Acids, Bases and Salts

Introduction

Have you ever wondered why lemon juice tastes sour, why soap feels slippery, or why your stomach needs antacid tablets when you feel acidity? The answer lies in Acids, Bases and Salts—three chemical families that affect our daily lives in countless ways. From the citric acid in your favorite fruits to the sodium hydroxide in your cleaning products, these substances are everywhere. This chapter teaches you how to identify them, how they react with each other, and why the pH scale matters for everything from digestion to agriculture. Whether you're searching for class 10 acids bases and salts notes, class 10 acids bases and salts MCQ, or class 10 acids bases and salts PYQ, this guide covers everything you need to ace your CBSE Board 2026 exam!

Table of Contents

Introduction
What are Acids?
What are Bases?
Indicators
The pH Scale
Salts
MCQs
Short Answer Questions
Long Answer Questions
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What are Acids?

An acid is a substance that makes a solution sour and releases hydrogen ions (H⁺) when mixed with water. These hydrogen ions are what give acids their characteristic properties—sour taste, ability to corrode metals, and turning blue litmus paper red.

Acids Around You – Real Examples

Natural Acids (found in food):

Citric acid – Lemon, orange, nimbu pani
Acetic acid – Vinegar used in salads and momos
Tartaric acid – Tamarind, imli chutney
Lactic acid – Curd, buttermilk, yogurt
Ascorbic acid – Amla, oranges (Vitamin C)

Mineral Acids (used in labs):

Hydrochloric acid (HCl) – Our stomach produces this to digest food
Sulphuric acid (H₂SO₄) – Car batteries, fertilizer factories
Nitric acid (HNO₃) – Making explosives and fertilizers

How Acids Behave

1. When acids meet metals:

Imagine putting a small zinc piece in dilute hydrochloric acid. You'll see bubbles forming—that's hydrogen gas!

Zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas.

Zn+2HCl→ZnCl2+H2↑Zn+2HCl→ZnCl2+H2↑

2. When acids meet carbonates:

If you add vinegar to baking soda, it fizzes immediately. The fizzing is carbon dioxide gas escaping.

Sodium carbonate reacts with hydrochloric acid to form sodium chloride, water, and carbon dioxide gas.

Na2CO3+2HCl→2NaCl+H2O+CO2↑Na2CO3+2HCl→2NaCl+H2O+CO2↑

3. When acids meet bases:

They neutralize each other and form salt and water.

Hydrochloric acid reacts with sodium hydroxide to form sodium chloride (common salt) and water.

HCl+NaOH→NaCl+H2OHCl+NaOH→NaCl+H2O

Quick Table: Acids in Daily Life

Substance	Acid Present	Where You See It
Lemon juice	Citric acid	Nimbu pani, pickles, lemon tea
Vinegar	Acetic acid	Kitchen, Chinese food stalls
Curd/Dahi	lactic acid	Breakfast, lunch, raita
Tamarind/Imli	Tartaric acid	Sambhar, chutney, candies
Stomach juice	Hydrochloric acid	Digests food, causes acidity
Ant sting	Formic acid	Causes burning sensation

Notes on Nationalism in India (First World War, Khilafat & Non-Cooperation Movement)

What are Bases?

A base is a substance that tastes bitter, feels slippery like soap, and releases hydroxide ions (OH⁻) when dissolved in water. Bases turn red litmus paper blue.

When a base dissolves well in water, it's called an alkali . So all alkalis are bases, but not all bases are alkalis.

Bases Around You – Real Examples

Sodium hydroxide (NaOH) – Drain cleaners, soap making
Calcium hydroxide [Ca(OH)₂] – Lime water, whitewash for walls
Magnesium hydroxide [Mg(OH)₂] – Milk of magnesia (antacid)
Ammonium hydroxide (NH₄OH) – Glass cleaners, floor cleaners
Toothpaste – Contains mild bases to fight mouth acid

How Bases Behave

1. When bases meet acids:

Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.

NaOH+HCl→NaCl+H2ONaOH+HCl→NaCl+H2O

2. When strong bases meet metals:

Sodium hydroxide reacts with zinc to form sodium zincate and hydrogen gas.

2NaOH+Zn→Na2ZnO2+H2↑2NaOH+Zn→Na2ZnO2+H2↑

3. When bases meet non-metallic oxides:

Sodium hydroxide reacts with carbon dioxide to form sodium carbonate and water.

2NaOH+CO2→Na2CO3+H2O2NaOH+CO2→Na2CO3+H2O

Quick Table: Bases in Daily Life

Substance	Base Present	Use in Daily Life
Soap	NaOH or KOH based	Bathing, washing clothes
Milk of magnesia	Mg(OH)₂	Antacid for stomach relief
Lime water/Chuna	Ca(OH)₂	Whitewashing walls, paan shops
Window cleaner	NH₄OH	Cleaning glass surfaces
Toothpaste	Mild bases	Neutralizing mouth acid

Class 10 Science Chapter-1: Chemical Reactions and Equations (Notes & PYQ).

Indicators – Color Detectives

Indicators are special substances that change color when they meet acids or bases. They help us quickly identify whether a solution is acidic, basic, or neutral without tasting it (which would be dangerous!).

Example:
When turmeric (haldi) accidentally falls on soap water while washing dishes, it turns reddish-brown. This happens because soap is basic and turmeric acts as a natural indicator.

Types of Indicators

1. Natural Indicators:

· Litmus (extracted from lichens)

· Turmeric

· Red cabbage juice

· China rose petals

2. Synthetic Indicators:

· Phenolphthalein

· Methyl orange

3. Olfactory Indicators:
These are substances whose smell changes in acidic and basic solutions (like vanilla essence, clove oil) .

Indicator Color Change Table

Indicator	Original Color	In Acid	In Base
Blue litmus paper	Blue	Turns red	Stays blue
Red litmus paper	Red	Stays red	Turns blue
Phenolphthalein	Colorless	Stays colorless	Turns pink
Methyl orange	Orange	Turns red	Turns yellow
Turmeric	Yellow	Stays yellow	Turns red-brown

Nationalism & Imperialism Class 10 Notes History.

What Happens When Acids and Bases Mix with Water?

When acids dissolve in water, they don't just float around. They break apart and release hydrogen ions (H⁺). These hydrogen ions quickly grab water molecules and form hydronium ions (H₃O⁺):

Hydrochloric acid dissolves in water to form hydronium ions and chloride ions.

HCl+H2O→H3O++Cl−

When bases dissolve in water, they release hydroxide ions (OH⁻):

Sodium hydroxide dissolves in water to form sodium ions and hydroxide ions.

NaOH→Na++OH−

Important point: Acids show their acidic properties only when water is present. Dry HCl gas doesn't turn blue litmus red, but HCl dissolved in water does !

Strong vs Weak Acids and Bases

Not all acids are equally strong. Some break apart completely in water (strong), while others only partially break apart (weak).

Strong acids (completely ionize):

· HCl (Hydrochloric acid)

· H₂SO₄ (Sulphuric acid)

· HNO₃ (Nitric acid)

Weak acids (partially ionize):

· CH₃COOH (Acetic acid/Vinegar)

· Citric acid (Lemon)

· Carbonic acid (Cold drinks)

Strong bases (completely ionize):

· NaOH (Sodium hydroxide)

· KOH (Potassium hydroxide)

· Ca(OH)₂ (Calcium hydroxide)

Weak bases (partially ionize):

· NH₄OH (Ammonium hydroxide)

· Mg(OH)₂ (Magnesium hydroxide)

The pH Scale

The pH scale is like a measuring tape for acids and bases. It runs from 0 to 14 and tells you how acidic or basic a solution is.

Think of it like a temperature scale, but instead of measuring hot or cold, it measures acidic or basic:

· pH 0-7 → Acidic (lower the number, stronger the acid)

· pH 7 → Neutral (neither acidic nor basic)

· pH 7-14 → Basic/Alkaline (higher the number, stronger the base)

pH Values Around You

Substance	Approx pH	Nature
Battery acid	0-1	Very strongly acidic
Stomach acid	1-2	Strongly acidic
Lemon juice	2-3	Acidic
Vinegar	3	Acidic
Tomato juice	4	Acidic
Coffee	5	Weakly acidic
Milk	6.5	Slightly acidic
Pure water	7	Neutral
Blood	7.4	Slightly basic
Baking soda solution	8-9	Basic
Soap solution	9-10	Basic
Milk of magnesia	10	Basic
Ammonia solution	11	Strongly basic
Bleach	12-13	Very strongly basic
Drain cleaner	14	Extremely basic

Why pH Matters in Daily Life

1. In Our Digestive System

Your stomach produces hydrochloric acid (pH 1-2) to kill bacteria and digest food. Sometimes excess acid forms, causing acidity and heartburn. That's when you take antacids (bases like Mg(OH)₂) to neutralize the acid and bring pH back to comfortable levels .

2. Tooth Decay

Bacteria in our mouth break down leftover food and produce acids. When mouth pH drops below 5.5, tooth enamel (made of calcium phosphate) starts dissolving, causing cavities .

Solution: Brush with toothpaste (slightly basic) to neutralize the acid and maintain healthy pH.

3. Soil pH and Plants

Most plants grow best when soil pH is between 6.5 to 7.5.

If soil becomes too acidic → Farmers add slaked lime [Ca(OH)₂] to increase pH

If soil becomes too basic → Farmers add organic compost to decrease pH

4. Acid Rain

Normal rainwater is slightly acidic with pH around 5.6 (because of dissolved CO₂). But when factories and vehicles release SO₂ and NO₂ into the air, rainwater becomes more acidic (pH < 5.6) .

Effects of acid rain:

· Damages the Taj Mahal and other monuments

· Harms fish and aquatic life in rivers and lakes

· Destroys crops and forests

5. Insect Stings

· Bee sting contains acid (formic acid) → Apply baking soda (base) for relief

· Wasp sting contains base → Apply vinegar (acid) for relief

Neutralization – When Acid and Base Cancel Each Other

When an acid meets a base, they react and neutralize each other's effects. The result is salt and water. This process is called neutralization reaction.

General pattern:

Acid+Base→Salt+WaterAcid+Base→Salt+Water

Examples in Daily Life

Stomach acidity: Excess HCl in stomach → Take antacid (Mg(OH)₂) → Neutralization → Relief
Acidic soil: Soil too acidic → Add lime [Ca(OH)₂] → pH increases → Better crop growth
Insect bite: Ant/bee sting (formic acid) → Apply baking soda paste → Neutralization → Pain reduces
Factory waste: Acidic waste water → Add base before releasing → Neutralized water → Safe for environment

Chemical Examples

Example 1:
Hydrochloric acid reacts with sodium hydroxide to form sodium chloride (table salt) and water.

HCl+NaOH→NaCl+H2OHCl+NaOH→NaCl+H2O

Example 2:
Sulphuric acid reacts with sodium hydroxide to form sodium sulphate and water.

H2SO4+2NaOH→Na2SO4+2H2OH2SO4+2NaOH→Na2SO4+2H2O

Visualising the Nation - Complete NCERT Notes Class 10 History.

Salts – The Result of Acid-Base Meetings

A salt is formed when the hydrogen of an acid is replaced by a metal or ammonium ion. Salts can be neutral, acidic, or basic depending on which acid and base they came from.

Types of Salts

Type of Salt	Made From	pH	Examples	Everyday Use
Neutral	Strong acid + Strong base	7	NaCl, K₂SO₄	Table salt, cooking
Acidic	Strong acid + Weak base	<7	NH₄Cl, CuSO₄	Fertilizers, batteries
Basic	Weak acid + Strong base	>7	Na₂CO₃, CH₃COONa	Washing soda, preservatives

Examples Explained

1. Neutral Salt (NaCl):

Sodium hydroxide (strong base) reacts with hydrochloric acid (strong acid) to form sodium chloride and water.

NaOH+HCl→NaCl+H2O

Result: NaCl solution has pH = 7 (neutral)

2. Acidic Salt (NH₄Cl):

Ammonium hydroxide (weak base) reacts with hydrochloric acid (strong acid) to form ammonium chloride and water.

NH4OH+HCl→NH4Cl+H2O

Result: NH₄Cl solution has pH < 7 (acidic)

3. Basic Salt (Na₂CO₃):

Sodium hydroxide (strong base) reacts with carbonic acid (weak acid) to form sodium carbonate and water.

2NaOH+H2CO3→Na2CO3+2H2O

Result: Na₂CO₃ solution has pH > 7 (basic)

The Making of Germany and Italy - Section 4 Complete Notes Class 10 History.

Water of Crystallization

Some salts trap water molecules inside their crystal structure. This fixed number of water molecules is called water of crystallization.

Examples:

Salt	Formula with Water	Water Molecules
Washing soda	Na₂CO₃·10H₂O	10
Gypsum	CaSO₄·2H₂O	2
Blue copper sulphate	CuSO₄·5H₂O	5
Plaster of Paris	CaSO₄·½H₂O	½ (shared between 2 units)

Experiment: Heating Copper Sulphate

Take blue copper sulphate crystals (CuSO₄·5H₂O) and heat them in a test tube.

What happens:

Blue crystals turn white
Water droplets appear on test tube walls

Hydrated copper sulphate loses water molecules on heating and becomes anhydrous white copper sulphate.

CuSO4⋅5H2O→heatCuSO4+5H2OCuSO4⋅5H2OheatCuSO4+5H2O

(Blue → White)

Now add water to white powder—it turns blue again!

Read the full blog here: The Age of Revolutions: 1830–1848 – Class 10 Notes.

Common Salt (NaCl) – The Chemical Factory

Common salt (sodium chloride) is not just for taste. It's the raw material for many important chemicals used in industries and homes.

The Salt Family Tree

Chemical	Formula	Key Use	How It's Made
Sodium hydroxide	NaOH	Making soaps	Chlor-alkali process
Bleaching powder	CaOCl₂	Cleaning water	Cl₂ + Ca(OH)₂
Baking soda	NaHCO₃	Baking, antacid	Solvay process
Washing soda	Na₂CO₃·10H₂O	Softening water	Heating baking soda
Plaster of Paris	CaSO₄·½H₂O	Casts, toys	Heating gypsum

1. Sodium Hydroxide (NaOH) – Caustic Soda

How it's made:
By passing electricity through concentrated salt solution (brine). This process is called Chlor-alkali process .

When electricity passes through brine solution, sodium hydroxide forms at the negative electrode, chlorine gas forms at the positive electrode, and hydrogen gas is released.

2NaCl+2H2O→electricity2NaOH+Cl2+H22NaCl+2H2Oelectricity2NaOH+Cl2+H2

Where it's used:

Making soaps and detergents
Paper manufacturing
Making artificial fibers (rayon)
Petroleum refining

2. Bleaching Powder (CaOCl₂)

How it's made:
By passing chlorine gas over dry slaked lime.

Chlorine gas reacts with calcium hydroxide to form bleaching powder and water.

Ca(OH)2+Cl2→CaOCl2+H2OCa(OH)2+Cl2→CaOCl2+H2O

Where it's used:

Bleaching cotton and linen in textile factories
Disinfecting drinking water in municipal water treatment plants

As an oxidizing agent in chemical industries
Bleaching wood pulp in paper factories

3. Baking Soda (NaHCO₃) – Sodium Hydrogen Carbonate

How it's made:
From common salt using Solvay process.

Sodium chloride reacts with water, carbon dioxide, and ammonia to form sodium hydrogen carbonate and ammonium chloride.

NaCl+H2O+CO2+NH3→NaHCO3+NH4ClNaCl+H2O+CO2+NH3→NaHCO3+NH4Cl

Where it's used:

In baking: When heated or mixed with acid (in cake batter), it releases CO₂ which makes cakes soft and fluffy

Sodium hydrogen carbonate decomposes on heating to give sodium carbonate, water, and carbon dioxide gas.

2NaHCO3→heatNa2CO3+H2O+CO2↑2NaHCO3heatNa2CO3+H2O+CO2↑

As antacid: Neutralizes excess stomach acid
In fire extinguishers: Releases CO₂ which cuts off oxygen supply
In soda-acid fire extinguishers

4. Washing Soda (Na₂CO₃·10H₂O) – Sodium Carbonate

How it's made:
By heating baking soda, then adding water.

Step 1: Heat baking soda

Sodium hydrogen carbonate decomposes on heating to give sodium carbonate, water, and carbon dioxide.

2NaHCO3→heatNa2CO3+H2O+CO2↑2NaHCO3heatNa2CO3+H2O+CO2↑

Step 2: Add water for crystallization

Anhydrous sodium carbonate absorbs water molecules to form washing soda crystals.

Na2CO3+10H2O→Na2CO3⋅10H2ONa2CO3+10H2O→Na2CO3⋅10H2O

Where it's used:

Removing permanent hardness of water

Cleaning agent for floors and bathrooms
Manufacturing glass, soap, and paper
In laundry detergents

5. Plaster of Paris (CaSO₄·½H₂O)

How it's made:
By heating gypsum at 373 K (100°C).

Gypsum loses most of its water molecules on heating at 373 K to form Plaster of Paris.

CaSO4⋅2H2O→373 KCaSO4⋅12H2O+112H2OCaSO4⋅2H2O373 KCaSO4⋅21H2O+121H2O

(Gypsum → Plaster of Paris)

Where it's used:

Making casts for broken bones (sets in 10-15 minutes)
Making toys, dolls, and decorative items
Making smooth surfaces on walls and ceilings
Making statues and sculptures

Why store in moisture-proof containers?

When Plaster of Paris absorbs water from air, it hardens and becomes useless:

Plaster of Paris absorbs water and converts back to hard gypsum.

CaSO4⋅12H2O+112H2O→CaSO4⋅2H2OCaSO4⋅21H2O+121H2O→CaSO4⋅2H2O

(Plaster of Paris → Gypsum - hardens)

The Making of Nationalism in Europe – Class 10 Notes

MCQs for Practice (CBSE Board PYQs)

1. Which of the following correctly represents hydrated copper sulphate? [CBSE 2023]
(a) CuSO₄·H₂O
(b) CuSO₄·3H₂O
(c) CuSO₄·5H₂O
(d) CuSO₄·7H₂O
Answer: (c)

2. Sodium hydrogen carbonate when added to acetic acid evolves a gas. Which statement is true? [CBSE 2023]
(a) The gas turns lime water milky
(b) The gas has a pungent smell
(c) The gas burns with a pop sound
(d) The gas has a pleasant smell
Answer: (a) The gas is CO₂ which turns lime water milky

3. Bee sting leaves an acid which causes pain. It can be treated by rubbing: [CBSE 2020]
(a) Vinegar
(b) Baking soda solution
(c) Lemon juice
(d) Acetic acid
Answer: (b) Baking soda (base) neutralizes the acid

4. When hydrochloric acid reacts with sodium carbonate, the products formed are: [CBSE 2021]
(a) NaCl + H₂O
(b) NaCl + H₂O + O₂
(c) NaCl + H₂O + CO₂
(d) NaCl + CO₂
Answer: (c)

5. Which salt does not contain water of crystallization? [CBSE 2022]
(a) Washing soda
(b) Gypsum
(c) Baking soda
(d) Copper sulphate
Answer: (c) Baking soda (NaHCO₃) has no water of crystallization

6. The pH of a solution is 8. On adding acid to it, the pH will: [CBSE 2020]
(a) Increase
(b) Decrease
(c) Remain same
(d) First increase then decrease
Answer: (b) Adding acid makes solution more acidic, so pH decreases

7. An aqueous solution turns red litmus blue. Excess addition of which substance will reverse the change? [CBSE 2021]
(a) HCl
(b) NaOH
(c) Ca(OH)₂
(d) NH₄OH
Answer: (a) HCl (acid) will neutralize the base and turn litmus red again

8. The correct formula of Plaster of Paris is: [CBSE 2019]
(a) CaSO₄
(b) CaSO₄·2H₂O
(c) CaSO₄·½H₂O
(d) 2CaSO₄·H₂O
Answer: (c)

9. Sodium carbonate is a basic salt because it is a salt of: [CBSE 2018]
(a) Strong acid and strong base
(b) Weak acid and weak base
(c) Strong acid and weak base
(d) Weak acid and strong base
Answer: (d) Weak acid (H₂CO₃) + Strong base (NaOH)

10. Tooth enamel is made up of: [CBSE 2017]
(a) Calcium phosphate
(b) Calcium carbonate
(c) Calcium hydroxide
(d) Calcium oxide
Answer: (a)

11. Fresh milk has pH 6. When it changes into curd, the pH will: [CBSE 2019]
(a) Increase
(b) Decrease
(c) Remain same
(d) Become zero
Answer: (b) Lactic acid forms, making it more acidic

12. During preparation of hydrogen chloride gas on a humid day, the gas is passed through a guard tube containing calcium chloride because: [CBSE 2018]
(a) CaCl₂ absorbs HCl gas
(b) CaCl₂ absorbs moisture from HCl gas
(c) CaCl₂ reacts with HCl
(d) CaCl₂ is hygroscopic
Answer: (b) CaCl₂ is hygroscopic and absorbs moisture

Short Answer Questions (1-2 Marks) - CBSE Board PYQs

Q1. What happens when a solution of an acid is mixed with a solution of a base in a test tube? [CBSE 2020]

Answer: When an acid solution is mixed with a base solution, neutralization reaction occurs. They react to form salt and water.

Example:

HCl+NaOH→NaCl+H2OHCl+NaOH→NaCl+H2O

Q2. Why does tooth decay start when the pH of mouth is lower than 5.5? [CBSE 2019]

Answer: Tooth enamel is made of calcium phosphate, which is the hardest substance in our body . It does not dissolve in water but gets corroded when pH in the mouth falls below 5.5. Bacteria in the mouth produce acids from leftover food, which lowers the pH and causes tooth decay.

Q3. What is meant by water of crystallization in a substance? Give an example. [CBSE 2021]

Answer: Water of crystallization is the fixed number of water molecules chemically combined with each formula unit of a salt in its crystalline form.

Example: Copper sulphate has 5 molecules of water of crystallization:
CuSO₄·5H₂O

Q4. Why does distilled water not conduct electricity whereas rainwater does? [CBSE 2018]

Answer: Distilled water is pure water without any dissolved ions, so it does not conduct electricity . Rainwater contains dissolved acids (from atmospheric pollution like SO₂ and NO₂) which provide H⁺ ions, making it a conductor of electricity.

Q5. Why should curd and sour substances not be kept in brass and copper vessels? [CBSE 2017]

Answer: Curd and sour substances contain acids which react with brass and copper vessels to form toxic metallic compounds that can cause food poisoning and harm health.

Q6. Name the substance which on treatment with chlorine yields bleaching powder. [CBSE 2016]

Answer: Slaked lime (Calcium hydroxide – Ca(OH)₂) reacts with chlorine to form bleaching powder:

Ca(OH)2+Cl2→CaOCl2+H2OCa(OH)2+Cl2→CaOCl2+H2O

Q7. How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide? [CBSE 2019]

Answer: When excess base is dissolved in sodium hydroxide solution, more OH⁻ ions are released into the solution. Therefore, the concentration of hydroxide ions (OH⁻) increases.

Long Answer Questions (3-5 Marks) - CBSE Board PYQs

Q1. (a) What happens when sodium carbonate reacts with dilute hydrochloric acid? Write chemical equation.
(b) State why calcium carbonate powder reacts with dilute HCl more vigorously than calcium carbonate marble chips. [CBSE 2020]

Answer:

(a) Sodium carbonate reacts with dilute HCl to form sodium chloride, water, and carbon dioxide gas with brisk effervescence:

Na2CO3+2HCl→2NaCl+H2O+CO2↑Na2CO3+2HCl→2NaCl+H2O+CO2↑

(b) Calcium carbonate powder has much larger surface area exposed to HCl compared to marble chips (lump). Greater surface area means more contact between reactant particles at the same time, leading to faster and more vigorous reaction .

Q2. What is meant by pH scale? Fresh milk has pH of 6. What happens to its pH when it changes into curd? Explain. [CBSE 2019]

Answer:

pH scale is a measure of hydrogen ion (H⁺) concentration in a solution. It ranges from 0 to 14:

pH < 7 → Acidic
pH = 7 → Neutral
pH > 7 → Basic

Fresh milk has pH 6 (slightly acidic). When milk changes into curd, bacteria produce lactic acid which increases the acidity of the curd . Therefore, the pH value decreases (becomes more acidic, pH becomes less than 6).

Q3. Write chemical formula of washing soda and baking soda. Which one will give CO₂ on heating? Write chemical equation. [CBSE 2018]

Answer:

Washing soda: Na₂CO₃·10H₂O
Baking soda: NaHCO₃

Baking soda (NaHCO₃) gives CO₂ on heating :

2NaHCO3→heatNa2CO3+H2O+CO2↑2NaHCO3heatNa2CO3+H2O+CO2↑

This property is used in baking to make cakes soft and spongy because CO₂ gas forms bubbles that make the cake fluffy.

Q4. (a) Define indicators. Give two examples.
(b) What color do the following indicators turn when added to a base?
(i) Methyl orange (ii) Litmus [CBSE 2017]

Answer:

(a) Indicators are substances which show different colors in acidic and basic solutions, helping us identify the nature of the solution .

Examples: Litmus solution, Phenolphthalein

(b) Color changes in base:
(i) Methyl orange turns yellow in base
(ii) Red litmus turns blue in base

Q5. State reason for the following:
(a) Plaster of Paris should be stored in a moisture-proof container.
(b) An aqueous solution of sodium chloride is neutral but that of sodium carbonate is basic. [CBSE 2020]

Answer:

(a) Plaster of Paris (CaSO₄·½H₂O) absorbs moisture from air and converts into hard gypsum (CaSO₄·2H₂O), losing its setting property and becoming useless:

CaSO4⋅12H2O+112H2O→CaSO4⋅2H2OCaSO4⋅21H2O+121H2O→CaSO4⋅2H2O

(b)

Sodium chloride (NaCl) is formed from strong acid (HCl) and strong base (NaOH). Both neutralize each other completely, hence its aqueous solution is neutral with pH = 7.

 Sodium carbonate (Na₂CO₃) is formed from weak acid (H₂CO₃) and strong base (NaOH). The strong base effect dominates, hence its solution is basic with pH > 7.

Q6. What is bleaching powder? How is it prepared? Write chemical equation. State two of its uses. [CBSE 2019]

Answer:

Bleaching powder is calcium oxychloride with chemical formula CaOCl₂.

Preparation:
Bleaching powder is prepared by passing chlorine gas over dry slaked lime (calcium hydroxide):

Ca(OH)2+Cl2→CaOCl2+H2OCa(OH)2+Cl2→CaOCl2+H2O

Uses:

Used for bleaching cotton and linen fabrics in textile industry
Used as a disinfectant for cleaning drinking water in water treatment plants

Q7. What is meant by the following terms? Give one example of each.
(a) Alkali (b) Neutralization (c) Salt [CBSE 2016]

Answer:

(a) Alkali: A base that is soluble in water is called an alkali .
Example: Sodium hydroxide (NaOH), Potassium hydroxide (KOH)

(b) Neutralization: The reaction between an acid and a base to form salt and water is called neutralization.

Example:

HCl+NaOH→NaCl+H2OHCl+NaOH→NaCl+H2O

(c) Salt: A compound formed by replacing the hydrogen of an acid with a metal or ammonium ion is called a salt.

Example: Sodium chloride (NaCl), Calcium sulphate (CaSO₄)

Q8. How is washing soda prepared from sodium chloride? Write chemical equations. State two uses of washing soda. [CBSE 2018]

Answer:

Preparation in 3 steps:

Step 1: Sodium chloride reacts with ammonia, water, and carbon dioxide to form baking soda (Solvay process):

NaCl+H2O+CO2+NH3→NaHCO3+NH4ClNaCl+H2O+CO2+NH3→NaHCO3+NH4Cl

Step 2: Baking soda is heated to get anhydrous sodium carbonate:

2NaHCO3→heatNa2CO3+H2O+CO2↑2NaHCO3heatNa2CO3+H2O+CO2↑

Step 3: Recrystallization with water gives washing soda:

Na2CO3+10H2O→Na2CO3⋅10H2ONa2CO3+10H2O→Na2CO3⋅10H2O

Uses:

Removing permanent hardness of water
Manufacturing of glass, soap, and paper

Q9. What happens when:
(a) Zinc granules react with dilute sulphuric acid
(b) Excess of carbon dioxide is passed through lime water
(c) Dilute hydrochloric acid is added to sodium carbonate
Write balanced chemical equations. [CBSE 2017]

Answer:

(a) Zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas:

Zn+H2SO4→ZnSO4+H2↑Zn+H2SO4→ZnSO4+H2↑

(b) Carbon dioxide passed through lime water turns it milky due to formation of calcium carbonate:

Ca(OH)2+CO2→CaCO3↓+H2OCa(OH)2+CO2→CaCO3↓+H2O

When excess CO₂ is passed, the milkiness disappears:

CaCO3+CO2+H2O→Ca(HCO3)2CaCO3+CO2+H2O→Ca(HCO3)2

(Soluble calcium bicarbonate forms)

(c) Dilute HCl reacts with sodium carbonate to form sodium chloride, water, and carbon dioxide with brisk effervescence:

Na2CO3+2HCl→2NaCl+H2O+CO2↑Na2CO3+2HCl→2NaCl+H2O+CO2↑

Q10. A white powder 'X' on strong heating gives a white residue 'Y' and a gas 'Z' which turns lime water milky. Residue 'Y' dissolves in water and the solution gives pink color with phenolphthalein. Identify X, Y and Z. Write balanced chemical equations for all reactions. [CBSE 2020]

Answer:

X = Ca(OH)₂ (Calcium hydroxide / Slaked lime)
Y = CaO (Calcium oxide / Quick lime)
Z = CO₂ (Carbon dioxide)

Explanations:

Reaction 1: Strong heating of calcium hydroxide:

Ca(OH)2→heatCaO+H2OCa(OH)2heatCaO+H2O

(White residue Y = CaO)

Reaction 2: Gas Z (CO₂) turns lime water milky:

Ca(OH)2+CO2→CaCO3↓+H2OCa(OH)2+CO2→CaCO3↓+H2O

(White precipitate makes it milky)

Reaction 3: Residue Y dissolves in water to form calcium hydroxide (base):

CaO+H2O→Ca(OH)2CaO+H2O→Ca(OH)2

(Basic solution turns phenolphthalein pink)

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Thursday, January 8, 2026