Common Salt

The white coloured crystal salt we use at our homes for cooking and other purposes is called common salt and has a chemical name of Sodium Chloride (NaCl).

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How is common Salt obtained?

Common Salt is usually obtained from 2 sources:

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  1. Salt obtained from Sea : Sea water contains many types of salts the majority being Sodium Chloride . Sodium Chloride (NaCl) is obtained by the process of evaporation of seawater.
    Salt obtained from Sea-Teachoo.jpg

  2. Salt obtained from Underground Deposits : Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt. Rock salt is mined like coal.

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Uses of Common Salt

We already know common salt is used for many day to day purposes such as cooking and even in preserving food items. But apart from this common salt has many industrial uses.



1. Caustic Soda ( Sodium Hydroxide)

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  • Formula : NaOH
  • How it’s made : When electricity is passed through salt water (called brine) , it forms Sodium Hydroxide (NaOH), Chlorine and Water.The process is called the chlor-alkali process.
  • Chemical equation of process :


2NaCl(aq) + 2H 2 O(l) → 2NaOH(aq) + Cl 2 (g) + H 2 (g)

Sodium Chloride + Water → Sodium Hydroxide + Hydrogen gas

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2.) Bleaching Powder (Calcium Oxychloride)

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  • Formula: CaOCl 2      
  • How it’s made: Chlorine produced in the process of making Caustic Soda  is used for the manufacture of Bleaching powder. The Chlorine reacts with dry slaked lime (Ca(OH) 2 ) to produce bleaching powder (CaOCl 2 ).
  • Chemical equation of process:


                                            Ca(OH) 2 + Cl 2 → CaOCl 2 + H 2

                Dry Slaked Lime + Chlorine Gas Bleaching Powder +Water


  • Uses of Bleaching Powder:
  1. Used for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry
  2. As an oxidising agent in many chemical industries
  3. Used for disinfecting drinking water to make it free of germs.
  4. Used for making wool unshrinkable
  5. Used in the manufacture of Chloroform.

Intext Question - Page 33 Q2

3.) Baking Soda (Sodium Hydrogen Carbonate)

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  • Formula: NaHCO 3    
  • How it’s made: It's produced on a large scale by reacting a cold and concentrated solution of sodium chloride with ammonia and carbon dioxide.
  • Chemical equation of process:


NaCl + H 2 O + CO 2 + NH 3 → NH 4 Cl + NaHCO 3  

Sodium Chloride + water + Carbon Dioxide + Ammonia Ammonium Chloride + Baking Soda


  • Uses of Baking Soda: 

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Intext Question - Page33 Q4

NCERT Question - Q12

4.) Washing Soda ( Hydrated Sodium Carbonate)

  • Formula: Na 2 CO 3 . 10H 2 O     
  • How it’s made: Anhydrous (dry) Sodium Carbonate is dissolved in water and recrystallized to get washing soda crystals containing 10 molecules of water of crystallisation.
  • Chemical equation of process:

Na 2 CO 3 + 10H 2 O →  Na 2 CO 3 .10H 2 O    

Sodium Carbonate + Water Hydrated Sodium Carbonate

  • Uses of washing soda: 
  1. Sodium carbonate (washing soda) is used in glass, soap and paper industries.
  2. It is used in the manufacture of sodium compounds such as borax.
  3. Sodium carbonate can be used as a cleaning agent for domestic purposes.
  4. It is used for removing permanent hardness of water.



Water of Crystallization and Hydrated Salts


What is water of crystallization?

  • Salts exist in the form of formula units
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  • To these formula units of a salt, a fixed number of molecules of water remain attached. This is known as the Water of Crystallization .

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For example:

Copper Sulphate (CuSO 4 ) is ideally white in colour and looks like a dry powder. This is because in this form it does not have any water. When we add water to these crystals they turn blue due to the formation of Copper pentahydrate (CuSO 4 . 5H 2 O). These water molecules present in the structure of salts and thus hydrating them is called water of crystallisation.

Heating of Hydrated Copper Sulphate to remove water of crystallisation.jpg

Similarly , as seen in the previous section , washing soda (Na 2 CO 3 .10H 2 O) also contains water of crystallisation. Salts with water in them are called hydrated salts. Salts from which the water of crystallisation have been removed are called unhydrated salts.

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Different Examples of Hydrated Salts and Water of Crystallization


Hydrated Salt Name


Unhydrated Salt Name


Copper Sulphate Pentahydrate

CuSO 4 .5H 2 O

Copper SulpHate

CuSO 4

Calcium Sulphate dihydrate (Gypsum)

CaSO 4 .2H 2 O

Calcium SulpHate

CaSO 4

Sodium Carbonate Decahydrate

Na 2 CO 3 .10H 2 O

Sodium Carbonate

Na 2 CO 3

Iron Sulphate Heptahydrate

FeSO 4 .7H 2 O

Iron Sulphate

FeSO 4



Plaster of Paris ( Calcium Sulphate Semi Hydrate- CaSO 4 .1/2H 2 O)

It is a hydrated salt containing 1/2 a molecule of water of crystallization. We can say that 2 molecules of Plaster Of Paris contains 1 molecule of water of crystallization.

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How is Gypsum used to prepare Plaster of paris?

On heating gypsum at 373 K, it loses water molecules and becomes Calcium Sulphate semi-hydrate (CaSO 4 .1/2H 2 O) which is Plaster of Paris. On adding water to the Plaster of Paris which is a powdery substance , it once again turns into Gypsum which is a hard mass. This is shown in the following reaction:


CaSO 4 .1/2H 2 O + 1 1 / 2 H 2 O → CaSO 4 .2H 2 O

Plaster Paris Powder.jpeg


Uses of Plaster of Paris:

  • Casts and patterns for molds and statues are made using Plaster of Paris
  • Plaster of Paris is used as the cement in ornamental casting
  • It is also used for making decorative materials
  • It is used as a fireproofing material 
  • It is used in making chalks
  • Incase of bone fracture or sprain, it is used to make a cast bandage to prevent movement of the affected part
  • It is used to fill small gaps on walls & roofs

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Intext Question - Pg 33 Q5

NCERT Question Q13


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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 13 years and a teacher from the past 17 years. He teaches Science, Economics, Accounting and English at Teachoo