Solved Examples

About This Section

This is a collection of 30+ solved examples covering every topic of Chapter 2. Here you will find —

• Simple questions on indicators and pH
• Reactions of acids/bases with metals/carbonates
• Numerical neutralisation problems
• pH-scale calculations
• Daily-life applications of pH
• pH and families of salts
• Questions on chemicals from common salt
• All major NCERT textbook questions

Study strategy:

1. Try to solve each example yourself first.
2. Compare with the solution.
3. Memorise key reactions.
4. Also do the NCERT chapter exercises.

Let's begin!

Example 1: NCERT — Phenolphthalein Colour Change

Phenolphthalein is colourless in dilute $HCl$ but turns pink in $NaOH$. Why?

Solution:

Principle: Phenolphthalein is a weak acid. It exists in two forms:

• In acidic/neutral medium: un-ionised — colourless.
• In basic medium: ionised ($A^-$) — pink/magenta.

Analysis:

In dilute $HCl$:

• Acidic environment → phenolphthalein un-ionised → colourless ✓

In dilute $NaOH$:

• Basic environment → phenolphthalein ionised → pink ✓

Practical use:

• Detecting endpoints in titration.
• Identifying bases.

[NCERT — repeatedly asked]

Example 2: NCERT — Zn + Dilute $H_2SO_4$

What are the observations when granulated zinc is treated with dilute $H_2SO_4$? Write the reaction. How do you identify the gas evolved?

Solution:

Observations:

1. Bubbles appear around the zinc granules.
2. The test tube becomes warm.
3. Zinc gradually dissolves.

Reaction:

$$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)\uparrow$$

Test for hydrogen:

• Pass the gas through a soap solution — soap bubbles form.
• Bring a burning candle close — the gas burns with a 'pop' sound.
• This is the famous test for $H_2$.

Example 3: NCERT — Test for $CO_2$

What happens when dilute $HCl$ is added to $Na_2CO_3$? How is the gas evolved identified?

Solution:

Reaction:

$$Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2\uparrow$$

Observation: brisk effervescence — colourless, odourless gas.

Test for $CO_2$ — the lime water test:

$$Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O$$

(white $CaCO_3$ — lime water turns milky)

This is the most famous test for $CO_2$.

Example 4: NCERT — Neutralisation

A solution of $NaOH$ in water reacts with $HCl$ to form sodium chloride and water. Write the balanced equation. What kind of reaction is it?

Solution:

Reaction:

$$NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)$$

Type of reaction:

• Neutralisation reaction — acid + base → salt + water.
• A special case of double displacement.

Ionic form:

$$H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$$

(Na⁺ and Cl⁻ are spectator ions)

Daily life:

• Treatment of acidity ($NaHCO_3 + HCl$).
• Adding lime to acidic soil.

Example 5: NCERT — $CuO$ + $HCl$

What are the observations when dilute $HCl$ is added to black $CuO$? Write the reaction.

Solution:

Observations:

1. The black $CuO$ slowly dissolves.
2. The solution turns bluish-green (copper chloride).

Reaction:

$$CuO(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + H_2O(l)$$

(black) (bluish-green)

Type of reaction:

• Acid + metallic oxide → salt + water.
• Proves that metallic oxides are basic.

Example 6: NCERT — Universal Indicator

State the colour of the universal indicator at pH 1, pH 7, and pH 12.

Solution:

pH	Colour	Nature
1	Red	Strong acid
7	Green	Neutral
12	Violet / blue-violet	Strong base

The universal indicator is a mixture of several indicators — it gives a different colour at each pH.

Example 7: NCERT — Calculating pH

If $[H^+] = 10^{-4}$ M, what is the pH? What is the nature of the solution?

Solution:

$$\text{pH} = -\log_{10}[H^+] = -\log(10^{-4}) = 4$$

Nature of solution: acidic (pH < 7).

Practical context: pH 4 is similar to a tomato in acidity.

Example 8: NCERT — From pH to Concentration

If a solution has pH = 9, what is its $[H^+]$?

Solution:

$$[H^+] = 10^{-\text{pH}} = 10^{-9} \text{ M}$$

Nature of solution: basic (pH > 7).

(Example: a baking soda solution is in this range.)

Example 9: NCERT — pH Difference

How much more acidic is a pH 3 solution than a pH 6 solution?

Solution:

Difference: $6 - 3 = 3$ pH units.

Acidity ratio: $10^3 = 1000$ times

Answer: A pH 3 solution is 1000 times more acidic than a pH 6 solution.

Principle: pH is a logarithmic scale.

Example 10: NCERT — Treating Acidity

Why is milk of magnesia given for acidity? Write the chemical reaction.

Solution:

Milk of magnesia = $Mg(OH)_2$ (basic).

How it works:

• Acidity = excess $HCl$ in the stomach.
• $Mg(OH)_2$ is basic — it neutralises the excess $HCl$.
• pH returns to normal — burning is reduced.

Reaction:

$$Mg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O$$

This is a neutralisation reaction.

Example 11: NCERT — Tooth Decay

How do teeth decay? Give three preventive measures.

Solution:

Mechanism of tooth decay:

1. We eat sugar.
2. Mouth bacteria convert sugar into acid (lactic acid).
3. Mouth pH drops below 5.5.
4. Tooth enamel ($Ca_3(PO_4)_2$) starts dissolving.
5. Cavities develop over time.

Three preventions:

1. Rinse the mouth after eating.
2. Brush with basic toothpaste.
3. Reduce sugar intake.

Example 12: NCERT — Treating a Bee Sting

Why do we apply baking soda on a bee sting?

Solution:

Cause: A bee's sting contains formic acid ($HCOOH$) — causing burning.

Action of baking soda:

• $NaHCO_3$ is basic.
• Neutralises the formic acid.
• Pain is reduced.

Reaction:

$$NaHCO_3 + HCOOH \rightarrow HCOONa + H_2O + CO_2\uparrow$$

Wasp: sting is basic → treat with vinegar/lemon (acid).

Example 13: NCERT — Acid Rain

What is acid rain? State two of its effects.

Solution:

Definition: Rain is called acid rain when its pH drops below 5.6.

Cause: Air pollutants ($SO_2$, $NO_2$) → acids in rainwater.

$$SO_3 + H_2O \rightarrow H_2SO_4$$

Two effects:

1. Fish die in rivers/lakes (low pH harms aquatic life).
2. Marble buildings (e.g. the Taj Mahal) dissolve ($CaCO_3 + H_2SO_4$).

Example 14: NCERT — Acidic Soil

A farmer's soil is acidic. What is the treatment?

Solution:

Problem: Most crops thrive at pH 6-7.5. In acidic soil (pH < 5.5), yields drop.

Treatment: Add slaked lime ($Ca(OH)_2$).

Reaction:

$$Ca(OH)_2 + 2H^+ \rightarrow Ca^{2+} + 2H_2O$$

(soil acid is neutralised — pH rises)

Other options: dolomite, quick lime.

Example 15: NCERT — pH of Salts

State the nature of the following salts — $NaCl$, $NH_4Cl$, $CH_3COONa$, $Na_2CO_3$.

Solution:

Salt	Parent Acid	Parent Base	Nature
$NaCl$	Strong	Strong	Neutral (pH=7)
$NH_4Cl$	Strong ($HCl$)	Weak ($NH_4OH$)	Acidic
$CH_3COONa$	Weak	Strong	Basic
$Na_2CO_3$	Weak ($H_2CO_3$)	Strong	Basic

Rule: Both strong → neutral; one strong/one weak → the weaker partner's nature.

Example 16: NCERT — Heating Copper Sulphate

Why does the colour of blue $CuSO_4$ crystals change on heating? Write the reaction.

Solution:

Reaction:

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

Explanation:

• The blue crystals contain 5 water molecules of crystallisation — responsible for the colour.
• On heating, the water vaporises — anhydrous $CuSO_4$ is white.
• On cooling and adding water — it turns blue again.

This is a famous example of the test for water.

Example 17: NCERT — Sources of Common Salt

State three main sources of common salt in India.

Solution:

Three sources:

1. Sea water (largest):

• Gujarat — ~ 75%
• Tamil Nadu, Odisha, Andhra Pradesh
• Method: sun-evaporation.

2. Salt lakes:

• Sambhar Lake (Rajasthan)
• Didwana Lake

3. Rock salt:

• Mandi district of Himachal Pradesh.

Historical: the Salt Satyagraha (1930) — part of the independence struggle.

Example 18: NCERT — Chlor-Alkali Process

What is the chlor-alkali process? Name three products.

Solution:

Definition: Electrolysis of an aqueous $NaCl$ solution.

Reaction:

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

Three products:

Product	Location
$NaOH$	In solution
$Cl_2$	At the anode
$H_2$	At the cathode

Name: 'chlor' = chlorine; 'alkali' = $NaOH$.

Example 19: NCERT — Bleaching Powder

How is bleaching powder made? Three uses.

Solution:

Formula: $CaOCl_2$

Preparation:

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

(slaked lime + chlorine)

Three uses:

1. Bleaching cotton/linen.
2. Bleaching paper pulp.
3. Water purification (drinking water, swimming pools).

Example 20: NCERT — Baking Soda

Formula, preparation, and three uses of baking soda.

Solution:

Formula: $NaHCO_3$

Preparation (part of Solvay):

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

On heating: $2NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2$

Three uses:

1. In baking — to raise cakes/biscuits.
2. Treating acidity.
3. In fire extinguishers.

Example 21: NCERT — Washing Soda

How does washing soda soften hard water?

Solution:

Reactions:

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

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

Principle: Hardness ions ($Ca^{2+}, Mg^{2+}$) are precipitated as insoluble carbonates. Water becomes soft.

Formula: $Na_2CO_3 \cdot 10H_2O$

Example 22: NCERT — POP

How is POP made? What happens when water is added?

Solution:

Formula: $CaSO_4 \cdot \frac{1}{2}H_2O$

Preparation (from gypsum):

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

On adding water: It turns back to gypsum and forms a hard solid.

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

Uses: plaster casts for broken bones, sculptures, ceilings.

Example 23: Numerical — Neutralisation

How much $HCl$ is needed to fully neutralise 40 g of $NaOH$? (Na=23, O=16, H=1, Cl=35.5)

Solution:

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

Molecular masses:

• $NaOH = 40$
• $HCl = 36.5$

Ratio: 40 g $NaOH$ + 36.5 g $HCl$

Given: 40 g $NaOH$ → $HCl = 36.5$ g needed.

Example 24: Numerical — Percentage of Water

What is the percentage of water in $Na_2CO_3 \cdot 10H_2O$? (Na=23, C=12, O=16, H=1)

Solution:

Molecular masses:

• $Na_2CO_3 = 46 + 12 + 48 = 106$
• $10H_2O = 180$
• Total $= 286$ g/mol

Percentage of water:

$$\% H_2O = \frac{180}{286} \times 100 = 62.9\%$$

Answer: ~ 63% water (a hydrated salt with high water content).

Example 25: A pH Mixed Question

A student mixes equal volumes of $0.1$ M $HCl$ and $0.1$ M $NaOH$. What will be the final pH?

Solution:

Analysis:

• Equal concentrations and volumes.
• $HCl$ fully ionises — gives all $H^+$.
• $NaOH$ fully ionises — gives all $OH^-$.

Reaction: $H^+ + OH^- \rightarrow H_2O$

Equal amounts of $H^+$ and $OH^-$ → complete neutralisation.

Final solution: only $NaCl + H_2O$ → neutral.

pH = 7

Lesson: Strong acid + strong base = neutral salt.

Example 26: NCERT — Five Questions

(a) What are indicators? (b) Range of the pH scale. (c) Effect of acid on blue litmus. (d) What is neutralisation? (e) Formula of caustic soda.

Solution:

(a) Indicators: substances that show the presence of an acid or base by changing colour or odour.

(b) Range of pH scale: 0 to 14.

(c) Blue litmus in acid: turns red.

(d) Neutralisation: acid + base → salt + water. Ionic: $H^+ + OH^- \rightarrow H_2O$.

(e) Formula of caustic soda: $NaOH$ (sodium hydroxide).

Example 27: A Reasoning Question

Dry $HCl$ does not turn blue litmus red. Why?

Solution:

Principle: Acidic properties come from $H^+$ (or $H_3O^+$) — and they form only in water.

In dry $HCl$:

• Only $HCl$ molecules.
• No ionisation — no separated $H^+$.
• So no acidic properties shown.

To prove:

• Dry $HCl$ on dry blue litmus — no change.
• Dry $HCl$ on wet blue litmus — turns red instantly.

Conclusion: Water is essential for acidic properties.

Example 28: NCERT — A Comparison

State the difference between strong and weak acids. Two examples each.

Solution:

Aspect	Strong Acid	Weak Acid
Ionisation	Complete (~ 100%)	Partial (< 5%)
$H^+$ concentration	High	Low
pH	1-3	4-6
Reaction with metals	Fast	Slow

Strong: $HCl$, $H_2SO_4$, $HNO_3$

Weak: $CH_3COOH$, $H_2CO_3$, $HCOOH$

Example 29: NCERT — Action of Bleaching Powder

How does bleaching powder bleach? Chemical reactions.

Solution:

Principle: Bleaching powder → free $Cl_2$ → oxidising agent → makes dyes colourless.

Step by step:

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

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

$$HOCl \rightarrow HCl + [O] \quad \text{(nascent oxygen)}$$

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

[O] is a strong oxidising agent — destroys dyes.

Example 30: NCERT — State the Differences

Five differences between baking soda and washing soda.

Solution:

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
pH	~ 8.5	~ 11-12
Use	In food, antacid	Laundry, glass industry

Connection: Heating baking soda gives $Na_2CO_3$ — the basis of washing soda.

Example 31: NCERT — Mixed Applications

The correct treatment for the following daily-life problems: (a) Stomach burning (b) Bee sting (c) Wasp sting (d) Acidic soil

Solution:

Problem	Nature	Treatment
(a) Stomach burning	Excess acid	Baking soda / milk of magnesia
(b) Bee sting	Acidic (formic)	Baking soda
(c) Wasp sting	Basic	Vinegar / lemon
(d) Acidic soil	Acid	Slaked lime

General rule: Treatment is opposite in nature to the problem.

Example 32: A Numerical Challenge

From 1 mol of $NaCl$ electrolysed, how much $NaOH$ and $Cl_2$ are obtained? (Na=23, Cl=35.5, O=16, H=1)

Solution:

Reaction:

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

Mole ratio: 2 mol NaCl → 2 mol NaOH + 1 mol Cl₂ + 1 mol H₂

Simplified: 1 mol NaCl → 1 mol NaOH + 0.5 mol Cl₂

Masses:

• 1 mol NaCl = 58.5 g
• 1 mol NaOH = 40 g
• 0.5 mol Cl₂ = 35.5 g

Answer:

• NaOH = 40 g
• Cl₂ = 35.5 g
• H₂ = 1 g

Example 33: NCERT — Final Multi-Topic

(a) Definition of neutralisation. (b) Introduction to the pH scale. (c) Use of caustic soda. (d) Why store POP away from moisture? (e) Example of water of crystallisation.

Solution:

(a) Neutralisation: acid + base → salt + water. Ionic: $H^+ + OH^- \rightarrow H_2O$.

(b) pH scale: 0-14. pH < 7 acidic; pH = 7 neutral; pH > 7 basic. $\text{pH} = -\log[H^+]$.

(c) Use of caustic soda: soap industry, paper manufacturing, petroleum refining.

(d) Why store POP away from moisture: moisture turns POP back into gypsum — and POP becomes useless.

(e) Example of water of crystallisation: $CuSO_4 \cdot 5H_2O$ (blue vitriol) — 5 water molecules.

Example 34: An Application Question

When lemon is added to tea-water, the colour becomes lighter. Why?

Solution:

Principle: Tea contains dyes (tannins) — which are pH-sensitive.

Analysis:

• Plain tea has pH ~ 5 (slightly acidic).
• Colour: deep brown-red.
• Adding lemon (citric acid) drops the pH further.
• Dyes shift to a more acidic form.
• Colour becomes lighter.

Demonstrates: Tea acts as a natural acid-base indicator.

Other examples: red cabbage juice, turmeric, purple flowers.

Example 35: A Closing Question

In summary, what are the five main topics of Chapter 2?

Solution:

Five main topics of Chapter 2:

1. Indicators:

• Natural (litmus, turmeric), synthetic (phenolphthalein, methyl orange), olfactory (onion, vanilla).

2. Reactions of acids/bases:

• With metals (release $H_2$)
• With carbonates (release $CO_2$)
• With each other (neutralisation)
• With oxides

3. Ionic nature:

• Acid = gives $H^+$
• Base = gives $OH^-$
• Dilution and safety
• Strong/weak

4. pH scale and daily life:

• 0-14 scale
• pH of stomach, blood, mouth
• Acidity, tooth decay, acid rain, soil, stings

5. Salts:

• Definition, families
• Four pH types
• Water of crystallisation
• Chemicals from common salt ($NaOH$, $CaOCl_2$, $NaHCO_3$, $Na_2CO_3 \cdot 10H_2O$, POP)

Most important for the board exam:

• Phenolphthalein colours
• Diluting concentrated acid (precaution)
• pH of daily things (stomach 1-2, blood 7.4)
• Treating acidity
• 4 chemicals from $NaCl$
• POP and gypsum

This chapter is one of the foundation chapters of chemistry — directly connected to everyday life.

[A complete summary for the board exam]