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NCERT Class 10 Science — Chapter 2

Acids, Bases and Salts

Arrhenius definitions (H+,OH−\text{H}^+, \text{OH}^- ions), indicators (litmus, phenolphthalein, methyl orange, olfactory), pH scale, neutralization, and industrial preparation of Bleaching Powder, Baking Soda, Washing Soda, and Plaster of Paris.

Quick Key Takeaways:
Acids and Bases in Water: Acids generate hydronium ions H3O+\text{H}_3\text{O}^+ in aqueous solution; Bases generate hydroxide ions OH−\text{OH}^-. Pure dry HCl\text{HCl} gas does not change dry litmus paper.
pH Scale (0−140-14): pH=−log⁡[H+]\text{pH} = -\log[\text{H}^+]. pH<7\text{pH} < 7 is acidic, pH=7\text{pH} = 7 is neutral, pH>7\text{pH} > 7 is basic. Rainwater with pH<5.6\text{pH} < 5.6 is acid rain.
Chlor-Alkali Process: Electrolysis of brine (2NaCl+2H2O→2NaOH+Cl2+H22\text{NaCl} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{Cl}_2 + \text{H}_2) yields NaOH\text{NaOH} (cathode), Cl2\text{Cl}_2 (anode), and H2\text{H}_2 (cathode).
Important Industrial Salts: Bleaching powder CaOCl2\text{CaOCl}_2, Baking soda NaHCO3\text{NaHCO}_3, Washing soda Na2CO3⋅10H2O\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}, Plaster of Paris CaSO4⋅12H2O\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}.
Chemistry Equation & Reaction HelperExplore neutralization, metal-acid, carbonate, and bleaching powder reactions
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Jump to Concept:1. 1. Acids, Bases, Indicators & The Universal pH Scale2. 2. Industrial Preparation of Important Salts & Water of Crystallization3. 3. High-Yield Solved Board Examination Questions (3-Mark & 5-Mark)4. 4. Practical Laboratory & Competency-Based Case Drill: Water of Crystallization in Hydrated Copper Sulphate Crystals5. 5. CBSE Examiner Marking Scheme, Scientific Notation & Deduction Traps
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1. Acids, Bases, Indicators & The Universal pH Scale

Fundamental Principles

Core scientific laws, chemical equations, anatomical structures, and visual model for Acids, Bases and Salts.

• Properties of Acids and Bases & Chemical Reactions
•
Reaction with Metals: Acid+Metal⟶Salt+H2(g)↑\text{Acid} + \text{Metal} \longrightarrow \text{Salt} + \text{H}_2(g)\uparrow
2HCl(aq)+Zn(s)⟶ZnCl2(aq)+H2(g)↑2\text{HCl}(aq) + \text{Zn}(s) \longrightarrow \text{ZnCl}_2(aq) + \text{H}_2(g)\uparrow
Base+Metal:2NaOH(aq)+Zn(s)⟶Na2ZnO2(aq) (Sodium Zincate)+H2(g)↑\text{Base} + \text{Metal}: 2\text{NaOH}(aq) + \text{Zn}(s) \longrightarrow \text{Na}_2\text{ZnO}_2(aq) \text{ (Sodium Zincate)} + \text{H}_2(g)\uparrow
•
Reaction with Carbonates & Hydrogencarbonates:
Na2CO3(s)+2HCl(aq)⟶2NaCl(aq)+H2O(l)+CO2(g)↑\text{Na}_2\text{CO}_3(s) + 2\text{HCl}(aq) \longrightarrow 2\text{NaCl}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)\uparrow
Ca(OH)2(aq)+CO2(g)⟶CaCO3(s)↓ (Milky)+H2O(l)\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \longrightarrow \text{CaCO}_3(s)\downarrow \text{ (Milky)} + \text{H}_2\text{O}(l)
(Excess CO2\text{CO}_2 dissolves milkiness by forming soluble Ca(HCO3)2(aq)\text{Ca(HCO}_3)_2(aq)).
•
Neutralization Reaction: Acid+Base⟶Salt+Water+Heat\text{Acid} + \text{Base} \longrightarrow \text{Salt} + \text{Water} + \text{Heat}
HCl(aq)+NaOH(aq)⟶NaCl(aq)+H2O(l)\text{HCl}(aq) + \text{NaOH}(aq) \longrightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)
• Indicators & pH Scale Colour Chart
•
Litmus: Acid →\rightarrow Red; Base →\rightarrow Blue.
•
Phenolphthalein: Acid →\rightarrow Colourless; Base →\rightarrow Pink.
•
Methyl Orange: Acid →\rightarrow Red/Pink; Base →\rightarrow Yellow.
•
Olfactory Indicators (Vanilla, Onion, Clove oil): Retain smell in acid; lose characteristic smell in base.
•
pH Scale: 0−20-2 (Strong Acid: HCl\text{HCl}, stomach acid); 77 (Pure Water); 1010 (Milk of Magnesia Mg(OH)2\text{Mg(OH)}_2); 1414 (Strong Base: NaOH\text{NaOH}).
📊 Acids & Bases: Universal pH Scale & Salt ArchitectureVisual Model
pH 0Strong Acid (HCl)pH 7 (Neutral)Pure WaterpH 14Strong Base (NaOH)Increasing H⁺ Concentration ←→ Increasing OH⁻ ConcentrationLitmus: Acid = Red | Base = Blue ── Phenolphthalein: Acid = Colourless | Base = Pink

Visual schematic mapping the logarithmic pH scale from 0 to 14, indicator color transitions, and the industrial Chlor-Alkali synthesis pathway.

2

2. Industrial Preparation of Important Salts & Water of Crystallization

Mechanisms & Experiments

Step-by-step chemical reaction mechanisms, experimental activities, and physiological pathways for Acids, Bases and Salts.

• Preparation, Equations & Uses of Common Salts
•
1. Bleaching Powder (CaOCl2\text{CaOCl}_2): Prepared by passing chlorine gas over dry slaked lime:
Ca(OH)2(s)+Cl2(g)⟶CaOCl2(s)+H2O(l)\text{Ca(OH)}_2(s) + \text{Cl}_2(g) \longrightarrow \text{CaOCl}_2(s) + \text{H}_2\text{O}(l)
Uses: Disinfectant in drinking water, bleaching cotton in textile mills.
•
2. Baking Soda (NaHCO3\text{NaHCO}_3 - Sodium Hydrogencarbonate):
NaCl+H2O+CO2+NH3⟶NH4Cl+NaHCO3\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \longrightarrow \text{NH}_4\text{Cl} + \text{NaHCO}_3
On heating during cooking: 2NaHCO3→ΔNa2CO3+H2O+CO2↑2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2\uparrow (CO2 makes cakes soft and spongy).
•
3. Washing Soda (Na2CO3⋅10H2O\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}): Recrystallization of sodium carbonate:
Na2CO3+10H2O⟶Na2CO3⋅10H2O\text{Na}_2\text{CO}_3 + 10\text{H}_2\text{O} \longrightarrow \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}
Uses: Glass, soap, paper industries; removing permanent hardness of water.
•
4. Plaster of Paris (CaSO4⋅12H2O\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}): Heating gypsum at 373 K (100°C):
CaSO4⋅2H2O→373 KCaSO4⋅12H2O+112H2O\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O}
On mixing with water: Sets hard into Gypsum within 10–15 minutes.
3

3. High-Yield Solved Board Examination Questions (3-Mark & 5-Mark)

Solved Board Questions

Standard CBSE board exam questions with complete scientific justifications and marking scheme step protocols.

• 3-Mark Standard Board Question: Why does dry HCl gas not change the colour of dry litmus paper, whereas dilute HCl does? Explain with the Arrhenius concept.
•
Reasoning: Acids show acidic behavior and release hydrogen ions (H+\text{H}^+) only in the presence of water.
•
Chemical Mechanism: When HCl\text{HCl} dissolves in water, polar water molecules assist in the ionization of HCl\text{HCl} to produce hydronium ions:
HCl+H2O⟶H3O++Cl−\text{HCl} + \text{H}_2\text{O} \longrightarrow \text{H}_3\text{O}^+ + \text{Cl}^-
•
In Dry Conditions: Dry HCl\text{HCl} gas contains un-ionized covalent molecules without free H+\text{H}^+ or H3O+\text{H}_3\text{O}^+ ions. Dry litmus paper also contains no moisture. Therefore, no colour change occurs.
•
Dilute HCl\text{HCl}: Contains abundant H3O+\text{H}_3\text{O}^+ ions that turn blue litmus paper red immediately.
• 5-Mark Comprehensive Question / Numerical: A compound 'A' of sodium is used as an antacid and in fire extinguishers.
(a) Identify compound 'A' and write its chemical formula.
(b) What happens when compound 'A' is heated? Give the balanced equation.
(c) How is compound 'A' converted into washing soda? Give balanced equations.
•
Part (a) Identification: Compound 'A' is Sodium Hydrogencarbonate (Baking Soda). Chemical Formula: NaHCO3\mathbf{\text{NaHCO}_3}.
•
Part (b) Effect of Heat: When heated, it decomposes thermally into sodium carbonate, water, and carbon dioxide gas:
2NaHCO3(s)→ΔNa2CO3(s)+H2O(l)+CO2(g)↑2\text{NaHCO}_3(s) \xrightarrow{\Delta} \text{Na}_2\text{CO}_3(s) + \text{H}_2\text{O}(l) + \text{CO}_2(g)\uparrow
•
Part (c) Conversion into Washing Soda:
The anhydrous sodium carbonate (soda ash) obtained by heating is dissolved in water and recrystallized with 10 molecules of water of crystallization:
Na2CO3(s)+10H2O(l)⟶Na2CO3⋅10H2O(s)\text{Na}_2\text{CO}_3(s) + 10\text{H}_2\text{O}(l) \longrightarrow \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}(s)
(Compound formed is Sodium Carbonate Decahydrate - Washing Soda).
4

4. Practical Laboratory & Competency-Based Case Drill: Water of Crystallization in Hydrated Copper Sulphate Crystals

Practical & Case Drill

Experimental observation analysis, chemical gas tests, and assertion-reason drills.

• Laboratory Activity Context: Water of Crystallization in Hydrated Copper Sulphate Crystals
Blue crystals of Copper Sulphate (CuSO4⋅5H2O\text{CuSO}_4 \cdot 5\text{H}_2\text{O}) are heated in a dry boiling test tube.
•
Q1: What colour change is observed during heating? →\rightarrow The blue crystals lose their 5 molecules of water of crystallization and turn into a white anhydrous powder (CuSO4\text{CuSO}_4).
•
Q2: What is observed on the inner walls of the test tube? →\rightarrow Tiny droplets of liquid water condense on the cooler upper walls of the tube.
•
Q3: What happens when 2-3 drops of water are added back to the white powder? →\rightarrow The white powder immediately rehydrates and restores its brilliant blue colour (CuSO4+5H2O→CuSO4⋅5H2OCuSO_4 + 5H_2O \rightarrow CuSO_4 \cdot 5H_2O).
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5. CBSE Examiner Marking Scheme, Scientific Notation & Deduction Traps

Important Solved Board Questions

Examiner step-marking allocations, mandatory scientific terminology, and common error avoidance.

• Step-by-Step Marking Rubric & Key Terminology
•
1 Mark: Exact chemical name and correct stoichiometric formula of salt.
•
1 Mark: Balanced equation of preparation with temperature conditions (e.g. 373 K for POP).
•
2 Marks: Explaining ionization with hydronium ion (H3O+\text{H}_3\text{O}^+) equations.
•
1 Mark: Stating specific practical applications (e.g. baking powder ingredient: NaHCO3+tartaric acid\text{NaHCO}_3 + \text{tartaric acid}).
• Common Error Deduction Traps
•
Trap 1: Heating gypsum above 373 K produces 'dead burnt plaster' (anhydrous CaSO4\text{CaSO}_4) which loses the setting property of POP.
•
Trap 2: Confusing Baking Soda (pure NaHCO3\text{NaHCO}_3) with Baking Powder (mixture of NaHCO3\text{NaHCO}_3 and mild edible acid like tartaric acid to neutralize bitter Na2CO3\text{Na}_2\text{CO}_3).
•
Trap 3: Forgetting water of crystallization formulas (5H2O5\text{H}_2\text{O} in copper sulphate, 10H2O10\text{H}_2\text{O} in washing soda, 12H2O\frac{1}{2}\text{H}_2\text{O} in POP).
Authentic Board Question (3 Marks)Topic: Acid-Metal Reactions & Ionisation Mechanism
(a) Write the balanced chemical equation for the reaction of dilute hydrochloric acid with zinc granules and describe the confirmation test for the gas evolved. (b) Explain why dry HCl gas does not change the colour of dry blue litmus paper.

Official CBSE Step-by-Step Marking Breakdown:

Part (a) Equation & Observation: Write balanced equation: Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)\text{Zn(s)} + 2\text{HCl(aq)} \to \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)} with effervescence.
1 Mark
Part (a) Hydrogen Gas Test: Bring a burning candle near the mouth of the test tube; the gas burns with a characteristic "pop sound".
1 Mark
Part (b) Hydronium Ion Explanation: Dry HCl\text{HCl} does not dissociate into H+/H3O+\text{H}^+\text{/}\text{H}_3\text{O}^+ ions in the absence of water; acidic properties require aqueous dissociation.
1 Mark
Model Student Answer (Target: Full 3/3 Marks):
(a) Reaction with Zinc Granules & Gas Test:
- Balanced Chemical Equation:
Zn(s)+2HCl(aq)⟶ZnCl2(aq)+H2(g)\text{Zn(s)} + 2\text{HCl(aq)} \longrightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}
- Confirmation Test: When a burning splinter or candle is brought near the mouth of the test tube containing the evolved gas, it burns with a characteristic "pop" sound, confirming the presence of Hydrogen (H2\text{H}_2) gas.

(b) Why Dry HCl\text{HCl} Gas Does Not Affect Dry Litmus:
Acids exhibit acidic behavior solely due to the presence of free hydrogen ions (H+\text{H}^+) or hydronium ions (H3O+\text{H}_3\text{O}^+).
HCl+H2O⟶H3O++Cl−\text{HCl} + \text{H}_2\text{O} \longrightarrow \text{H}_3\text{O}^+ + \text{Cl}^-
In the complete absence of moisture/water, dry HCl\text{HCl} exists as covalent molecules and cannot dissociate to produce H+\text{H}^+ ions. Consequently, it shows no acidic properties and cannot turn dry blue litmus red.
Examiner Mark Deduction Traps:
•Always specify state symbols (s,aq,g)(s, aq, g) in the chemical equation to avoid losing ½ mark.
•Mention "hydronium ions (H₃O⁺) in presence of water" for Part (b).

High-Frequency Conceptual Doubts & FAQs

Curated answers to the most common questions asked by Class 10 students.
Acids exhibit acidic character and produce hydrogen/hydronium ions (H+\text{H}^+ or H3O+\text{H}_3\text{O}^+) ONLY in the presence of water: HCl+H2O→H3O++Cl−\text{HCl} + \text{H}_2\text{O} \to \text{H}_3\text{O}^+ + \text{Cl}^-. In the absence of moisture, dry HCl\text{HCl} gas cannot dissociate into ions and therefore does not change dry litmus colour.

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