Baking Soda, Washing Soda, Plaster of Paris

"Baking soda," "Washing soda," and "Plaster of Paris" are common household chemicals with significant industrial and educational importance. In JEE Chemistry, understanding the properties, preparation, uses, and chemical differences of substances is vital. This guide covers all aspects relevant to competitive exams and practical applications.

1.0Baking Soda

Baking soda is a white crystalline compound with a slightly salty taste. Its chemical formula is NaHCO3 and is generally known as sodium bicarbonate.

Preparation

Baking soda is primarily prepared by the Solvay process, which also produces washing soda.

Solvay Process Steps:

1. Ammonia reacts with water and carbon dioxide to form ammonium carbonate.
2. Ammonium carbonate reacts with carbon dioxide and water to form ammonium hydrogen carbonate.
3. Ammonium hydrogen carbonate reacts with sodium chloride to produce sodium bicarbonate (baking soda) and ammonium chloride:

NH₃ + H₂O + CO₂ → NH₄HCO₃
NH₄HCO₃ + NaCl → NaHCO₃↓ + NH₄Cl

The sodium bicarbonate (NaHCO₃) is filtered, washed, and dried.

Properties

• Physical: White crystalline powder, sparingly soluble in water.
• Chemical:
• Mildly alkaline.
• Decomposes on heating to form sodium carbonate, water, and carbon dioxide gas:
  2 NaHCO₃ (s) → Na₂CO₃ (s) + CO₂ (g) + H₂O (g)
• Reacts with acids to produce carbon dioxide:
  NaHCO₃ + HCl → NaCl + CO₂ + H₂O

Uses

• Baking powder (a leavening agent in cooking and baking).
• Antacid (to treat acid indigestion).
• Fire extinguisher (for small fires).
• Cleaning agent (due to mild abrasiveness and alkalinity).
• Used in making effervescent drinks.

2.0Washing Soda

Washing soda is a whitish, odourless powder. Its chemical formula is Na2CO3.10H2Oand is better known as sodium carbonate decahydrate. It is hygroscopic in nature, that is, it absorbs moisture from the air. It is highly soluble in water and forms a strongly alkaline water solution.

Preparation

Washing soda is obtained from sodium bicarbonate (baking soda):

1. Thermal decomposition of sodium bicarbonate gives sodium carbonate:
   2 NaHCO₃ (s) → Na₂CO₃ (s) + CO₂ (g) + H₂O (g)
2. Sodium carbonate is then dissolved in water and recrystallized to form washing soda:
   Na₂CO₃ (s) + 10 H₂O (l) → Na₂CO₃·10H₂O (s)

Properties

• Physical: White crystalline solid, soluble in water, efflorescent (loses water of crystallization on exposure to air).
• Chemical:
• Strongly alkaline in solution.
• Reacts with acids to release carbon dioxide.
• Used to remove permanent hardness of water.

Uses

• Used as a cleansing agent for domestic and industrial purposes.
• Water softening (removes Ca²⁺ and Mg²⁺ ions).
• Used in the glass, soap, and paper industries.
• In the manufacture of sodium compounds, such as borax.

3.0Plaster of Paris

Chemical Nature and Formula

Plaster of Paris is a chemical compound consisting of fine white powder, which hardens when exposed to moisture and allowed to dry. The chemical formula of Plaster of Paris is CaSO4 · 1/2H2O  and is better known as calcium sulphate hemihydrate.

Preparation

Plaster of Paris is obtained by heating gypsum (calcium sulfate dihydrate) at about 373 K (100°C):

CaSO₄·2H₂O (s) → CaSO₄·½H₂O (s) + 1½ H₂O (g)

Properties

• Physical: White powder that sets into a hard mass upon addition of water.
• Chemical:
• Reacts with water to reform gypsum (sets into a hard solid):
  CaSO₄·½H₂O + 1½ H₂O → CaSO₄·2H₂O
• Exothermic reaction (gives off heat during setting).

Uses

• Making casts for broken bones in orthopedics.
• Molds and sculptures in art and architecture.
• False ceilings and decorative panels.
• Used in dentistry.

4.0Differences Between Baking Soda, Washing Soda, and Plaster of Paris

5.0Important Reactions and Equations

Baking Soda

• Thermal decomposition:
  
  2 NaHCO₃ → Na₂CO₃ + CO₂ + H₂O
• Acid reaction:
  
  NaHCO₃ + HCl → NaCl + CO₂ + H₂O

Washing Soda

• Formation from soda ash:
  
  Na₂CO₃ + 10 H₂O → Na₂CO₃·10H₂O
• Water softening:
  2 N
  Na₂CO₃ + CaCl₂ → CaCO₃↓ +aCl

Plaster of Paris

• Preparation:
  CaSO₄·2H₂O → CaSO₄·½H₂O + 1½ H₂O
• Setting reaction:
  CaSO₄·½H₂O + 1½ H₂O → CaSO₄·2H₂O