Salts — Definition, Family, and pH

What Is a Salt?

We have seen many reactions producing salts — particularly in neutralisation. Now let's study salts in depth.

Definition: A compound formed by the neutralisation reaction between an acid and a base — in which the cation comes from the base and the anion from the acid — is called a salt.

General Form

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

Example:

$$HCl + NaOH \rightarrow NaCl + H_2O$$

Here — $NaCl$ is the salt.

• $Na^+$ (cation) — from the base $NaOH$
• $Cl^-$ (anion) — from the acid $HCl$

Some Common Examples of Salts

Salt	Formula	From which acid	From which base
Sodium chloride	$NaCl$	$HCl$	$NaOH$
Potassium sulphate	$K_2SO_4$	$H_2SO_4$	$KOH$
Calcium chloride	$CaCl_2$	$HCl$	$Ca(OH)_2$
Ammonium chloride	$NH_4Cl$	$HCl$	$NH_4OH$
Sodium nitrate	$NaNO_3$	$HNO_3$	$NaOH$
Copper sulphate	$CuSO_4$	$H_2SO_4$	$Cu(OH)_2$

General Properties of Salts

1. Solid state (mostly, at normal temperatures).
2. Crystalline — ordered structure.
3. High melting point — being ionic compounds.
4. Conduct electricity — when molten or in aqueous solution.
5. Soluble in water (most salts).

Salts in Daily Life

• Common salt — $NaCl$ — in the kitchen
• Washing soda — $Na_2CO_3 \cdot 10H_2O$
• Baking soda — $NaHCO_3$
• Plaster of Paris — $CaSO_4 \cdot \frac{1}{2} H_2O$
• Bleaching powder — $CaOCl_2$

(Detailed study of these in Section 8.)

[Board Important] Definition of salt and 5 examples — a 2-3 mark question.

Family of Salts

Salts that share a common acid or base are members of the same family.

Chloride Family

All salts containing $Cl^-$ — they all come from $HCl$.

Members:

• $NaCl$ — sodium chloride
• $KCl$ — potassium chloride
• $CaCl_2$ — calcium chloride
• $MgCl_2$ — magnesium chloride
• $NH_4Cl$ — ammonium chloride

Sulphate Family

All salts containing $SO_4^{2-}$ — they all come from $H_2SO_4$.

Members:

• $Na_2SO_4$ — sodium sulphate
• $K_2SO_4$ — potassium sulphate
• $CaSO_4$ — calcium sulphate (gypsum)
• $CuSO_4$ — copper sulphate
• $FeSO_4$ — ferrous sulphate

Sodium Family

All salts containing $Na^+$ — they all come from $NaOH$.

Members:

• $NaCl$ — sodium chloride
• $Na_2SO_4$ — sodium sulphate
• $NaNO_3$ — sodium nitrate
• $Na_2CO_3$ — sodium carbonate
• $NaHCO_3$ — sodium bicarbonate

Potassium Family

All salts containing $K^+$ — all from $KOH$.

Members:

• $KCl$, $K_2SO_4$, $KNO_3$, $K_2CO_3$, $KHCO_3$

A Useful Cation-Anion Matrix

Many salts arise from the matrix of cations and anions:

	$Cl^-$	$SO_4^{2-}$	$NO_3^-$
$Na^+$	$NaCl$	$Na_2SO_4$	$NaNO_3$
$K^+$	$KCl$	$K_2SO_4$	$KNO_3$
$Ca^{2+}$	$CaCl_2$	$CaSO_4$	$Ca(NO_3)_2$
$NH_4^+$	$NH_4Cl$	$(NH_4)_2SO_4$	$NH_4NO_3$

[Board Tip] Name five salts of the chloride family.

pH of Salts

Are all salts neutral? No!

The pH of a salt depends on the strengths of its parent acid and base.

Four Types of Salts

1. Strong Acid + Strong Base → Neutral Salt (pH = 7)

Example: $NaCl$

• Acid: $HCl$ (strong)
• Base: $NaOH$ (strong)
• pH: 7 (neutral)

More examples: $KCl, K_2SO_4, NaNO_3, Na_2SO_4$.

2. Strong Acid + Weak Base → Acidic Salt (pH < 7)

Example: $NH_4Cl$

• Acid: $HCl$ (strong)
• Base: $NH_4OH$ (weak)
• pH: < 7 (acidic, ~ 5)

More examples: $(NH_4)_2SO_4, FeCl_3, CuSO_4, ZnCl_2$.

3. Weak Acid + Strong Base → Basic Salt (pH > 7)

Example: $CH_3COONa$ (sodium acetate)

• Acid: $CH_3COOH$ (weak)
• Base: $NaOH$ (strong)
• pH: > 7 (basic, ~ 9)

More examples: $Na_2CO_3, NaHCO_3, K_2CO_3$.

4. Weak Acid + Weak Base → Mixed Salt

Example: $CH_3COONH_4$ (ammonium acetate)

• Both parents are weak
• pH: depends — usually near 7

Why Does This Happen?

Hydrolysis of the salt: When salts dissolve in water, their ions can react with water.

Example — $Na_2CO_3$:

$$Na_2CO_3 + H_2O \rightleftharpoons 2Na^+ + 2OH^- + H_2CO_3$$

Here — the carbonate ion takes $H^+$ from water, increasing $OH^-$ in the solution — making it basic.

Table — Salts and Their pH

Salt	Parent Acid	Parent Base	pH	Nature
$NaCl$	Strong ($HCl$)	Strong ($NaOH$)	7	Neutral
$NH_4Cl$	Strong	Weak	< 7	Acidic
$CH_3COONa$	Weak	Strong	> 7	Basic
$Na_2CO_3$	Weak ($H_2CO_3$)	Strong ($NaOH$)	> 7	Basic
$K_2SO_4$	Strong	Strong	7	Neutral
$CuSO_4$	Strong	Weak	< 7	Acidic

[Board Important] Classifying salts by pH — asked every year.

Common Salt — Sodium Chloride ($NaCl$)

This is the most common and important salt.

Preparation

$$NaOH + HCl \rightarrow NaCl + H_2O$$

(Strong acid + strong base → neutral salt; pH = 7)

Sources

1. Sea water: The largest source. Sea water is evaporated to give sea salt.

2. Underground: Rock salt — crystals of ancient dried-up seas. In India: Rajasthan and Himachal.

3. Salt lakes: Sambhar Lake (Rajasthan) — a famous source.

Physical Properties

• Colour: white (pure form), slightly brown (with impurities).
• Taste: salty.
• Crystalline.
• Melting point: 801°C.
• Highly soluble in water.

Daily-Life Uses

1. Food: for taste and seasoning.

2. Food preservation: pickles, fish, meat — salt controls bacteria.

3. Raw material for the chemical industry:

• Making $NaOH$ (chlor-alkali process).
• Making $Na_2CO_3$ (washing soda).
• Making $NaHCO_3$ (baking soda).
• Producing $Cl_2$ gas.

4. In the soap industry.

5. De-icing: spread on roads in winter to melt ice (lowers the freezing point).

6. In the body: $NaCl$ balance is essential (electrolyte).

Salt Production in India

• Gujarat — the largest producer (~ 75% of the country).
• Tamil Nadu, Rajasthan, Odisha, Andhra Pradesh — other producing states.

During India's independence movement, the 'Salt Satyagraha' (1930) made salt historically important.

[Board Important] Preparation, sources, and uses of sodium chloride.

Are Salt Crystals Really Dry?

A curious question that NCERT raises.

Water Inside Crystalline Salts

Many salt crystals contain chemically bound water — called the water of crystallisation.

Activity — Heating Copper Sulphate

Take some blue $CuSO_4$ crystals in a test tube. Heat.

Observations:

1. Blue crystals turn white.
2. Water droplets appear on the inner walls of the tube.

Reaction:

$$CuSO_4 \cdot 5H_2O \xrightarrow{\Delta} CuSO_4 + 5H_2O$$

(blue) (white)

On cooling and adding water to the white powder — it turns blue again. This is the famous test for water.

Some Famous Hydrated Salts

Salt	Formula	Water molecules	Colour
Copper sulphate	$CuSO_4 \cdot 5H_2O$	5	Blue
Sodium carbonate	$Na_2CO_3 \cdot 10H_2O$	10	White
Gypsum	$CaSO_4 \cdot 2H_2O$	2	White
Ferrous sulphate	$FeSO_4 \cdot 7H_2O$	7	Green
Alum	$K_2SO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O$	24	White

Importance of Water of Crystallisation

1. Colour: the colour of most hydrated salts is due to water. On dehydration, the colour changes.
2. Crystal structure: water molecules play a key role in the geometry.
3. Industrial use: plaster of Paris from gypsum (Section 8).
4. Detection of water: anhydrous $CuSO_4$ (white) detects moisture.

A Curious Naming Convention

The number of water molecules can even be a fraction:

• Plaster of Paris: $CaSO_4 \cdot \frac{1}{2} H_2O$ — half a water molecule!
• (How can it be half a water molecule?) — one water per two formula units. So half per formula unit.

[Repeatedly asked in boards] What is the water of crystallisation? Three examples.

🧠 Memory Capsule

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

1. Definition of Salt

A compound formed by the neutralisation of an acid with a base.

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

2. Families of Salts

• Chloride family: all from $HCl$ ($NaCl, KCl, CaCl_2, ...$)
• Sulphate family: all from $H_2SO_4$ ($Na_2SO_4, CuSO_4, ...$)
• Sodium family: all from $NaOH$ ($NaCl, Na_2SO_4, Na_2CO_3, ...$)
• Potassium family: all from $KOH$ ($KCl, K_2SO_4, ...$)

3. Four pH Types of Salts

Parent	Salt's nature	Example
Strong acid + strong base	Neutral (pH = 7)	$NaCl, K_2SO_4$
Strong acid + weak base	Acidic (pH < 7)	$NH_4Cl, CuSO_4$
Weak acid + strong base	Basic (pH > 7)	$Na_2CO_3, CH_3COONa$
Weak + weak	Mixed	$CH_3COONH_4$

4. Common Salt ($NaCl$) — Essential Facts

• Preparation: $NaOH + HCl \rightarrow NaCl + H_2O$
• Sources: sea water, rock salt, salt lakes.
• Uses: food, preservation, raw material for the chemical industry.
• Industry: for making $NaOH, Na_2CO_3, NaHCO_3, Cl_2$.

5. Water of Crystallisation

Chemically bound water within a salt's crystal.

Must-memorise examples:

• $CuSO_4 \cdot 5H_2O$ (blue vitriol)
• $Na_2CO_3 \cdot 10H_2O$ (washing soda)
• $CaSO_4 \cdot 2H_2O$ (gypsum)
• $CaSO_4 \cdot \frac{1}{2} H_2O$ (plaster of Paris)
• $FeSO_4 \cdot 7H_2O$ (green vitriol)

6. Board's 'Golden' Questions

1. What is a salt? Five examples.
2. Three members each of the chloride/sulphate/sodium family.
3. Four types of salt pH and examples.
4. What is the water of crystallisation?
5. Why does copper sulphate change colour on heating?
6. Preparation, sources, and uses of $NaCl$.

The Bottom Line: Acid + base = salt + water. The pH of a salt depends on the strengths of its parents.

Solved Examples

Example 1: Definition of Salt

What is a salt? Give five examples and their parent acids and bases.

Solution:

Definition: An ionic compound formed by the neutralisation of an acid with a base — in which the cation comes from the base and the anion from the acid.

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

Five examples:

#	Salt	Formula	Parent Acid	Parent Base
1	Sodium chloride	$NaCl$	$HCl$	$NaOH$
2	Potassium sulphate	$K_2SO_4$	$H_2SO_4$	$KOH$
3	Calcium chloride	$CaCl_2$	$HCl$	$Ca(OH)_2$
4	Sodium nitrate	$NaNO_3$	$HNO_3$	$NaOH$
5	Ammonium chloride	$NH_4Cl$	$HCl$	$NH_4OH$

Key properties:

• Solid, crystalline.
• Ionic compounds.
• Soluble in water.
• Conduct electricity (in solution or molten).

[Board 3-mark question]

Example 2: Families of Salts

Write five members of the chloride family. Write five members of the sodium family.

Solution:

Chloride family (all contain $Cl^-$ — all from $HCl$):

1. Sodium chloride — $NaCl$
2. Potassium chloride — $KCl$
3. Calcium chloride — $CaCl_2$
4. Magnesium chloride — $MgCl_2$
5. Ammonium chloride — $NH_4Cl$

Sodium family (all contain $Na^+$ — all from $NaOH$):

1. Sodium chloride — $NaCl$
2. Sodium sulphate — $Na_2SO_4$
3. Sodium nitrate — $NaNO_3$
4. Sodium carbonate — $Na_2CO_3$
5. Sodium bicarbonate — $NaHCO_3$

Special note: Sodium chloride ($NaCl$) belongs to both families (it has both sodium and chloride).

[Board 2-3 marks]

Example 3: NCERT — Preparation of Sodium Chloride

How is sodium chloride formed? State its sources and three uses.

Solution:

Preparation:

$$NaOH + HCl \rightarrow NaCl + H_2O$$

This is a neutralisation reaction — strong acid + strong base.

Salt's nature: neutral (pH = 7).

Sources:

1. Sea water — the largest source.

• Sea water is held in large open basins for sun-evaporation.
• The salt left behind is collected.

1. Rock salt — found underground.

• Crystals of ancient dried-up seas.
• Mined out.
• In India: Mandi district of Himachal Pradesh.

1. Salt lakes — like the Sambhar Lake (Rajasthan).

Three uses:

1. Food industry: to add salty flavour, food preservation.
2. Raw material for the chemical industry: in making $NaOH$, $Na_2CO_3$, $NaHCO_3$, $Cl_2$.
3. Daily use: in soap-making, melting ice on winter roads.

[NCERT textbook question]

Example 4: NCERT — pH of Salts

State the pH (acidic / basic / neutral) of these salts — $NaCl, NH_4Cl, CH_3COONa, KCl, Na_2CO_3, K_2SO_4$.

Solution:

Principle: A salt's pH depends on the strengths of its parent acid and base.

Salt	Parent Acid	Parent Base	pH	Nature
$NaCl$	$HCl$ (strong)	$NaOH$ (strong)	7	Neutral
$NH_4Cl$	$HCl$ (strong)	$NH_4OH$ (weak)	< 7	Acidic
$CH_3COONa$	$CH_3COOH$ (weak)	$NaOH$ (strong)	> 7	Basic
$KCl$	$HCl$ (strong)	$KOH$ (strong)	7	Neutral
$Na_2CO_3$	$H_2CO_3$ (weak)	$NaOH$ (strong)	> 7	Basic
$K_2SO_4$	$H_2SO_4$ (strong)	$KOH$ (strong)	7	Neutral

Quick rules to remember:

• Both strong → neutral
• One strong, one weak → the nature of the weaker partner dominates
• Strong acid + weak base = acidic
• Weak acid + strong base = basic

[NCERT textbook question — repeatedly asked]

Example 5: NCERT — Copper Sulphate Experiment

What are the observations when blue copper sulphate crystals are heated? Write the reaction. What is the explanation?

Solution:

Observations:

1. The blue crystals turn white.
2. Water droplets appear on the walls of the test tube.
3. On cooling and adding water — the colour returns to blue.

Reaction:

$$CuSO_4 \cdot 5H_2O \xrightarrow{\Delta} CuSO_4 + 5H_2O$$

(blue) (white — anhydrous)

Explanation:

Blue colour — due to water molecules:

• Copper sulphate crystals contain 5 water molecules chemically bound — this is the water of crystallisation.
• This water gives copper its characteristic blue colour.

On heating:

• The heat drives off the water molecules (vaporisation).
• Anhydrous $CuSO_4$ — white.

On cooling and adding water:

• Anhydrous $CuSO_4$ + water → hydrated $CuSO_4 \cdot 5H_2O$
• The colour returns to blue.

Practical use:

• Detecting the presence of water: anhydrous $CuSO_4$ is white, but turns blue with even a single drop of water. A very sensitive test.

This is a famous example of water of crystallisation.

[NCERT textbook question — important]

Example 6: Water of Crystallisation — Five Examples

What is the water of crystallisation? Give five examples.

Solution:

Definition: A definite number of water molecules that are chemically bound in the crystal of a salt — known as the water of crystallisation.

This water is not just 'trapped' in the crystal — it is an essential part of the crystal structure.

Five examples:

#	Salt	Formula	Water molecules	Colour
1	Copper sulphate (blue vitriol)	$CuSO_4 \cdot 5H_2O$	5	Blue
2	Washing soda	$Na_2CO_3 \cdot 10H_2O$	10	White
3	Gypsum	$CaSO_4 \cdot 2H_2O$	2	White
4	Ferrous sulphate (green vitriol)	$FeSO_4 \cdot 7H_2O$	7	Green
5	Plaster of Paris	$CaSO_4 \cdot \frac{1}{2} H_2O$	1/2	White

Special note — Plaster of Paris:

• '$\frac{1}{2} H_2O$' means — one water per two $CaSO_4$ units.
• It can also be written as $2CaSO_4 \cdot H_2O$.
• Made by heating gypsum: $CaSO_4 \cdot 2H_2O \xrightarrow{\Delta} CaSO_4 \cdot \frac{1}{2} H_2O + \frac{3}{2} H_2O$

(detailed in Section 8)

[Board 3-5 marks]

Example 7: Why is the pH of $Na_2CO_3$ Basic?

Why is an aqueous solution of sodium carbonate basic? How would you prove it?

Solution:

Analysis:

Parents of $Na_2CO_3$:

• Parent acid: carbonic acid ($H_2CO_3$) — a weak acid (very partial ionisation).
• Parent base: sodium hydroxide ($NaOH$) — a strong base.

Rule: Weak acid + strong base → basic salt

Therefore, the aqueous solution of $Na_2CO_3$ is basic.

Hydrolysis in water:

When $Na_2CO_3$ dissolves:

$$Na_2CO_3 + H_2O \rightarrow 2Na^+ + CO_3^{2-}$$

Then $CO_3^{2-}$ takes $H^+$ from water:

$$CO_3^{2-} + H_2O \rightleftharpoons HCO_3^- + OH^-$$

(the weak acid's ion grabs $H^+$ from water, leaving more $OH^-$)

Result: excess $OH^-$ in solution → basic.

How to prove?

Using indicators:

1. Take an aqueous solution of $Na_2CO_3$.
2. Dip red litmus — turns blue ✓ (proof of base).
3. Add phenolphthalein — turns pink ✓.
4. With a pH meter — gives pH ~ 11.

Practical use:

• Because $Na_2CO_3$ is basic, it is good for washing clothes (removes grease).
• Hence the name 'washing soda'.

[Board 5-mark question]

Example 8: Why is the pH of $NH_4Cl$ Acidic?

Why is an aqueous solution of ammonium chloride acidic?

Solution:

Analysis:

Parents of $NH_4Cl$:

• Parent acid: $HCl$ — a strong acid.
• Parent base: $NH_4OH$ — a weak base.

Rule: Strong acid + weak base → acidic salt

So the solution of $NH_4Cl$ is acidic.

Hydrolysis in water:

$$NH_4Cl + H_2O \rightarrow NH_4^+ + Cl^-$$

Then $NH_4^+$ reacts with water:

$$NH_4^+ + H_2O \rightleftharpoons NH_4OH + H^+$$

(the weak base's cation gives $H^+$ to water — making the solution acidic)

Result: excess $H^+$ in solution → acidic.

To prove:

• Blue litmus → turns red.
• pH < 7 (about 5).

Practical use:

• $NH_4Cl$ is used in dry cell batteries.
• An acidic medium is required for current flow.

[Board 3-mark question]

Example 9: Group Classification

In the following salts, which are (a) neutral, (b) acidic, (c) basic? — $NaCl, NH_4NO_3, CH_3COOK, K_2SO_4, Na_2CO_3$

Solution:

Analysis of each:

$NaCl$: $HCl$ (strong) + $NaOH$ (strong) → neutral ✓

$NH_4NO_3$: $HNO_3$ (strong) + $NH_4OH$ (weak) → acidic ✗

$CH_3COOK$ (potassium acetate): $CH_3COOH$ (weak) + $KOH$ (strong) → basic ✓

$K_2SO_4$: $H_2SO_4$ (strong) + $KOH$ (strong) → neutral ✓

$Na_2CO_3$: $H_2CO_3$ (weak) + $NaOH$ (strong) → basic ✓

Final classification:

Nature	Salt
Neutral (pH = 7)	$NaCl$, $K_2SO_4$
Acidic (pH < 7)	$NH_4NO_3$
Basic (pH > 7)	$CH_3COOK$, $Na_2CO_3$

Lesson: The pH is decided by the strengths of the parent acid and base.

[Board 5-mark — mixed question]

Example 10: NCERT — Sambhar Lake

What are the main sources of salt production in India? What is the importance of Sambhar Lake?

Solution:

Main sources of salt production in India:

1. Sea water (largest):

• Gujarat — ~ 75% of national output.
• Tamil Nadu, Odisha, Andhra Pradesh — other coastal states.
• Method: sea water held in vast basins, sun-evaporated.

2. Salt lakes:

• Sambhar Lake (Rajasthan) — the most famous.
• Didwana Lake (Rajasthan).

3. Rock salt:

• Mandi district of Himachal Pradesh.
• Khewra mine in Pakistan (world-famous).

Importance of Sambhar Lake:

1. Geographic:

• Near Jaipur in Rajasthan.
• India's largest inland salt lake.
• Area: ~ 230 sq km.

2. Salt production:

• Country's second-largest inland salt producer.
• Method:
• Lake fills during rains.
• Sun's heat evaporates water.
• Salt crystals form.
• These are collected and refined.

3. Ecological:

• Flamingos visit the lake.
• 'Ramsar Site' — a protected international wetland.

4. Historical:

• Salt production since the 7th century.
• Important during India's independence struggle.

[Board + India-focused question]

Example 11: Using Anhydrous $CuSO_4$ for Water Detection

A student needs to test whether a white substance contains water. How can they do it using copper sulphate?

Solution:

Method: anhydrous copper sulphate test.

Step by step:

Step 1 — Make anhydrous $CuSO_4$:

• Heat blue $CuSO_4 \cdot 5H_2O$.
• It turns white (the five water molecules leave).

$$CuSO_4 \cdot 5H_2O \xrightarrow{\Delta} CuSO_4 + 5H_2O$$

Step 2 — Test:

• Place a few drops of the test substance on some white $CuSO_4$.
• If water is present:
• White $CuSO_4$ → turns blue.
• $CuSO_4$ + $H_2O$ → $CuSO_4 \cdot 5H_2O$
• If no water: no change.

Practical examples:

• Older method for making dry alcohol.
• Detecting moisture in foodstuffs.
• A simple lab test for dryness.

Why does it work?

• Anhydrous $CuSO_4$ has a strong tendency to absorb water.
• Even a single drop is taken up.
• The colour change is immediate — the indicator.

How sensitive is this test?

• Very — it can detect a few microlitres of water.
• The most common water test at the Class 10 level.

[Board daily-life question]

Example 12: A Curious Classification Question

For the following salts, identify the parent acid and base: (a) $Mg(NO_3)_2$ (b) $K_3PO_4$ (c) $(NH_4)_2SO_4$ (d) $FeCl_3$ (e) $Na_2SO_3$

Solution:

Method: Split the salt into cation and anion — identify their parent acid/base.

(a) $Mg(NO_3)_2$ (magnesium nitrate):

• Cation: $Mg^{2+}$ → base $Mg(OH)_2$
• Anion: $NO_3^-$ → acid $HNO_3$
• Reaction: $Mg(OH)_2 + 2HNO_3 \rightarrow Mg(NO_3)_2 + 2H_2O$

(b) $K_3PO_4$ (potassium phosphate):

• Cation: $K^+$ → base $KOH$
• Anion: $PO_4^{3-}$ → acid $H_3PO_4$
• Reaction: $3KOH + H_3PO_4 \rightarrow K_3PO_4 + 3H_2O$

(c) $(NH_4)_2SO_4$ (ammonium sulphate):

• Cation: $NH_4^+$ → base $NH_4OH$
• Anion: $SO_4^{2-}$ → acid $H_2SO_4$
• Reaction: $2NH_4OH + H_2SO_4 \rightarrow (NH_4)_2SO_4 + 2H_2O$

(d) $FeCl_3$ (ferric chloride):

• Cation: $Fe^{3+}$ → base $Fe(OH)_3$
• Anion: $Cl^-$ → acid $HCl$
• Reaction: $Fe(OH)_3 + 3HCl \rightarrow FeCl_3 + 3H_2O$

(e) $Na_2SO_3$ (sodium sulphite):

• Cation: $Na^+$ → base $NaOH$
• Anion: $SO_3^{2-}$ → acid $H_2SO_3$
• Reaction: $2NaOH + H_2SO_3 \rightarrow Na_2SO_3 + 2H_2O$

Key takeaway:

• Salt = cation (from base) + anion (from acid)
• Most salts can be analysed using this principle.

[Board 5-mark question]

Example 13: Industrial Uses of NaCl

Which chemical products are made from common salt ($NaCl$)? (A preview of Section 8.)

Solution:

$NaCl$ — the raw material of the chemical industry.

1. Sodium hydroxide ($NaOH$ — caustic soda):

Method: the chlor-alkali process — electrolysis of $NaCl$ solution.

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

Products: $NaOH$, $Cl_2$, $H_2$ — all useful.

2. Sodium carbonate ($Na_2CO_3$ — washing soda):

Method: the Solvay Process — $NaCl$ + $CO_2$ + $NH_3$ + $H_2O$ → $NaHCO_3$ → on heating, $Na_2CO_3$.

3. Sodium hydrogen carbonate ($NaHCO_3$ — baking soda):

An intermediate of the Solvay Process.

4. Chlorine gas ($Cl_2$):

From the chlor-alkali process. Uses — water purification, bleaching, PVC.

5. Bleaching powder ($CaOCl_2$):

From the action of $Cl_2$ on slaked lime.

6. Hydrogen chloride ($HCl$):

A multi-billion-dollar industry from $NaCl$:

Global production of common salt is ~ 270 million tonnes/year — of which ~ 60% goes into the chemical industry.

[Board + introduction to Section 8]

Example 14: Numerical — Molecular Mass of a Salt

Find the molecular mass of $CuSO_4 \cdot 5H_2O$. What percentage of its mass is water? (Cu=64, S=32, O=16, H=1)

Solution:

Calculating molecular mass:

Mass of $CuSO_4$:

• $Cu = 64$
• $S = 32$
• $4 \times O = 4 \times 16 = 64$
• Total: $64 + 32 + 64 = 160$

Mass of $5H_2O$:

• $H_2O = 2 + 16 = 18$
• $5 \times 18 = 90$

Total mass of $CuSO_4 \cdot 5H_2O$:

• $160 + 90 = 250$

Percentage of water:

$$\% H_2O = \frac{90}{250} \times 100 = 36\%$$

So $CuSO_4 \cdot 5H_2O$ is 36% water by mass — i.e., 100 g of blue vitriol contains 36 g of water and 64 g of anhydrous $CuSO_4$.

Practical use:

• To check the purity of a salt — measure the water percentage.
• Test by mass loss on heating.

[Board 3-mark question]

Example 15: NCERT — A Mixed Question

Classify the following salts by their parent acid/base and state their nature: (a) $Na_2CO_3$ (b) $CuSO_4$ (c) $KCl$ (d) $NH_4NO_3$ (e) $CaCl_2$ (f) $NH_4Cl$

Solution:

Salt	Parent Acid	Parent Base	Type	pH
(a) $Na_2CO_3$	$H_2CO_3$ (weak)	$NaOH$ (strong)	W+S	basic (~ 11)
(b) $CuSO_4$	$H_2SO_4$ (strong)	$Cu(OH)_2$ (weak)	S+W	acidic
(c) $KCl$	$HCl$ (strong)	$KOH$ (strong)	S+S	neutral (7)
(d) $NH_4NO_3$	$HNO_3$ (strong)	$NH_4OH$ (weak)	S+W	acidic
(e) $CaCl_2$	$HCl$ (strong)	$Ca(OH)_2$ (moderate)	S+M	nearly neutral
(f) $NH_4Cl$	$HCl$ (strong)	$NH_4OH$ (weak)	S+W	acidic

Final classification into three types:

Neutral: $KCl$, $CaCl_2$

Acidic: $CuSO_4$, $NH_4NO_3$, $NH_4Cl$

Basic: $Na_2CO_3$

Lesson: The pH of a salt is predicted by the strengths of its parents.

[NCERT important — 5-mark question]

Example 16: A Closing Question

(a) What is a salt? Definition. (b) Preparation and two uses of sodium chloride. (c) What is the water of crystallisation? Three examples. (d) Why is the pH of $Na_2CO_3$ greater than 7?

Solution:

(a) Definition of a salt:

An ionic compound formed by neutralisation of an acid with a base — in which the cation comes from the base and the anion from the acid.

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

(b) Sodium chloride:

Preparation: $$NaOH + HCl \rightarrow NaCl + H_2O$$

Two uses:

1. In food — taste, preservation.
2. In the chemical industry — raw material for $NaOH, Na_2CO_3, NaHCO_3, Cl_2$.

(c) Water of crystallisation:

A definite number of water molecules chemically bound in a salt's crystal.

Three examples:

• $CuSO_4 \cdot 5H_2O$ (blue)
• $Na_2CO_3 \cdot 10H_2O$ (washing soda)
• $CaSO_4 \cdot 2H_2O$ (gypsum)

(d) Why pH of $Na_2CO_3$ > 7:

Reason:

• Parent acid = $H_2CO_3$ (carbonic acid) — weak
• Parent base = $NaOH$ — strong
• Rule: weak acid + strong base → basic salt

In water: $$CO_3^{2-} + H_2O \rightleftharpoons HCO_3^- + OH^-$$

(The carbonate ion takes $H^+$ from water, leaving more $OH^-$ — making the solution basic.)

[Board 5-mark question]