Plaster of Paris, Baking Soda and Washing Soda

Plaster of Paris, baking soda, and washing soda are important inorganic compounds that have a wide range of applications in construction, medicine, cleaning, food preparation, and various industries.

Plaster of Paris

Plaster of Paris is a quick-setting white powder made from gypsum. It is chemically known as calcium sulphate hemihydrate (CaSO₄·½H₂O). When mixed with water, it forms a paste that quickly hardens into a solid mass. The name “Plaster of Paris” comes from the large deposits of gypsum found near Paris.

Formula

The chemical formula of Plaster of Paris is CaSO₄.1/2H₂O.

Preparation of Plaster of Paris

Plaster of Paris is (POP) prepared from the chemical compound, calcium sulfate dihydrate, which is also known as, gypsum( CaSO₄.2H₂O). POP is manufactured by heating the element gypsum at a very high temperature of about 373 K. During heating, it loses water of crystallization to form Plaster of Paris.

CaSO₄ · 2H₂O + Heat ⇢ CaSO4 · 1/2 H₂O + 3/2 H₂O (Discharged as steam)

Since the presence of moisture may slow down the setting of plaster by bringing about the hydration process. Therefore, it is stored in moisture-proof containers.

Properties of Plaster of Paris

• It is a white, soft powder
• On mixing with water, it forms gypsum again and hardens quickly
• It expands slightly on setting, giving a smooth finish
• It does not crack easily
• It has a relatively low density and good moulding ability

Uses of Plaster of Paris

• Architecture and Decoration: Plaster of Paris is widely used to make decorative items, false ceilings, and designs on walls and monuments. It is also used to give smooth finishing to surfaces and to imitate materials like wood or stone.
• Medical Use: It is used to make casts and moulds for supporting fractured bones. These casts help keep the bones fixed during the healing process.
• Mould Making: Plaster of Paris is used to prepare moulds for statues, toys, ceramics, and other artistic items due to its ability to take fine shapes.
• Fire Protection: It is used in fire-resistant materials because it does not catch fire easily and can provide insulation against heat.

Washing Soda (Sodium Carbonate)

Washing soda is a white crystalline substance chemically known as sodium carbonate decahydrate (Na₂CO₃·10H₂O). It is a strongly alkaline compound used widely for cleaning and industrial purposes. The anhydrous form of sodium carbonate is called soda ash.

Formula

The chemical formula of Sodium Carbonate or Washing Soda is Na₂CO₃.10H₂O, where 10H₂O is the water of crystalization.

Preparation of Sodium Carbonate

Solvay method is required for the synthesis of sodium carbonate, and the steps involved in sodium carbonate manufacturing are described below:

Step 1: Purification of Brine

A solution of salt and water, called brine is purified in the first step. This process is important to remove the impurities and therefore increase the yield of the final product. Evaporation produces concentrated brine, while precipitation removes impurities such as calcium, magnesium, and other minerals coming from the rock salts. The concentrated brine solution is filtered before being mixed with ammonia in the ammonia tower.

Step 2: Formation of Sodium Hydrogen Carbonate

In the second step, purified brine is transferred into another chamber, and ammonia passes through it. This ammonia reacts with salt water to give ammoniated brine and heat. In the next step, this ammoniated brine reacts with Carbon dioxide (produced via decomposing CaCO₃ at 900°C) to give sodium bicarbonate and ammonium chloride. These conditions are controlled in such a way as to maximise the precipitation of sodium bicarbonate, which is extracted using a rotary pump.

NH₃ + CO₂ + NaCl + H₂O → NaHCO₃ + NH₄Cl

Step 3: Formation of Sodium Carbonate

In this step sodium carbonate (Na₂CO₃) is produced by the thermal decomposition of sodium bicarbonate (NaHCO₃) at 300°C. As a result, sodium carbonate crystals form.

2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

Step 4: Recovery of Ammonia

This process involves the recovery of ammonia from the previous steps. In this step, a distillation column is used to recover ammonia. First NH₄Cl solution is treated with lime water (Ca(OH)₂) to obtain the ammonia as the final product. The Solvay process uses this ammonia again, and CaCl₂ is obtained as a by-product.

2NH₄Cl + Ca(OH)₂ → 2NH₃ + CaCl₂ + H₂O

Properties of Sodium Carbonate

• It's a crystalline solid that's white in colour.
• It comes in monohydrated (Na₂CO₃.10H₂O), anhydrous (Na₂CO₃), heptahydrate (Na₂CO₃.7H₂O), and decahydrate (Na₂CO₃.10H₂O) forms.
• Strongly alkaline in nature.
• It loses its water in the presence of heat and forms an anhydrous salt (soda ash).

Na₂CO₃.10H₂O → Na₂CO₃.H₂O → Na₂CO₃ (at 373 K)

Uses of Sodium Carbonate

• In industry and the home, it is used as a cleaning agent.
• It's used in the paper, textile, soap, and detergent sectors, among others.
• It is utilised in the water softening process.
• It's a material used in the production of glass.
• In laundries, it is one of the most significant agents.

Baking Soda (Sodium Bicarbonate)

Sodium bicarbonate, commonly known as baking soda, is a widely used chemical compound with the formula NaHCO₃. It is related to naturally occurring substances like natron, which was used in ancient times for cleaning purposes. Today, baking soda is valued for its versatility, usefulness, and low cost.

Formula

The chemical formula of Sodium Bicarbonate or Baking Soda is NaHCO₃.

Preparation of Sodium Bicarbonate: Solvay Process

Ernest Solvay, an Italian chemist developed this method in 1861 to produce sodium bicarbonate. The steps following the formation of sodium bicarbonate are given below,

Production of Sodium Hydrogen Carbonate

Baking soda is prepared by the Solvay process. In this process, carbon dioxide is passed through ammoniated brine to form sodium hydrogen carbonate.

CO₂ + H₂O + NH₃ + NaCl → NaHCO₃ + NH₄Cl

2 NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

The carbon dioxide that is created is recycled to make NaHCO₃.

Properties of Sodium Bicarbonate

• It is a white crystalline solid and odourless.
• It is mildly basic (alkaline) in nature.
• It is non-combustible.
• It is soluble in water.
• It decomposes on heating to form sodium carbonate, carbon dioxide, and water (it does not melt).

Uses of Sodium Bicarbonate

• Acts as an antacid, relieving stomach acidity and indigestion.
• Used in fire extinguishers, where it releases carbon dioxide gas.
• Functions in the baking industry as a raising agent, producing carbon dioxide to make dough rise.
• Serves as a cleaning agent for removing dirt and stains.
• Found in personal care products such as toothpaste and cosmetics.
• Acts as a neutralizing agent, reducing the effect of acids.