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NCERT Class 10 Science — Chapter 1

Chemical Reactions and Equations

Balanced chemical equations, combination, decomposition (thermal, electrolytic, photolytic), displacement, double displacement (precipitation), redox reactions, corrosion, and rancidity.

Quick Key Takeaways:
Law of Conservation of Mass: Mass cannot be created nor destroyed in a chemical reaction. The number of atoms of each element must remain equal on both sides: LHS=RHS\text{LHS} = \text{RHS}.
Types of Chemical Reactions: Combination (A+B→ABA + B \rightarrow AB), Decomposition (AB→Δ,hν,e−A+BAB \xrightarrow{\Delta, h\nu, e^-} A + B), Single Displacement (A+BC→AC+BA + BC \rightarrow AC + B), Double Displacement (AB+CD→AD+CBAB + CD \rightarrow AD + CB).
Redox Reactions: Oxidation is the gain of oxygen or loss of electrons/hydrogen; Reduction is the loss of oxygen or gain of electrons/hydrogen. Oxidizing agent gets reduced; Reducing agent gets oxidized.
Precipitation & Gas Tests: White precipitate BaSO4(s)\text{BaSO}_4(s) in double displacement; CO2\text{CO}_2 turns lime water milky; H2\text{H}_2 gas burns with a 'pop' sound.
Chemistry Equation & Reaction HelperBalance and verify all 26 NCERT Class 10 chemical equations with live atom audits
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Jump to Concept:1. 1. Chemical Reaction Types, Law of Conservation & Balancing Protocols2. 2. Corrosion, Rancidity & Experimental Activity Protocols3. 3. High-Yield Solved Board Examination Questions (3-Mark & 5-Mark)4. 4. Practical Laboratory & Competency-Based Case Drill: Electrolysis of Acidified Water (NCERT Activity 1.7)5. 5. CBSE Examiner Marking Scheme, Scientific Notation & Deduction Traps
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1. Chemical Reaction Types, Law of Conservation & Balancing Protocols

Fundamental Principles

Core scientific laws, chemical equations, anatomical structures, and visual model for Chemical Reactions and Equations.

• Core Types of Chemical Reactions with Balanced Equations
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1. Combination Reaction: Two or more reactants combine to form a single product.
CaO(s)+H2O(l)⟶Ca(OH)2(aq)+Heat\text{CaO}(s) + \text{H}_2\text{O}(l) \longrightarrow \text{Ca(OH)}_2(aq) + \text{Heat}
(Quicklime reacting vigorously with water to form slaked lime - highly exothermic).
•
2. Decomposition Reactions (Requires Energy Input):
- Thermal Decomposition: 2FeSO4(s)→ΔFe2O3(s)+SO2(g)+SO3(g)2\text{FeSO}_4(s) \xrightarrow{\Delta} \text{Fe}_2\text{O}_3(s) + \text{SO}_2(g) + \text{SO}_3(g) (Green crystals turn reddish-brown with choking sulphur smell);
2Pb(NO3)2(s)→Δ2PbO(s)+4NO2(g)+O2(g)2\text{Pb(NO}_3)_2(s) \xrightarrow{\Delta} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g) (White powder emits brown fumes of NO2\text{NO}_2).
- Electrolytic Decomposition: 2H2O(l)→Electricity2H2(g)+O2(g)2\text{H}_2\text{O}(l) \xrightarrow{\text{Electricity}} 2\text{H}_2(g) + \text{O}_2(g) (Volume ratio H2:O2=2:1\text{H}_2 : \text{O}_2 = 2 : 1).
- Photolytic Decomposition: 2AgCl(s)→Sunlight2Ag(s)+Cl2(g)2\text{AgCl}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g) (White turns grey in black-and-white photography).
• Displacement, Double Displacement & Redox Reactions
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3. Displacement Reaction: More reactive metal displaces less reactive metal:
Fe(s)+CuSO4(aq)⟶FeSO4(aq)+Cu(s)\text{Fe}(s) + \text{CuSO}_4(aq) \longrightarrow \text{FeSO}_4(aq) + \text{Cu}(s)
(Blue copper sulphate turns pale green; reddish-brown copper deposits on iron nail).
•
4. Double Displacement (Precipitation): Exchange of ions between reactants:
Na2SO4(aq)+BaCl2(aq)⟶BaSO4(s)↓+2NaCl(aq)\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \longrightarrow \text{BaSO}_4(s)\downarrow + 2\text{NaCl}(aq)
(Forms insoluble white precipitate of barium sulphate BaSO4\text{BaSO}_4).
•
5. Redox Reaction (Simultaneous Oxidation & Reduction):
CuO(s)+H2(g)→ΔCu(s)+H2O(l)\text{CuO}(s) + \text{H}_2(g) \xrightarrow{\Delta} \text{Cu}(s) + \text{H}_2\text{O}(l)
- CuO\text{CuO} loses Oxygen   ⟹  \implies Reduced to Cu\text{Cu} (Oxidizing Agent: CuO\text{CuO}).
- H2\text{H}_2 gains Oxygen   ⟹  \implies Oxidized to H2O\text{H}_2\text{O} (Reducing Agent: H2\text{H}_2).
📊 Chemical Reactions: Redox Oxidation-Reduction ArchitectureVisual Model
REDOX REACTION: CuO + H₂ ──Δ──> Cu + H₂OCuO+H₂──Δ──>Cu+H₂OREDUCTION (Loss of O)OXIDATION (Gain of O)

Visual schematic mapping electron transfer, oxygen loss/gain, oxidizing vs reducing agents, and decomposition reaction pathways.

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2. Corrosion, Rancidity & Experimental Activity Protocols

Mechanisms & Experiments

Step-by-step chemical reaction mechanisms, experimental activities, and physiological pathways for Chemical Reactions and Equations.

• Corrosion & Rancidity Prevention Mechanisms
•
Corrosion of Iron (Rusting): 4Fe+3O2+2xH2O⟶2Fe2O3⋅xH2O4\text{Fe} + 3\text{O}_2 + 2x\text{H}_2\text{O} \longrightarrow 2\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O} (Hydrated ferric oxide - reddish-brown flaky layer).
•
Corrosion of Copper: Reacts with moist CO2\text{CO}_2 in air to form basic copper carbonate [CuCO3⋅Cu(OH)2][\text{CuCO}_3 \cdot \text{Cu(OH)}_2] (green coating).
•
Corrosion of Silver: Reacts with H2S\text{H}_2\text{S} in air to form silver sulphide Ag2S\text{Ag}_2\text{S} (black coating).
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Rancidity: Slow oxidation of unsaturated fats/oils producing foul odor and taste. Prevented by: (1) Flushing packaging with inert Nitrogen gas (N2\text{N}_2), (2) Adding antioxidants (BHA, BHT), (3) Vacuum packing, (4) Refrigeration.
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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: A shiny brown-coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed. Write the balanced chemical equation.
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Element 'X': Copper metal (Cu\text{Cu}), which has a characteristic shiny reddish-brown appearance.
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Black Compound: Copper(II) Oxide (CuO\text{CuO}), formed when copper reacts with atmospheric oxygen upon heating.
•
Balanced Chemical Equation:
2Cu(s)+O2(g)→Δ2CuO(s)2\text{Cu}(s) + \text{O}_2(g) \xrightarrow{\Delta} 2\text{CuO}(s)
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Reverse Reaction with Hydrogen: If hydrogen gas is passed over this heated black CuO\text{CuO}, the black coating turns brown again as reverse reduction takes place: CuO(s)+H2(g)→ΔCu(s)+H2O(l)\text{CuO}(s) + \text{H}_2(g) \xrightarrow{\Delta} \text{Cu}(s) + \text{H}_2\text{O}(l)
• 5-Mark Comprehensive Question / Numerical: (a) Write balanced chemical equations for thermal decomposition of Lead Nitrate and Ferrous Sulphate crystals.
(b) Identify the substance oxidized, reduced, oxidizing agent, and reducing agent in: MnO2+4HCl→MnCl2+2H2O+Cl2\text{MnO}_2 + 4\text{HCl} \rightarrow \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2.
•
Part (a) Equations:
1. Thermal Decomposition of Lead Nitrate:
2Pb(NO3)2(s)→Δ2PbO(s)+4NO2(g)+O2(g)2\text{Pb(NO}_3)_2(s) \xrightarrow{\Delta} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g)
(Observation: White solid forms yellow residue of PbO\text{PbO} and pungent brown fumes of NO2\text{NO}_2 gas).
2. Thermal Decomposition of Green Ferrous Sulphate:
2FeSO4(s)→ΔFe2O3(s)+SO2(g)+SO3(g)2\text{FeSO}_4(s) \xrightarrow{\Delta} \text{Fe}_2\text{O}_3(s) + \text{SO}_2(g) + \text{SO}_3(g)
(Observation: Green crystals FeSO4⋅7H2O\text{FeSO}_4 \cdot 7\text{H}_2\text{O} lose water of crystallization, turn reddish-brown with burning sulphur smell).
•
Part (b) Redox Analysis of MnO2+4HCl→MnCl2+2H2O+Cl2\text{MnO}_2 + 4\text{HCl} \rightarrow \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2:
- Substance Oxidized: HCl\text{HCl} (loses hydrogen to form Cl2\text{Cl}_2).
- Substance Reduced: MnO2\text{MnO}_2 (loses oxygen to form MnCl2\text{MnCl}_2).
- Oxidizing Agent: MnO2\text{MnO}_2 (supplies oxygen/causes oxidation of HCl\text{HCl}).
- Reducing Agent: HCl\text{HCl} (removes oxygen from MnO2\text{MnO}_2).
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4. Practical Laboratory & Competency-Based Case Drill: Electrolysis of Acidified Water (NCERT Activity 1.7)

Practical & Case Drill

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

• Laboratory Activity Context: Electrolysis of Acidified Water (NCERT Activity 1.7)
During the electrolysis of water in a voltameter using graphite electrodes and a few drops of dilute sulphuric acid, gas bubbles collect in test tubes inverted over both electrodes.
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Q1: Name the gases evolved at the cathode and anode. →\rightarrow Cathode (negative electrode) collects Hydrogen gas (H2\text{H}_2); Anode (positive electrode) collects Oxygen gas (O2\text{O}_2).
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Q2: Why is the volume of gas collected at one electrode double that at the other? →\rightarrow Water molecule contains 2 hydrogen atoms for every 1 oxygen atom (2H2O→electricity2H2+O22\text{H}_2\text{O} \xrightarrow{\text{electricity}} 2\text{H}_2 + \text{O}_2). Hence, the molar volume ratio of H2:O2\text{H}_2 : \text{O}_2 is strictly 2:12 : 1.
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Q3: What happens when a burning candle is brought near each test tube? →\rightarrow Near cathode tube (H2\text{H}_2), the gas burns with a distinctive 'pop' sound. Near anode tube (O2\text{O}_2), the candle flame burns more brightly (supports combustion).
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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: Writing complete chemical equations with correct physical state symbols (s,l,g,aqs, l, g, aq).
•
1 Mark: Mentioning specific sensory observations (e.g. brown fumes of NO2\text{NO}_2, pale green solution).
•
2 Marks: Correctly identifying oxidizing agent, reducing agent, substance oxidized, and reduced.
•
1 Mark: Stating the reaction conditions over the arrow (Δ\Delta, sunlight, electric current).
• Common Error Deduction Traps
•
Trap 1: Forgetting to balance equations before answering redox or mole ratio questions.
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Trap 2: Confusing cathode (negative, attracts H+\text{H}^+ to form H2\text{H}_2) with anode (positive, attracts OH−\text{OH}^- to form O2\text{O}_2).
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Trap 3: Misidentifying the oxidizing agent as the substance that gets oxidized (the oxidizing agent gets reduced).
Authentic Board Question (3 Marks)Topic: Corrosion & Acid-Base Reaction
A copper vessel acquires a green coating when exposed to moist air for a long time. (a) Identify the green compound formed. (b) Write the balanced chemical reaction. (c) How can this green layer be cleaned with lemon juice?

Official CBSE Step-by-Step Marking Breakdown:

Part (a): Compound Identification: Identify Basic Copper Carbonate [CuCO3⋅Cu(OH)2][\text{CuCO}_3 \cdot \text{Cu(OH)}_2].
1 Mark
Part (b): Balanced Equation: 2Cu+H2O+CO2+O2→CuCO3⋅Cu(OH)22\text{Cu} + \text{H}_2\text{O} + \text{CO}_2 + \text{O}_2 \to \text{CuCO}_3 \cdot \text{Cu(OH)}_2.
1 Mark
Part (c): Cleaning Action: Lemon contains citric acid which reacts with basic copper carbonate to form soluble copper citrate, washing the layer away.
1 Mark
Model Student Answer (Target: Full 3/3 Marks):
(a) The green substance formed on the surface of copper is Basic Copper Carbonate [CuCO3⋅Cu(OH)2][\text{CuCO}_3 \cdot \text{Cu(OH)}_2].

(b) Balanced Chemical Reaction:
2Cu(s)+H2O(l)+CO2(g)+O2(g)⟶CuCO3⋅Cu(OH)2(s)2\text{Cu(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} + \text{O}_2\text{(g)} \longrightarrow \text{CuCO}_3 \cdot \text{Cu(OH)}_2\text{(s)}

(c) Cleaning Mechanism: Basic copper carbonate is basic in nature. Lemon juice contains citric acid. When rubbed with lemon juice, the acid undergoes a neutralisation reaction with the basic copper carbonate, converting it into water-soluble copper citrate salt which easily washes away, restoring the reddish-brown shine of copper.
Examiner Mark Deduction Traps:
•Write both components of the basic carbonate [CuCO3+Cu(OH)2][\text{CuCO}_3 + \text{Cu(OH)}_2] to earn full 1 mark for identification.
•Always mention the word "neutralisation reaction" when explaining acid cleaning.

High-Frequency Conceptual Doubts & FAQs

Curated answers to the most common questions asked by Class 10 students.
During respiration, glucose reacts with oxygen in cells to release energy: C6H12O6+6O2→6CO2+6H2O+Energy (ATP)\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \to 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy (ATP)}. In photosynthesis, green plants absorb solar energy to synthesize glucose: 6CO2+6H2O+Sunlight→C6H12O6+6O26\text{CO}_2 + 6\text{H}_2\text{O} + \text{Sunlight} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2. Releasing energy makes respiration exothermic; absorbing energy makes photosynthesis endothermic.

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