O(steam)→ ZnO(s)+ H₂(g)

f) Few metallic oxides + water → alkali
Na₂O(s)+H₂O(l)→2NaOH(aq)

g) Ammonia + water → ammonium hydroxide
NH₃(g)+H₂O(l)→NH₄OH(aq)

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Salts

A salt is a combination of an anion of an acid and a cation of a base.

Examples – KCl,NaNO₃,CaSO₄,etc.

Salts are usually prepared by the neutralisation reaction of an acid and a base.

Common salt

Sodium Chloride (NaCl) is referred to as common salt because it’s used all over the world for cooking.

Family of salts

Salts having the same cation or anion belong to the same family. For example, NaCl, KCl, LiCl.

pH of salts

A salt of a strong acid and a strong base will be neutral in nature. pH = 7 (approx.).
A salt of a weak acid and a strong base will be basic in nature. pH > 7.
A salt of a strong acid and a weak base will be acidic in nature. pH < 7.
The pH of a salt of a weak acid and a weak base is determined by conducting a pH test.

Preparation of Sodium hydroxide

Chemical formula – NaOH
Also known as – caustic soda
Preparation (Chlor-alkali process):
Electrolysis of brine (solution of common salt, NaCl) is carried out.
At anode: Cl₂ is released
At cathode: H₂ is released
Sodium hydroxide remains in the solution.

Bleaching powder

Chemical formula – Ca(OCl)Cl or CaOCl₂
Preparation – Ca(OH)₂(aq)+Cl₂(g)→CaOCl₂(aq)+H₂O(l)

On interaction with water – bleaching powder releases chlorine which is responsible for bleaching action.

Baking soda

Chemical name – Sodium hydrogen carbonate
Chemical formula – NaHCO₃
Preparation (Solvay process) – 
a. Limestone is heated: CaCO₃→CaO+CO₂
b. CO₂ is passed through a concentrated solution of sodium chloride and ammonia:
NaCl(aq)+NH₃(g)+CO₂(g)+H₂O(l)→NaHCO₃(aq)+NH₄Cl(aq)

Uses:
1. Textile industry
2. Paper industry
3. Disinfectant

Washing soda

Chemical name – Sodium hydrogen carbonate
Chemical formula – NaHCO₃
Preparation (Solvay process) – 
a. Limestone is heated: CaCO₃→CaO+CO₂
b. CO_{_2} is passed through a concentrated solution of sodium chloride and ammonia:
NaCl(aq)+NH₃(g)+CO₂(g)+H₂O(l)→NaHCO₃(aq)+NH₄Cl(aq)

Uses
1. In glass, soap and paper industries
2. Softening of water
3. Domestic cleaner

Crystals of salts

Certain salts form crystals by combining with a definite proportion of water. The water that combines with the salt is called water of crystallisation.

Plaster of paris

Gypsum, CaSO₄.2H₂O (s) on heating at 100°C (373K) gives CaSO₄. ½ H₂O and 3/2 H₂O
CaSO₄. ½ H₂O is plaster of paris.
CaSO₄. ½ H₂O means two formula units of CaSO₄ share one molecule of water.
Uses – cast for healing fractures.