Baking Soda, Washing Soda, and Plaster of Paris for CBSE Class 10 Science

Among the salts derived from common salt and naturally occurring minerals, three compounds stand out for their widespread domestic and industrial importance: Baking Soda, Washing Soda, and Plaster of Paris (POP). Whether it is making a bakery cake light and fluffy, softening hard municipal water, or immobilizing a fractured limb with a rigid orthopedic cast, these three compounds illustrate how specific chemical modifications unlock remarkable functional properties.

In CBSE Class 10 Science, these salts are among the highest-yield topics in board examinations. Students are frequently asked to state their chemical formulas, describe preparation methods, write balanced thermal decomposition equations, and distinguish between baking soda and baking powder.

What You Will Learn

• Chemical name, formula, preparation, and uses of Baking Soda ($\text{NaHCO}_3$)
• The chemical distinction between baking soda and baking powder
• Chemical name, formula, and preparation of Washing Soda ($\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$)
• The scientific concept of Water of Crystallisation
• Plaster of Paris ($\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$): preparation from gypsum and setting mechanism
• Comprehensive board exam comparison matrix and error-prevention tips

1. Baking Soda ($ext{NaHCO}_3$)

Chemical Identity

• Chemical Name: Sodium hydrogen carbonate (or Sodium bicarbonate)
• Chemical Formula: $\text{NaHCO}_3$

Preparation (Solvay Process Principle)

Baking soda is manufactured industrially by reacting cold, concentrated brine with ammonia and carbon dioxide:

$\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \longrightarrow \text{NH}_4\text{Cl} \text{ [Ammonium chloride]} + \text{NaHCO}_3 \text{ [Sodium hydrogen carbonate]}$

Sodium hydrogen carbonate has low solubility in cold water, allowing it to precipitate out, after which it is filtered, washed, and dried.

Properties of Baking Soda:

1. Mild Non-Corrosive Base: It is a white crystalline solid that acts as a mild, non-corrosive basic salt.
2. Action of Heat: When heated during cooking, it decomposes to release carbon dioxide gas: $2\text{NaHCO}_3(s) \xrightarrow{\quad \Delta \quad} \text{Na}_2\text{CO}_3(s) \text{ [Sodium carbonate]} + \text{H}_2\text{O}(g) + \text{CO}_2(g) \uparrow$

2. Baking Soda vs. Baking Powder (CBSE High-Frequency Question)

$\text{NaHCO}_3 + \text{H}^+ \text{ (from edible acid)} \longrightarrow \text{CO}_2 \uparrow + \text{H}_2\text{O} + \text{Sodium salt of acid}$

Important: <u>The carbon dioxide gas produced during this reaction gets trapped in the wet dough. As the gas expands on heating, it causes bread and cakes to rise, making them light, soft, and spongy.</u>

Uses of Baking Soda:

1. Making baking powder for bakery products.
2. As an active ingredient in antacids to neutralize excess stomach acid.
3. In soda-acid fire extinguishers (reacts with $\text{H}_2\text{SO}_4$ to release rapid streams of $\text{CO}_2$ gas).

3. Washing Soda ($ext{Na}_2 ext{CO}_3 \cdot 10 ext{H}_2 ext{O}$)

Chemical Identity

• Chemical Name: Sodium carbonate decahydrate
• Chemical Formula: $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$

Preparation

1. Heating baking soda produces anhydrous sodium carbonate (soda ash): $2\text{NaHCO}_3 \xrightarrow{\quad \Delta \quad} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$
2. Recrystallisation from Water: Dissolving anhydrous sodium carbonate in water and recrystallising it yields washing soda: $\text{Na}_2\text{CO}_3 + 10\text{H}_2\text{O} \longrightarrow \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \text{ [Washing soda]}$

Uses of Washing Soda:

1. Used in glass, soap, and paper industries.
2. Used in the manufacture of sodium compounds such as borax.
3. Used as a cleaning agent for domestic laundry purposes.
4. Used for removing permanent hardness of water.

4. What is Water of Crystallisation?

Are the crystals of salts really dry? Many salts that appear completely dry actually contain fixed amounts of chemically bonded water.

Formal Definition

Water of crystallisation is the fixed number of water molecules chemically attached to each formula unit of a salt in its crystalline form.

Classic Examples:

1. Copper Sulphate ($\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$): Blue crystals contain $5$ water molecules. On heating, the water is driven off, turning the salt into white anhydrous $\text{CuSO}_4$: $\text{CuSO}_4 \cdot 5\text{H}_2\text{O} \text{ [Blue]} \xrightarrow{\quad \Delta \quad} \text{CuSO}_4 \text{ [White]} + 5\text{H}_2\text{O}$ Adding a few drops of water immediately restores the vibrant blue colour!
2. Ferrous Sulphate ($\text{FeSO}_4 \cdot 7\text{H}_2\text{O}$): Green crystals contain $7$ water molecules.
3. Gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$): Contains $2$ water molecules.

5. Plaster of Paris ( ext{CaSO}_4 \cdot rac{1}{2} ext{H}_2 ext{O})

Chemical Identity

• Chemical Name: Calcium sulphate hemihydrate
• Chemical Formula: $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$
  (Note: Two formula units of $\text{CaSO}_4$ share one molecule of water: $2\text{CaSO}_4 \cdot \text{H}_2\text{O}$)

Preparation from Gypsum

On heating gypsum at strictly $373\text{ K}$ ($100^\circ\text{C}$), it loses three-fourths of its water of crystallisation and forms Plaster of Paris:

$\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \text{ [Gypsum]} \xrightarrow{\quad 373\text{ K} \quad} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} \text{ [POP]} + 1\frac{1}{2}\text{H}_2\text{O}$

Important Temperature Caution: <u>The temperature must NOT exceed $373\text{ K}$. If heated above $373\text{ K}$, gypsum loses all its water of crystallisation to form anhydrous calcium sulphate ($\text{CaSO}_4$), commonly called "dead burnt plaster", which completely loses the property of setting with water!</u>

The Setting Reaction with Water

Plaster of Paris is a white powder. On mixing with water, it rehydrates and sets into a hard, solid mass of gypsum within 10 to 15 minutes:

$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \longrightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \text{ [Hard Solid Gypsum]}$

Uses of Plaster of Paris:

1. Orthopedic Casts: Doctors use POP to set fractured bones in the correct position.
2. Decorative Works: Making ornate ceiling mouldings, false ceilings, and cornices.
3. Sculpting & Toys: Making casts, moulds, statues, and toys.
4. Smooth Surfaces: Making wall surfaces smooth before painting.

6. Comprehensive Summary Table

Exam Tip: Why is Plaster of Paris written as $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$? Because half a water molecule cannot physically exist, this formula indicates that two formula units of $\text{CaSO}_4$ share one single molecule of water ($2\text{CaSO}_4 \cdot \text{H}_2\text{O}$).

Common Mistake: Storing Plaster of Paris in paper packets or damp places. POP absorbs atmospheric moisture and slowly hardens into useless gypsum. It must ALWAYS be stored in moisture-proof containers!