4. About Salts

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. Sodium chloride (NaCl) is also known as Common or Table Salt. It is formed after the reaction between sodium hydroxide and hydrochloric acid. It is a neutral salt. The pH value of sodium chloride is about 7. Sodium chloride is used to enhance the taste of food. Sodium chloride is used in the manufacturing of many chemicals.

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. 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. 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.

About  salts

1. Strong acid  +  strong base  →     neutral salt :  pH = 7
2. Salt of strong   +  weak base  →    Acidic salt :    pH <7
3. Salt of strong base + weak acid → Basic salt  :  pH  >7

Chemical from common salt

1;  sodium hydroxide  ( NaOH)

  When electricity is passed through an aqueous solution of NaCl (called brine) it decomposes to form NaOH.

This process is called chlor- alkali process.

2NaCl  +   2H₂O      →        2NaOH + Cl₂    + H₂

 At cathode  :→     H₂    gas

 At anode :→    Cl₂ gas

Near cathode :→   NaOH solution is formed

Uses

1. Cl₂ :→   Water treatment , PVC , pesticides.
2. H₂ :→      Fuels,  margarine.
3. Hcl : →      Medicines, cleaning steels.
4.  NaOH :→   De- greasing metals, soaps and paper making.

(2) Bleaching powder  ( CaOCl₂)

   Bleaching powder is produced by the action of chlorine on dry slaked lime   ( Ca(OH)₂).

 Cl₂  +  Ca(OH)₂ →       CaOCl₂  +   H₂O

 USES :→   * Bleaching cotton and linen in textile industry

• Bleaching washed clothes in laundry .
• Oxidising in chemical industries.
•  Make drinking water free from germs

(3)   Baking Soda ( NaHCO₃)

 The chemical name of the compound is sodium hydrogen carbonate.

NaCl+H₂O+CO₂+NH₃   →   NH₄Cl  +  NaHCO₃

                                     (Ammonium Chloride) (Sodium hydrogen carbonate)

• IT is a mild non- corrosive base.
• When it is heated during cooking.

2 NaHCO₃  →    Na₂CO₃  +   H₂O  +  CO₂

USES:-

1. For making baking powder, which is a mixture of baking soda ( sodium hydrogen carbonate ) and tartaric acid. Following reaction takes place.

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

2. Sodium hydrogen carbonate is an ingredient in antacids.
3. Used in soda-acid , fire extinguisher

 (4)  Washing Soda  :-   ( Na₂ CO₃IOH₂O)

  Recrystallisation of sodium carbonate gives washing sod. It is a basic salt.

  Na₂CO₃ + IOH₂O     →    Na₂CO₃. IOH₂O

   USES;   

1. In glass , soap and paper industries.
2. Manufacture of borax
3. Cleaning agent for domestic purpose.
4. For removing permanent hardness of water.

 (5)  Plaster of paris : CaSO₄ .2H₂ O. 

On heating gypsum ( CaSo₄.2H₂O) at 373k, ut loses water molecules and becomes plaster of paris (POP) .

It is a white powder and on mixing with waater it changes to gypsum.

 Removing water of crystallization.

 WATER OF CRYSTALLISATION ;

It is a fixed number of water molecules present in one formula unit of a salt.

For Example; 

• CuSO₄.5H₂O   has 5 water molecules.
• Na₂CO₃.10H₂  has 10 water molecules.
•  CaSO₄.2H₂O  has 2 water molecules.