Bleaching Powder, Baking Soda, Washing Soda & Plaster of Paris - Science Class 10

Bleaching Powder

We know that chlorine is produced during the electrolysis of aqueous sodium chloride (brine). This chlorine gas is used for the manufacture of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂]. Bleaching powder is represented as CaOCl₂, though the actual composition is quite complex. 
 

Preparation of Bleaching Powder

(a) For bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry. 

(b) As an oxidising agent in many chemical industries, and 

(c) For disinfecting drinking water to make it free of germs.

Baking Soda

The chemical name of baking soda is sodium hydrogen carbonate or sodium bicarbonate. Baking soda (or sodium bicarbonate) is represented by the formula NaHCO₃. The soda commonly used in the kitchen for making tasty crispy pakoras is baking soda. Some time it is added for faster cooking. It is produced using sodium chloride as one of the raw materials.

 
It can be used to neutralise an acid because it is mild non-corrosive base due to the hydrolysis of HCO₃^- ion. 
The following reaction takes place when it is heated during cooking.

Uses of Sodium Hydrogen-carbonate (NaHCO₃)

Carbon dioxide so produced during the reaction is responsible for making the bread and cake to rise making them soft and spongy.

(b) As an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.

(c) It is used in soda-acid fire extinguisher.

Washing Soda (Sodium Carbonate)

Step-I 

The CO₂ required in this reaction is obtained by heating limestone. 

Step-II Dry sodium hydrogencarbonate is heated strongly to produce sodium carbonate. 

Step-III Washing soda (Na₂CO₃.10H₂O) is obtained by crystallisation from a saturated solution of soda ash (Na₂CO₃) 

Manufacturing Process of Sodium Carbonate (Solvay Process)

Uses of Washing Soda

(b) Washing soda is used for softening hard water.

(c) Sodium carbonate (soda ash) is used for the manufacture of detergents.

(d) Sodium carbonate is used for the manufacture of many important compounds, such as borax (Na₂B₄O₇),  Hypo (Na₂S₂O₃.5H₂O), etc. 

(e) Sodium carbonate is also used in paper and paint industries.

Are the Crystals of Salts really Dry? 

Water of Crystallization

It is fixed number of water molecules present in crystalline salt,
Example: 

Hydrous to Anhydrous Copper Sulphate

Plaster of Paris: (CaSO₄.H₂O)

In plaster of paris one molecule of water is shared by two CaSO₄ as 

Preparation of Plaster of Paris

Plaster of paris is obtained by heating gypsum (CaSO₄.2H₂O) at 373K (or 100°C).

or 

During the preparation of plaster of paris, temperature should be controlled carefully. Otherwise, anhydrous calcium sulphate (CaSO₄) will be formed. Anhydrous calcium sulphate does not set into hard mass when mixed with water. So, if temperature is not controlled carefully, the plaster of paris obtained will have poor setting property.   Sculptures using Plaster of Paris

Properties of Plaster Paris

• Plaster of paris is a white, odourless powder.
• At ordinary room temperature, plaster of paris absorbs water and a large amount of heat is liberated.
• When mixed with a limited amount of water (50% by mass), if forms a plastic mass, evolves heat and quickly sets to a hard porous mass with in minutes. This is called the setting process.
• During setting, a slight expansion in volume occurs. It is due to this that it fills the mould completely and gives sharp impression. The reaction during process is CaSO₄.0.5H₂O(s) + 1.5H₂O H₂O(l) → CaSO₄ . 2H₂O(s)
  Plaster of paris     Water     Gypsum (Hard mass) 

Uses of Plaster of Paris

• Plaster of paris is used in making casts and patterns for moulds and statue.
• Plaster of paris is used as cement in ornamental casting and for making decorative materials.
• Plaster of paris is used as a fire proofing material and for making chalks.
• Plaster of paris is used in hospitals for immobilising the affected part in case of bone fracture of strain.
• Plaster of paris (POP) is used to fill small gaps on walls & roofs.

Efflorescence

Efflorescence

• Certain hydrated crystalline salts when exposed to atmosphere lose their water of crystallisation spontaneously and change into amorphous powder.
• The spontaneous loss of water of crystallization, wholly or partly, when crystals with water of crystallization are exposed to air is called efflorescence and the substances exhibiting efflorescence are called efflorescent substance.
• For Example : Washing soda (Na₂CO₃.10H₂O), Glauber's salt (Na₂SO₄.10H₂O), blue vitriol (CuSO₄.5H₂O)

Deliquescence

Hygroscopic Substances

Silica Gel is an example of Hygroscopic Substances

• Certain substance absorb water from the atmosphere without undergoing change in physical state. Such substances are known as hygroscopic substance.
• Anhydrous sodium carbonate, anhydrous copper sulphate, concentrated sulphuric acid are examples of hygroscopic substances.