CLASS 10 CHAPTER 2 – Acids, Bases and Salts

Introduction

The taste of the food that we eat depends on the presence of acids and bases in them.
Acids are sour in taste and change the colour of blue litmus to red.
Bases are bitter in taste and change the colour of red litmus to blue.
Litmus and turmeric are natural indicators. Some other examples of natural indicators are red cabbage leaves, coloured petals of some flowers like hydrangea, petunia, geranium.

Acids

Acids are those which produce hydrogen ions in water. Example sulphuric _{acid, hydrochloric acid etc.}
Acids turn blue litmus to red which is used as the confirmation test for acid.

Reaction with acids

Reaction of acid with metal

H₂SO₄ + Mg → MgSO₄+ H₂

2HCl + Zn → ZnCl₂ + H₂

Reaction of acids with metal carbonates and bicarbonates

2HCl + CaCO₃→ CaCl₂+ CO₂+ H₂O

H₂SO₄ + Mg (HCO₃)₂ → MgSO₄ + 2H₂O + CO₂

Bases do not react with metal carbonates and bicarbonates

The reaction of an acid with a base

Acids and bases react together to form Salt and water.

HCl + NaOH → NaCl + H₂O

Neutralisation Reaction: The reaction of an acid with a base is called Neutralisation Reaction.

Acid + Base → Salt + Water

The reaction of metallic oxides with Acids

H₂SO₄ + MgO → MgSO₄ + H₂O

2HCl + Mg (OH) ₂ → MgCl₂ + 2H₂O

The reaction of non – metallic oxides with Base

Non-metal oxides are acidic in nature

Base + Nonmetal oxide → salt + water + heat

2NaOH + CO₂→ Na₂CO₃ + H₂O

What happens to acid and base in water?

The acid in water produces an H⁺ion, which cannot exist alone therefore it combines with the water molecule to give hydronium ion. Example:

HCl + H₂O → H₃O + Cl^–

Bases produce OH^–ions in water. Bases that are soluble in water are known as alkalis. Example:

NaOH + H₂O → Na⁺(aq) + OH^–(aq)

KOH + H₂O → K⁺(aq) + OH^–(aq)

Now after knowing that all acids give H⁺(aq) and all bases give OH^–(aq), the neutralisation reaction can be written as follows:

Acid + Base → Salt + Water

HX + MOH → MX + HOH

H⁺(aq) + OH^–(aq) → H₂O (l)

Caution while adding acid to water

Mixing an acid or a base with water results in a decrease of ions (H₃O/ OH^–) per unit volume. This process is called dilution.
The process of dissolving an acid or a base in water is highly exothermic in nature.
If water is added to a concentrated acid, a huge amount of heat is generated which may cause the mixture to splash out and cause burns. Therefore, It should be kept in mind that acid should always be added to water slowly with constant stirring.

pH Scale

The scale developed for measuring hydrogen ion concentration in a solution is known as the pH scale, where p stands for potenz in German.
pH scale ranges from 0 (very acidic) to 14 (very alkaline).
The pH of a neutral solution is 7.
Values less than 7 on the pH scale refers to an acidic solution. pH value from 7 – 14 refers to an alkaline solution.

Acids producing more H⁺ions are strong acids while those producing less H⁺are said to be weak acids.
Similarly, bases giving more OH^–ions are strong bases and those giving less OH^–ions are called weak bases.

pH in our day-to-day life

Our body works within the pH range of 7.0 – 7.8. the pH of our saliva is around 6.5 – 7.5.
When the pH of rainwater drops below 5.6, it is called acid rain. This lowers the pH of river water, because of which survival of aquatic life becomes difficult.
Our stomach produces hydrochloric acid during digestion which causes no harm. But during indigestion stomach produces too much acid causing pain and irritation.
the pH of tomato juice ranges from 4.1 to 4.6 whereas the pH of carrot juice is around 6.4.
Bacteria present in the mouth releases acid by degradation of remaining sugar and food particles. This reduces the pH below 5.5 and corrodes calcium phosphate present in the tooth enamel.
Bee- sting leaves acid that causes pain and irritation. Nettle leaves inject methanoic acid causing burning pain. The use of a mild base such as baking soda on the stung area gives relief.

Chemicals from common salt

A salt formed by the reaction of hydrochloric acid and sodium hydroxide is called sodium chloride. (common salt) hence it is a neutral salt.
Seawater contains many dissolved salts. Sodium chloride is separated from these salts.
Common salt is the main raw material for many materials of daily use such as sodium hydroxide, washing soda, baking soda, bleaching powder and many more.

Sodium Hydroxide

When electricity is passed through an aqueous solution of sodium chloride also called brine solution, it breaks into sodium hydroxide and chlorine.
The process is known as the chloralkali process. Chlor for chlorine and alkali for sodium hydroxide.

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Chlorine gas is given off at the anode and hydrogen is released at the cathode.

Bleaching Powder

The chlorine produced during the electrolysis of brine solution is used for the manufacture of bleaching powder.
The reaction of chlorine gas on dry slaked lime [Ca (OH)₂] produces bleaching powder.

Ca(OH)₂ + Cl₂ → CaOCl₂+ H₂O

Uses of bleaching powder

It is used as a bleaching agent for cotton and linen in the textile industry.
To bleach wood pulp in paper industries.
To bleach washed clothes in the laundry.
It is used as a disinfectant for drinking water.
It is used as an oxidising agent in chemical industries.

Baking Soda

It is prepared by passing CO₂gas through a concentrated solution of sodium chloride and ammonia.

NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃

Its chemical name is Sodium Hydrogen Carbonate
It is a mild non- corrosive basic salt.
When heated during cooking following reaction takes place:

2NaHCO₃ → Na₂ CO₃ + H₂O+CO₂

Uses of Baking Soda

For making baking powder which is produced by the reaction of mild edible oil like tartaric acid with baking soda.
When baking powder is mixed with water or heated, the following reaction occurs:

NaHCO₃ + H⁺→ CO₂ + H₂O + Sodium salt of acid

This CO₂ produced makes bread and cake rise making them soft and spongy.
Being alkaline it is an active ingredient of antacids that acts by neutralising excess acid in the stomach.
It is also used in soda-acid fire extinguishers.

Washing Soda

This is another derivative of sodium chloride. The heating baking soda produces sodium carbonate.

2NaHCO₃→ NaCO₃+ H₂O + CO₂

Recrystallization of sodium carbonate yields washing soda.

Na₂CO₃+ 10 H₂O→ Na₂CO₃ .10H₂O

Uses of washing soda

Washing soda is used in glass, paper and soap industries.
It is used in the manufacturing of sodium compounds like borax.
It is also used as a cleaning agent for domestic purposes.
It is also used for removing the permanent hardness of the water.

Water of Crystallization

The fixed number of water molecules present in one formula unit of salt is known as the water of crystallization.
For example, there are 5 water molecules in one formula unit of copper sulphate. Therefore, the chemical formula of hydrated copper sulphate is written as CuSO₄ .5H₂
Gypsum has two molecules of water as water of crystallization and hence its chemical formula is written as CuSO₄ .2H₂
Gypsum on heating loses water molecules and becomes calcium sulphate hemihydrate. This is known as plaster of Paris.

Uses of plaster of Paris :

It is used by doctors for supporting fractured bones in the right position.
When mixed with water, it changes again to gypsum giving a hard solid mass.

CaSO₄ .0.5 H₂ O +1.5 H₂O → CaSO₄ .2H₂O

Plaster of Paris Gypsum

It is used for making toys, materials for decoration and for making surfaces smooth.