Salt

Acidic, Basic and Neutral materials of Class 7

Salt

In neutralisation reaction a new substance is formed. This is called salt. Salt may be acidic, basic or neutral in nature. Thus, neutralisation can be defined as follows: The reaction between an acid and a base is known as neutralisation. Salt and water are produced in this process with the evolution of heat.

Acid+Base----------Salt+Water (Heat is evolved)

The following reaction is an example:

Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) Sodium chloride (NaCl) + Water

“A substance formed by neutralization of an acid with a base is called a salt.”

e.g.

Ca(OH)₂ + H₂SO₄® CaSO₄ + H₂0

2Cu(OH)₂ + 4HNO₃® 2Cu(NO₃)₂ + 2H₂O

NaOH + HCI ®NaCl + H₂O

(a) Naming of salts:

Salts obtained from sulphuric acid are known as sulphates. e.g. CaSO₄, Na₂SO₄ etc.
Salts obtained from nitric acid are known as nitrates. e.g. KNO₃, NaNO₃ etc.
Salts obtained from hydrochloric acid are known as chlorides. e.g. CaCl₂, KCl etc.
Salts obtained from phosphoric acid are known as phosphates. e.g. Ca₃(PO₄)₂, Na₃PO₄ etc.
Salts obtained from carbonic acid are known as carbonates. e.g. CaCO₃, BaCO₃ etc.

(b) Preparation of salts

By neutralization of an acid by base:

2NaOH + H₂SO₄¾® Na₂SO₄ + 2H₂O

By reaction of an acid with a metal:

Zn + dil. 2HCl ¾® ZnCl₂ + H₂

Zn + dil. H₂SO₄¾® ZnSO₄ + H₂

2Na + dil. 2HCl ¾® 2NaCl + H₂

By the action of acid an carbonates

Na₂CO₃ + dil. 2HCl ¾® 2NaCl + H₂O + CO₂

ZnCO₃ + dil. 2HCl¾® ZnCl₂ + H₂O + CO₂

By the action of acid on metal oxides:

Na₂O + dil. 2HCl¾® 2NaCl + H₂O

ZnO + dil. H₂SO₄¾® ZnSO₄ + H₂O

(c) PROPERTIES OF SALTS :

Salts are mostly solids with high melting point and high boiling point.
Salts are usually soluble in water.
Solutions of salts in water conduct electricity.
The crystals of some salts have some water molecules associated with them. These water molecules are known as water of crystallization. Such salts are called hydrated salts.The crystals which have lost their water of crystallization are called anhydrous.
Some salts have the tendency to absorb water from air and turn into a solution, such substances are called deliquescent. They are stored in air tight container and in dry places.

(d) CLASSIFICATION OF SALTS:

Salts have been classified on two bases:

Chemical formulae of salts
Nature of salts

(i) Classification based on chemical formulae of salts:

Normal Salts: A normal salt is the one which does not contain any ionisable hydrogen atom or hydroxyl group. This means that it has been formed by the complete neutralisation of an acid by a base.
e.g. NaCI, KCI, NaNO₃, K₂SO₄ etc.

Acidic salts: An acidic salt still contains some replaceable hydrogen atoms. This means that the neutralisation of acid by the base is not complete.
e.g. Sodium hydrogen sulphate (NaHSO₄), sodium hydrogen carbonate (NaHCO₃) etc.

Basic salts: A basic salt still contains some replaceable hydroxyl groups. This means that the neutralisation of base by the acid is not complete.
e.g. Basic lead nitrate Pb(OH)NO₃, basic lead chloride, Pb(OH)CI etc.

(ii) Classification based on nature of salts:

Neutral salt solutions: The solution of a salt formed by neutralisation of a strong acid with a strong base is neutral in nature. Such solutions do not change the colour of litmus solution.
e.g. NaCI, KCI, NaNO₃, Na₂SO₄ etc.

Acidic salt solutions: The solution of a salt formed by neutralisation of a strong acid with a weak base is acidic in nature. Such solutions change the colour of blue litmus solution to red.
e.g. (NH₄)₂SO₄,NH₄Cl etc.

In both these salts, the base NH₄OH weak while the acids H₂SO₄ and HCI are strong.

Basic salt solutions : The solution of a salt formed by neutralisation of a weak acid with a strong base is basic in nature. Such solutions change the colour of red litmus solution to blue.
e.g. Na₂CO₃, K₃PO₄ etc.

In both the salts, bases NaOH and KOH are strong while the acids H₂CO₃ and H₃PO₄ are weak.

(e) USES OF SALTS:

As a table salt.
In the manufacture of butter and cheese.
In leather industry.
In the manufacturing of washing soda and baking soda.
For the preparation of sodium hydroxide by electrolysis of brine.
In cold countries rock salt is spread on ice to melt it.

USES OF SALTS IN DAILY LIFE

Common salt : It is used in large amount as table salt. It is used for preservation of food in pickling. It is used to preserve raw hides. It is used in the manufacture of chemicals such as chlorine and caustic soda (NaOH).
Baking soda : It is used for making baking powder. It is used for preparing antacid tablets, for controlling acidity of stomach.
Washing soda : It is used by washermen to wash clothes. It is used in manufacture of glass and caustic soda. It is used in fire extinguishers. It is used in the manufacture of detergents.
Chile saltpetre : It is used in the manufacture of nitric acid, gun powder and fire works.
Nitre : It is used as a fertiliser. It is also used in the manufacture of gun powder, fire works and nitric acid.
Smelling salt : It provides relief from common cold.
Epsom salt : It is used as a laxative by patients suffering from constipation.
Green vitriol: It is used in making blue black inks and in curing leather.
Blue vitriol : It is used as fungicide in agriculture. It is used in dyeing and printing industry. It is used in electroplating copper metal.
Plaster of Paris : It is used for making statues. It is used for setting broken bones.
Potash alum : It helps in rapid settling of suspended impurities in water. It is used for the purification of water.