Chemicals from Common Salt

Sodium Hydroxide ($NaOH$) — Caustic Soda

Sodium hydroxide is also called caustic soda — 'caustic' meaning 'burning' — because it damages skin, wood, and similar materials.

Preparation — The Chlor-Alkali Process

$NaOH$ is produced by the electrolysis of an aqueous solution of common salt (called brine).

Reaction:

$$2NaCl(aq) + 2H_2O(l) \xrightarrow{\text{electricity}} 2NaOH(aq) + Cl_2(g) + H_2(g)$$

Three useful products:

• At the anode: chlorine gas ($Cl_2$)
• At the cathode: hydrogen gas ($H_2$)
• In solution near the cathode: sodium hydroxide ($NaOH$)

Uses of Each Product

$Cl_2$ (chlorine gas):

• Water purification (drinking water).
• Bleaching in the paper industry.
• Making PVC plastic.
• Producing bleaching powder ($CaOCl_2$).

$H_2$ (hydrogen gas):

• Making ammonia (Haber process).
• As a fuel.
• Hydrogenation (making vegetable ghee).

$NaOH$ (caustic soda):

• Soap and detergent industry.
• Paper manufacture.
• Making artificial fibres.
• Petroleum refining.
• Making dyes.

[Board Important] Three products of the chlor-alkali process and their uses.

Bleaching Powder ($CaOCl_2$)

Formula: $CaOCl_2$ — calcium oxychloride Common name: bleaching powder

Preparation

Method: Action of chlorine gas on dry slaked lime.

Reaction:

$$Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$$

(slaked lime) (chlorine) (bleaching powder)

Physical Properties

• Colour: pale yellow-white
• Smell: strong chlorine-like odour
• State: solid powder
• Property: decomposes when exposed to moisture and $CO_2$

Uses

1. Textile industry:

• Bleaching cotton and linen.
• Releases $Cl_2$ — destroys dyes.

2. Paper industry:

• Bleaching paper pulp.

3. Water purification:

• Killing bacteria in drinking water.
• In swimming pools.

4. Disinfectant:

• In hospitals and homes.

5. Chemical industry:

• Making chloroform ($CHCl_3$).

How Does Bleaching Work?

In contact with moisture/$CO_2$, bleaching powder releases $Cl_2$:

$$CaOCl_2 + H_2O \rightarrow Ca(OH)_2 + Cl_2$$

The free $Cl_2$ is a strong oxidising agent — it makes dyes colourless and disinfects water.

Precautions

• Store in airtight container (away from moisture and $CO_2$).
• Avoid direct skin contact (causes burns).
• If splashed in eyes, wash immediately with water.

[Board Important] Formula, preparation, and three uses of bleaching powder.

Baking Soda ($NaHCO_3$)

Formula: $NaHCO_3$ — sodium hydrogen carbonate Common name: baking soda

Preparation — Part of the Solvay Process

Produced by the action of $CO_2$ and $NH_3$ on a $NaCl$ solution.

Reaction:

$$NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3$$

(low solubility allows $NaHCO_3$ to be filtered out as a precipitate.)

Physical Properties

• Colour: white powder
• Smell: odourless
• Taste: mildly basic (bitter-salty)
• pH: ~ 8.5 (slightly basic)
• Soluble in water

Decomposition on Heating

$$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2\uparrow$$

(That is, on heating it releases $CO_2$ — the basis of most of its uses.)

Uses

1. Food industry — baking:

• For making cakes, biscuits, bread.
• On heating, $CO_2$ is released — this raises the dough.
• Food becomes light and spongy.

2. Home remedy for acidity: $$NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$$ (stomach $HCl$ is neutralised)

3. Fire extinguishers:

• $NaHCO_3$ + dilute acid → $CO_2$
• $CO_2$ smothers the fire.

4. Treatment of insect stings:

• For bee, ant stings (acidic).

5. Component of baking powder:

• Baking powder = $NaHCO_3$ + tartaric acid.
• On contact with water, $CO_2$ is released.

Baking Powder vs. Baking Soda

Feature	Baking Soda	Baking Powder
Composition	$NaHCO_3$	$NaHCO_3$ + tartaric acid
Use	Alone in some cakes/biscuits	Cakes, biscuits, bread
Action	Releases $CO_2$ on heating	Releases $CO_2$ on adding water

A Curious Fact

Why are cakes spongy? The $CO_2$ released by baking soda forms bubbles — the dough rises and becomes light.

[Repeatedly asked in boards] Formula, preparation, and four uses of baking soda.

Washing Soda ($Na_2CO_3 \cdot 10H_2O$)

Formula: $Na_2CO_3 \cdot 10H_2O$ — sodium carbonate decahydrate Common name: washing soda or soda ash

Preparation

Step 1: Heat baking soda:

$$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2$$

Step 2: Recrystallise anhydrous $Na_2CO_3$ from water:

$$Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O$$

(10 water molecules of crystallisation bind — this is washing soda.)

Physical Properties

• Colour: white, crystalline
• pH: ~ 11-12 (basic)
• Highly soluble in water
• 10 water molecules of crystallisation — $Na_2CO_3 \cdot 10H_2O$

Uses

1. In laundry:

• Being basic, removes oil and dirt.
• Softens hard water (precipitates Ca²⁺/Mg²⁺).

2. Glass industry:

• Main raw material for glass-making.
• Forms $Na_2O \cdot SiO_2$.

3. Paper industry:

• In the treatment of paper pulp.

4. Soap and detergent industry.

5. Manufacture of borax.

6. In the laboratory:

• As a reagent.

Softening Hard Water

Hard water contains $Ca^{2+}$ and $Mg^{2+}$ ions — does not lather well with soap.

Adding washing soda:

$$Na_2CO_3 + CaCl_2 \rightarrow CaCO_3\downarrow + 2NaCl$$ $$Na_2CO_3 + MgCl_2 \rightarrow MgCO_3\downarrow + 2NaCl$$

(hardness ions ($Ca^{2+}, Mg^{2+}$) precipitate — water becomes soft.)

[Board Important] Formula, preparation of washing soda, and how it softens hard water.

Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$)

Formula: $CaSO_4 \cdot \frac{1}{2}H_2O$ — calcium sulphate hemihydrate (or written as $2CaSO_4 \cdot H_2O$)

Common name: Plaster of Paris (POP) (once mined near Paris — hence the name.)

Preparation

Method: Carefully heat gypsum ($CaSO_4 \cdot 2H_2O$) at 373 K (100°C).

Reaction:

$$CaSO_4 \cdot 2H_2O \xrightarrow{373 \text{ K}} CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O$$

(gypsum) (POP)

Caution: Above 373 K, all the water leaves (giving anhydrous $CaSO_4$) — this does not behave like POP.

Physical Properties

• Colour: white
• State: solid powder
• On adding water — sets quickly

Special Property — Reaction with Water

When water is added to POP, it turns back to gypsum, but in doing so it forms a hard solid.

Reaction:

$$CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$$

(POP) (gypsum — hard)

Uses

1. Medicine:

• For making plaster casts for broken bones.
• POP + water sets into a cast — keeps the bone stable.

2. Art and sculpture:

• For making statues, toys.
• Decorative wall panels.

3. Construction industry:

• False ceilings.
• Wall decorations.
• Filling cracks.

4. Laboratory:

• For drying gases (moisture absorber).

5. Fire-resistant materials.

Storage Caution

Keep POP away from moisture in airtight containers — even small amounts of water cause it to set into hard gypsum and become useless.

A Curious Fact

POP and gypsum — both are forms of $CaSO_4$ — but differ in number of water molecules.

• Gypsum: $CaSO_4 \cdot 2H_2O$ (2 water molecules)
• POP: $CaSO_4 \cdot \frac{1}{2}H_2O$ (½ water per unit)
• Anhydrous: $CaSO_4$ (no water — does not behave like POP)

[Board Important] Formula, preparation (from gypsum), and three uses of POP.

🧠 Memory Capsule

A one-glance recap to revisit just before the board exam.

1. Five Important Chemicals — From Common Salt

Chemical	Formula	Common Name
Sodium hydroxide	$NaOH$	Caustic soda
Bleaching powder	$CaOCl_2$	—
Baking soda	$NaHCO_3$	—
Washing soda	$Na_2CO_3 \cdot 10H_2O$	Soda ash
Plaster of Paris	$CaSO_4 \cdot \frac{1}{2}H_2O$	POP

2. The Chlor-Alkali Process

$$2NaCl + 2H_2O \xrightarrow{\text{electricity}} 2NaOH + Cl_2 + H_2$$

Three products: $NaOH$, $Cl_2$, $H_2$ — all useful.

3. Bleaching Powder

• Preparation: $Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$
• Uses: bleaching (cotton, paper), water purification, disinfectant.

4. Baking Soda

• Preparation (Solvay): $NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3$
• On heating: $2NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2$
• Uses: baking, antacid, fire extinguisher.

5. Washing Soda

• Preparation: Heat baking soda → $Na_2CO_3$ + recrystallise.
• Uses: laundry, glass industry, softening hard water.

6. Plaster of Paris

• Preparation: $CaSO_4 \cdot 2H_2O \xrightarrow{373 K} CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O$
• With water — sets to gypsum: plaster casts, statues, ceilings.

7. Famous Reactions

Chemical	Reaction
Heating baking soda	$2NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2$
Baking soda + acid	$NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$
Bleaching powder + water	$CaOCl_2 + H_2O \rightarrow Ca(OH)_2 + Cl_2$
Washing soda + hard water	$Na_2CO_3 + CaCl_2 \rightarrow CaCO_3\downarrow + 2NaCl$
POP + water	$CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$

8. Board's 'Golden' Questions

1. Three products of the chlor-alkali process and their uses.
2. Preparation and three uses of bleaching powder.
3. Difference between baking soda and washing soda.
4. How is POP made? Three uses.
5. Why are cakes spongy?
6. Baking powder vs. baking soda.

The Bottom Line: From common salt, 5 vital chemicals — caustic soda, bleaching powder, baking soda, washing soda, and Plaster of Paris — all useful in everyday life.

Solved Examples

Example 1: NCERT — The Chlor-Alkali Process

What is the chlor-alkali process? Name its three products, state where they are formed, and give one use of each.

Solution:

Definition: Electrolysis of an aqueous solution of common salt (brine) — yielding three important chemicals.

Reaction:

$$2NaCl(aq) + 2H_2O(l) \xrightarrow{\text{electricity}} 2NaOH(aq) + Cl_2(g) + H_2(g)$$

Three products and their locations:

Product	Location	One use
$Cl_2$ gas	At the anode (+)	Water purification; PVC; bleaching powder
$H_2$ gas	At the cathode (−)	Ammonia synthesis; fuel; vegetable ghee
$NaOH$	In solution near the cathode	Soap industry; paper manufacture

'Chlor' = chlorine; 'alkali' = base ($NaOH$) — hence the name.

[NCERT textbook question]

Example 2: NCERT — Bleaching Powder

What is bleaching powder? State its preparation, formula, and three uses.

Solution:

Bleaching powder ($CaOCl_2$) — calcium oxychloride.

Preparation:

Action of chlorine gas on dry slaked lime ($Ca(OH)_2$).

Reaction:

$$Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$$

(slaked lime) (chlorine) (bleaching powder)

Formula: $CaOCl_2$

Three uses:

1. Bleaching in the textile industry:

• Bleaching cotton and linen white.
• $Cl_2$ release destroys dyes.

2. In the paper industry:

• Bleaching paper pulp.

3. Water purification:

• Disinfecting drinking water.
• In swimming pools.

Other uses:

• Disinfectant (in hospitals and homes).
• Manufacturing chloroform.

Caution: Store in an airtight container (away from moisture and $CO_2$).

[NCERT textbook question]

Example 3: NCERT — Baking Soda

What is baking soda? State its preparation, formula, and four uses.

Solution:

Baking soda ($NaHCO_3$) — sodium hydrogen carbonate.

Common name: baking soda.

Preparation (part of the Solvay Process):

Action of $CO_2$ and $NH_3$ on a $NaCl$ solution.

Reaction:

$$NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3$$

(low solubility allows $NaHCO_3$ to precipitate out.)

Formula: $NaHCO_3$

On heating:

$$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2$$

Four uses:

1. Food industry — baking:

• For cakes, biscuits, bread. On heating, $CO_2$ is released — the dough rises and becomes spongy.

2. Home remedy for acidity: $$NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$$ (stomach $HCl$ is neutralised)

3. In fire extinguishers:

• $NaHCO_3$ + acid → $CO_2$ — extinguishes fires.

4. Treating insect stings:

• For bee, ant stings (acidic).

[NCERT textbook question — repeatedly asked]

Example 4: NCERT — Washing Soda

What is washing soda? Give its formula, preparation, and explain how it softens hard water.

Solution:

Washing soda ($Na_2CO_3 \cdot 10H_2O$) — sodium carbonate decahydrate.

Common name: soda ash.

Formula: $Na_2CO_3 \cdot 10H_2O$ (10 molecules of crystallisation water).

Preparation (in two steps):

Step 1: Heat baking soda:

$$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2$$

Step 2: Recrystallise anhydrous $Na_2CO_3$ from water:

$$Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O$$

How does it soften hard water?

Hard water contains $Ca^{2+}$ and $Mg^{2+}$ ions — does not lather well with soap.

On adding washing soda:

$$Na_2CO_3 + CaCl_2 \rightarrow CaCO_3\downarrow + 2NaCl$$ $$Na_2CO_3 + MgCl_2 \rightarrow MgCO_3\downarrow + 2NaCl$$

Result:

• Hardness ions ($Ca^{2+}, Mg^{2+}$) precipitate as insoluble carbonates.
• Only $Na^+$ remains in the water — soft water.
• Soap now lathers well.

Other uses:

• Glass industry.
• Paper, soap, detergent.
• Borax manufacture.

[NCERT textbook question]

Example 5: NCERT — Plaster of Paris

What is Plaster of Paris? State its preparation, formula, and three uses.

Solution:

Plaster of Paris (POP) — calcium sulphate hemihydrate.

Formula: $CaSO_4 \cdot \frac{1}{2}H_2O$ (or $2CaSO_4 \cdot H_2O$).

Origin of the name: Once mined near Paris — hence 'Plaster of Paris'.

Preparation:

Carefully heat gypsum ($CaSO_4 \cdot 2H_2O$) at 373 K (100°C).

Reaction:

$$CaSO_4 \cdot 2H_2O \xrightarrow{373 K} CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O$$

Caution: Do not exceed 373 K — above this, all the water leaves (giving anhydrous $CaSO_4$), which does not behave like POP.

Special property:

When water is added, POP turns back into gypsum — and in this process forms a hard solid.

$$CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$$

(hard — useful for making casts)

Three uses:

1. Medicine — plaster casts for broken bones:

• POP + water sets into a cast.
• Keeps the bone immobile — helping it heal.

2. Art and sculpture:

• Statues, toys, models.
• Suitable for fine surfaces.

3. Construction industry:

• False ceilings.
• Wall decorations.
• Filling cracks.

Other uses:

• Drying agent in laboratories.
• In dentistry.

Caution: Store POP in airtight containers, away from moisture.

[NCERT textbook question — repeatedly asked]

Example 6: Baking Soda vs. Washing Soda

State five differences between baking soda and washing soda.

Solution:

Comparison Table:

Aspect	Baking Soda	Washing Soda
Chemical name	Sodium hydrogen carbonate	Sodium carbonate decahydrate
Formula	$NaHCO_3$	$Na_2CO_3 \cdot 10H_2O$
Crystal water	None	10 water molecules
pH	~ 8.5 (weakly basic)	~ 11-12 (more basic)
Use	In food, antacid	Laundry, glass industry
On heating	Gives $Na_2CO_3 + H_2O + CO_2$	Loses water on mild heating
Edible?	Yes (in food)	No (not caustic, but not edible)

Connection:

• Both relate to $Na_2CO_3$.
• Heating baking soda yields $Na_2CO_3$ (the precursor to washing soda).

Practical difference:

• Baking soda in the kitchen.
• Washing soda in laundry.

[Board 5-mark question]

Example 7: NCERT — Why Is a Cake Spongy?

When baking soda is used in cake-making, why does the cake become spongy? Write the chemical reaction.

Solution:

Principle: Baking soda releases $CO_2$ gas on heating — which forms bubbles in the dough, making it rise.

Reaction (on heating):

$$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2\uparrow$$

Step by step what happens:

1. While preparing cake batter — baking soda or baking powder is added.
2. In the oven, on heating — baking soda decomposes.
3. $CO_2$ bubbles form throughout the dough.
4. The dough rises, becoming light and fluffy.
5. After baking, these bubbles remain trapped — giving a spongy structure.

Action of baking powder:

• Baking powder = $NaHCO_3$ + tartaric acid.
• On adding water, tartaric acid reacts with $NaHCO_3$ — giving immediate $CO_2$.
• No need to wait for the oven — works at room temperature too.

Why is tartaric acid added?

• Without it, only $Na_2CO_3$ remains — which has a bitter taste.
• Tartaric acid + $Na_2CO_3$ = sodium tartrate (tasteless).
• So the food's taste is preserved.

[NCERT textbook question]

Example 8: A Mixed Question — Uses of the Five Chemicals

Give one main use each of the following: (a) $NaOH$ (b) $CaOCl_2$ (c) $NaHCO_3$ (d) $Na_2CO_3 \cdot 10H_2O$ (e) $CaSO_4 \cdot \frac{1}{2}H_2O$

Solution:

Chemical	Main use
(a) $NaOH$ (caustic soda)	In the soap and detergent industry
(b) $CaOCl_2$ (bleaching powder)	Bleaching cotton/paper; water purification
(c) $NaHCO_3$ (baking soda)	Raising cakes/biscuits; treating acidity
(d) $Na_2CO_3 \cdot 10H_2O$ (washing soda)	Laundry; softening hard water; glass industry
(e) $CaSO_4 \cdot \frac{1}{2}H_2O$ (POP)	Plaster casts for broken bones; sculpture; ceilings

Big picture — most are linked to common salt:

NaCl (common salt)
   |
   |--> NaOH (chlor-alkali) -----> Cl_2 ----> CaOCl_2 (bleach)
   |                                |
   |                                |
   |--> NaHCO_3 (Solvay) -------> Na_2CO_3 -----> Na_2CO_3 . 10H_2O
                                                  (washing soda)

While POP comes from gypsum (only indirectly related to common salt).

[Board 5-mark question]

Example 9: NCERT — POP and Gypsum

What is the difference between gypsum and Plaster of Paris? Why is POP stored in airtight containers?

Solution:

Comparison Table:

Aspect	Gypsum	Plaster of Paris
Chemical name	Calcium sulphate dihydrate	Calcium sulphate hemihydrate
Formula	$CaSO_4 \cdot 2H_2O$	$CaSO_4 \cdot \frac{1}{2}H_2O$
Water molecules	2	1/2
State	Natural mineral	Made from gypsum
Reaction with water	No change	Forms hard gypsum

Conversion between the two:

$$\text{Gypsum} \xrightarrow[\text{373 K}]{\Delta} \text{POP} + \frac{3}{2}H_2O$$

$$\text{POP} + \frac{3}{2}H_2O \rightarrow \text{Gypsum}$$

Why store POP in airtight containers?

Reason:

• POP absorbs moisture (hygroscopic).
• In humid air, POP picks up water and turns back into gypsum.
• Once gypsum is formed, it no longer behaves like POP.
• It becomes useless.

Reaction (in moist air):

$$CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$$

(POP) (gypsum — hardened, useless)

Precautions:

• Airtight container.
• Store in a dry place.
• Extra care during humid weather.

[NCERT textbook question]

Example 10: Acidity and Baking Soda

Why is baking soda taken to relieve acidity? Write the chemical reaction. What if you take too much baking soda?

Solution:

Treatment of acidity:

Excess $HCl$ in stomach → burning. Baking soda is basic — it neutralises the acid.

Reaction:

$$NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2\uparrow$$

Benefits:

1. Stomach pH returns to normal.
2. Burning is reduced.
3. Relief is felt.

If too much baking soda?

Immediate problem — burping:

• A lot of $CO_2$ is formed.
• Released as burps.
• Sometimes stomach burning.

Long-term consequences:

• Alkalosis — blood pH rises.
• High blood pressure — high sodium intake.
• Kidney problems.
• Mineral imbalance.

Better alternatives:

1. Milk of magnesia ($Mg(OH)_2$):

• Basic but sodium-free.
• No effect on blood pressure.

2. Calcium carbonate tablets ($CaCO_3$):

• Stable, safe.

3. For long-term issues — see a doctor.

Precaution:

• Do not take baking soda daily.
• Not more than 1-2 teaspoons.

[Board + safety-related question]

Example 11: NCERT — Why Is It Called 'Caustic' Soda?

Why is sodium hydroxide called 'caustic soda'?

Solution:

Meaning of 'caustic': burning, damaging — from the Latin causticus.

Why is $NaOH$ caustic?

1. Effect on skin:

• $NaOH$ is a very strong base.
• Direct skin contact — severe burning.
• Type of burn: a chemical burn.
• It denatures the skin's proteins.

2. Effect on cellulose:

• Paper, wood, cotton — all organic.
• $NaOH$ breaks them down.
• That's why it's used in the paper/textile industry.

3. Effect on fats/oils:

• It saponifies fats.
• Hence its use in soap-making.

Precautions:

• Keep away from skin.
• Wear goggles and gloves.
• In an accident, immediately wash with plenty of water.
• Work under professional supervision.

Applications where its causticity is useful:

1. Soap industry: Fat + $NaOH$ → soap + glycerol (saponification)

2. Paper manufacture: Making paper pulp from wood.

3. Petroleum refining.

4. Drain cleaning:

• Dissolves hair and grease in drains.
• Caution: dangerous for home use.

Lesson: The name 'caustic' captures its strongly basic, corrosive nature.

Example 12: NCERT — Hard Water and Soda

What is hard water? How does washing soda soften it? Write the chemical reactions.

Solution:

Hard water:

Water containing higher amounts of $Ca^{2+}$ and $Mg^{2+}$ ions — which does not lather well with soap.

Problems caused by hard water:

1. Less lather with soap — more soap is needed.
2. A 'rough' precipitate (scum) forms — leaves marks on cloth.
3. Deposits in pipes — eventual blockage.
4. 'Scale' in boilers — reduced efficiency.

How does washing soda work?

Principle: Washing soda ($Na_2CO_3$) precipitates the hardness ions ($Ca^{2+}, Mg^{2+}$) as insoluble carbonates.

Reactions:

(1) For $Ca^{2+}$ hardness: $$Na_2CO_3 + CaCl_2 \rightarrow CaCO_3\downarrow + 2NaCl$$

(2) For $Mg^{2+}$ hardness: $$Na_2CO_3 + MgCl_2 \rightarrow MgCO_3\downarrow + 2NaCl$$

(3) For $Ca(HCO_3)_2$ hardness: $$Na_2CO_3 + Ca(HCO_3)_2 \rightarrow CaCO_3\downarrow + 2NaHCO_3$$

Result:

• $Ca^{2+}, Mg^{2+}$ ions are removed as insoluble carbonates.
• Only $Na^+$ stays in the water — soft water.
• Soap now lathers well.

Other methods to soften water:

• Ion-exchange resins.
• Distillation.

[Board 5-mark question]

Example 13: Numerical — POP and Gypsum

How much POP is produced from 100 g of gypsum? (Ca=40, S=32, O=16, H=1)

Solution:

1. Reaction: $$CaSO_4 \cdot 2H_2O \xrightarrow{373 K} CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O$$

2. Molecular masses:

• Gypsum $CaSO_4 \cdot 2H_2O$ = $40 + 32 + 64 + 36 = 172$ g/mol
• POP $CaSO_4 \cdot \frac{1}{2}H_2O$ = $40 + 32 + 64 + 9 = 145$ g/mol

1. Mole ratio: 1 mol gypsum → 1 mol POP

2. Mass ratio: 172 g → 145 g

3. From 100 g of gypsum: $$\text{POP} = \frac{145}{172} \times 100 = 84.3 \text{ g}$$

Answer: 84.3 g of POP is produced.

Reason for the difference: The remaining mass ($100 - 84.3 = 15.7$ g) leaves as water.

Verification:

• Water lost: $172 - 145 = 27$ g per mole of gypsum.
• That is, $\frac{27}{172} \times 100 = 15.7$ g per 100 g gypsum. ✓

Special note: Heating gypsum at exactly 373 K is essential — too high a temperature drives off all the water.

Example 14: NCERT — Action of Bleaching Powder

How does bleaching powder bleach? Write the chemical reaction.

Solution:

Bleaching: the process of making coloured substances colourless.

Action of bleaching powder $CaOCl_2$:

Step 1: In contact with water or moisture, bleaching powder releases free $Cl_2$:

$$CaOCl_2 + H_2O \rightarrow Ca(OH)_2 + Cl_2$$

Step 2: Free $Cl_2$ reacts with water to form hypochlorous acid ($HOCl$):

$$Cl_2 + H_2O \rightarrow HOCl + HCl$$

Step 3: Decomposition of $HOCl$ — releasing nascent oxygen ($[O]$):

$$HOCl \rightarrow HCl + [O]$$

Step 4: Nascent oxygen is a strong oxidising agent. It oxidises dye molecules into colourless products.

Simplified equation:

$$\text{Coloured dye} + [O] \rightarrow \text{Colourless product}$$

Applications of Bleaching

1. Textile industry:

• Cotton, linen — making cloth white.

2. Paper industry:

• Whitening paper pulp.

3. Pulp to white paper.

Bleaching Powder vs. Direct Chlorine

Using chlorine gas directly is dangerous. Bleaching powder acts as a 'storage' form — releasing $Cl_2$ as needed.

[Board 3-5 marks]

Example 15: Safety Question — Caustic Soda

What precautions must be taken while working with caustic soda ($NaOH$) in the laboratory?

Solution:

The danger of caustic soda:

• Very strong base (pH ~ 14).
• 'Burns' the skin (chemical burn).
• Risk of blindness if it gets into the eyes.
• Destroys clothes.
• Dissolves paper and wood.

Necessary precautions:

1. Personal Protective Equipment (PPE):

• Safety goggles — mandatory.
• Rubber gloves.
• Long-sleeved lab coat.
• Closed shoes.

2. While working:

• Dilution with cold water: always add caustic soda to water (never water to it).
• Add slowly.
• Stir continuously.
• Heat is released — keep the beaker cool.

3. Storage:

• Airtight container.
• Away from moisture.
• Away from acids.
• Out of children's reach.

4. Emergencies:

• On skin: immediately rinse with plenty of water for 15 minutes.
• In eyes: rinse the eye with water; see a doctor.
• If swallowed: see a doctor immediately — give milk or water.
• Never induce vomiting.

5. Waste disposal:

• Do not pour directly into drains.
• Dilute and neutralise (with vinegar) before disposal.

6. Heed the labels:

• 'Danger — Corrosive' label.
• Read the Safety Data Sheet (SDS).

[Board + lab safety]

Example 16: Closing Question — The Common Salt Family

How many important chemicals are derived from common salt? Give their formulae, common names, and one use each.

Solution:

From common salt directly or indirectly, several chemicals are made — at the Class 10 level, mainly four:

#	Chemical	Formula	Common Name	One use
1	Sodium hydroxide	$NaOH$	Caustic soda	Soap industry
2	Sodium hydrogen carbonate	$NaHCO_3$	Baking soda	In cakes/biscuits
3	Sodium carbonate decahydrate	$Na_2CO_3 \cdot 10H_2O$	Washing soda	Laundry
4	Bleaching powder	$CaOCl_2$	—	Water purification

Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$) — not from common salt — but from gypsum.

Connection diagram:

         NaCl
           |
  +-----------------+
  |                 |
  |                 |
NaOH (electricity)  NaHCO_3 (Solvay)
  |                 |
  | + Cl_2          |
  |                 |
Cl_2              Na_2CO_3 (on heating)
  |                 |
  | + Ca(OH)_2      |
  |                 |
CaOCl_2          Na_2CO_3 . 10H_2O

Key differences relevant to boards:

• NaOH: strong base — soap, petroleum.
• NaHCO₃: weak base — in food, antacid.
• Na₂CO₃ . 10H₂O: moderate base — in laundry.
• CaOCl₂: oxidising agent — bleaching, water purification.
• POP: sets hard with water — bone casts, sculpture.

[Board 5-mark multi-topic question]