NCERT Class 10 Science Chapter 1 – Chemical Reactions and Equations

NCERT Class 10 Science Chapter 1 introduces students to the concept of chemical reactions and their symbolic representation through chemical equations. In NCERT Class 10 Science Chapter 1, learners understand how substances change during reactions and how to write and balance chemical equations correctly.

The chapter begins with everyday examples such as rusting of iron, burning of magnesium, souring of milk, and digestion of food, explaining how chemical changes differ from physical changes. Students learn that observable changes like evolution of gas, change in colour, change in temperature, and formation of precipitate indicate a chemical reaction.

A major focus of NCERT Class 10 Science Chapter 1 is writing balanced chemical equations. The chapter explains the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction. The hit-and-trial method is used to balance equations without changing chemical formulae.

Another important section of NCERT Class 10 Science Chapter 1 covers types of chemical reactions:

• Combination reactions (e.g., CaO + H₂O → Ca(OH)₂)
• Decomposition reactions (thermal, electrolytic, photochemical)
• Displacement reactions
• Double displacement reactions (precipitation reactions)
• Oxidation and reduction (Redox reactions)

The chapter also explains practical applications such as corrosion (rusting of iron) and rancidity (oxidation of fats and oils). These examples connect chemistry concepts with real-life situations.

For board exams and foundation-level competitive exams, NCERT Class 10 Science Chapter 1 is important because it builds the base for advanced chemistry topics in higher classes. Students should always cross-check concepts with the official NCERT textbook available on the NCERT website to ensure accurate and updated learning.

For structured preparation of NCERT Class 9–12 for UPSC, BPSC and State PCS examinations, strengthen your basics with our complete NCERT Book Notes PDF for Class 9-12, available inside the NCERT foundation course level-2.

Access Complete NCERT Book Notes PDF

Introduction

• A chemical reaction takes place when the nature and identity of a substance change, such as when milk turns sour, iron rusts, grapes ferment, food cooks, food digests, and during respiration
• In these situations, a chemical change occurs because new substances are formed and the original substances lose their original properties
• In Activity 1.1, a magnesium ribbon burns in air with a dazzling white flame and forms a white powder called magnesium oxide, showing formation of a new compound
• In Activity 1.2, when zinc reacts with dilute sulphuric acid, hydrogen gas is evolved and the temperature increases, indicating gas evolution and heat production
• In Activity 1.3, mixing lead nitrate solution and potassium iodide solution produces a yellow precipitate of lead iodide, showing formation of an insoluble substance
• The main observable signs of a chemical reaction are change in state, change in colour, evolution of gas, and change in temperature
• Chemical reactions involve breaking and making of bonds between atoms, resulting in the formation of new substances
• This chapter explains the symbolic representation of chemical reactions and different types of chemical reactions

1.1 Chemical Equations

• A chemical equation represents a chemical reaction symbolically, showing the reactants, products, and direction of the reaction using an arrow
• A word equation expresses a reaction using names of substances, for example:
  Magnesium + Oxygen → Magnesium oxide, where substances on the left are reactants and those on the right are products
• A chemical equation written using symbols and formulae is more precise, for example:
  Mg + O₂ → MgO, which is called a skeletal chemical equation because it is not balanced
• In a chemical reaction, atoms do not disappear or appear; according to the Law of Conservation of Mass, the total number of atoms of each element must remain the same before and after reaction
• A chemical equation is said to be balanced when the number of atoms of each element on the left-hand side (LHS) equals the number on the right-hand side (RHS)
• The common method used to balance equations is the hit-and-trial method, where coefficients are adjusted without changing the chemical formulae, as shown in the balancing of
  Fe + H₂O → Fe₃O₄ + H₂
• While balancing, only coefficients are changed and never the chemical formulae, because changing formulae alters the substance itself
• To make equations more informative, physical states are mentioned using symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution, for example:
  3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
• Sometimes reaction conditions such as heat, pressure, catalyst, sunlight, or chlorophyll are written above or below the arrow, for example in photosynthesis:
  6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ (in presence of sunlight and chlorophyll)
• Writing balanced and state-specified equations helps in understanding the quantitative relationship between reactants and products in a reaction

1.2 Types of Chemical Reactions

• During a chemical reaction, atoms of one element do not change into atoms of another element and no atoms are created or destroyed
• Chemical reactions involve breaking and making of bonds between atoms, leading to formation of new substances
• Based on the nature of change, chemical reactions are classified into different types such as Combination, Decomposition, Displacement, Double Displacement, and Oxidation-Reduction reactions

1.2.1 Combination Reaction

• A combination reaction is one in which two or more substances combine to form a single product
• In Activity 1.4, calcium oxide (quick lime) reacts with water to form calcium hydroxide (slaked lime) releasing heat:
  CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (1.13)
• Burning of coal is a combination reaction:
  C(s) + O₂(g) → CO₂(g) (1.15)
• Formation of water from hydrogen and oxygen is also a combination reaction:
  2H₂(g) + O₂(g) → 2H₂O(l) (1.16)
• Reactions in which heat is released along with products are called exothermic reactions
• Respiration is an exothermic reaction where glucose reacts with oxygen to produce energy:
  C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy (1.18)

1.2.2 Decomposition Reaction

• A decomposition reaction is the opposite of combination reaction, where a single substance breaks down into two or more simpler substances
• Decomposition reactions require energy in the form of heat, light, or electricity
• Thermal decomposition example:
  CaCO₃(s) → CaO(s) + CO₂(g)
• Electrolytic decomposition of water:
  2H₂O(l) → 2H₂(g) + O₂(g)
• Photochemical decomposition example:
  2AgCl(s) → 2Ag(s) + Cl₂(g) (in presence of sunlight)
• Reactions in which energy is absorbed are called endothermic reactions

1.2.3 Displacement Reaction

• A displacement reaction occurs when a more reactive element displaces a less reactive element from its compound
• In Activity 1.9, when an iron nail is dipped in copper sulphate solution, the blue colour fades and iron becomes brown due to copper deposition:
  Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) (1.24)
• Other examples include:
  Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) (1.25)
  Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s) (1.26)
• In displacement reactions, the more reactive metal replaces the less reactive metal

1.2.4 Double Displacement Reaction

• A double displacement reaction involves exchange of ions between two reactants
• In Activity 1.10, mixing sodium sulphate solution and barium chloride solution forms a white precipitate of barium sulphate:
  Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (1.27)
• A reaction producing an insoluble solid (precipitate) is called a precipitation reaction
• These reactions involve exchange of positive and negative ions between compounds

1.2.5 Oxidation and Reduction

• In Activity 1.11, heating copper powder forms black copper(II) oxide due to addition of oxygen:
  2Cu + O₂ → 2CuO (1.28)
• Passing hydrogen over heated copper oxide reduces it back to copper:
  CuO + H₂ → Cu + H₂O (1.29)
• Oxidation is defined as gain of oxygen or loss of hydrogen
• Reduction is defined as loss of oxygen or gain of hydrogen
• Reactions in which oxidation and reduction occur simultaneously are called redox reactions
• Example of redox reaction:
  ZnO + C → Zn + CO (1.31)

1.3 Have You Observed the Effects of Oxidation Reactions in Everyday Life?

• Oxidation reactions occur in daily life and often cause damage to materials or spoilage of food
• When a metal reacts with substances like moisture, air, or acids, it undergoes corrosion, which slowly destroys the metal surface

1.3.1 Corrosion

• Corrosion is the process in which a metal is attacked by substances in its environment such as oxygen and moisture
• Rusting of iron is the most common example, where iron forms a reddish-brown coating called rust
• Other examples include the black coating on silver and the green coating on copper, showing oxidation of these metals
• Corrosion causes serious damage to bridges, ships, car bodies, railings, and iron structures, leading to heavy economic loss
• To prevent corrosion, methods like painting, oiling, greasing, or galvanising are used

1.3.2 Rancidity

• When fats and oils are oxidised, they become rancid, causing unpleasant smell and taste in food
• Rancidity can be prevented by adding antioxidants to food products
• Storing food in air-tight containers reduces exposure to oxygen and slows oxidation
• Food packets such as chips are flushed with nitrogen gas to prevent oxidation and increase shelf life

Exam Oriented Facts

• Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction
• A skeletal chemical equation is an unbalanced chemical equation
• The method used for balancing equations in NCERT is the hit-and-trial method
• Physical state symbols used in equations are: (s) solid, (l) liquid, (g) gas, (aq) aqueous
• Combination reaction example:
  CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (1.13)
• Exothermic reaction example:
  C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (Respiration) (1.18)
• Decomposition reaction example (Photochemical):
  2AgCl(s) → 2Ag(s) + Cl₂(g) (in presence of sunlight)
• Displacement reaction example:
  Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) (1.24)
• Double displacement (precipitation) reaction example:
  Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (1.27)
• Oxidation means gain of oxygen or loss of hydrogen
• Reduction means loss of oxygen or gain of hydrogen
• Redox reaction example:
  CuO + H₂ → Cu + H₂O (1.29)
• Corrosion example: Rusting of iron, formation of reddish-brown rust
• Rancidity is caused due to oxidation of fats and oils, prevented by antioxidants and nitrogen flushing

Understanding NCERT Class 10 Science Chapter 1 – Chemical Reactions and Equations helps students master the basics of chemical transformations and reaction types.

NCERT Class 10 Science Chapter 1 forms the foundation for topics like acids, bases, metals, carbon compounds, and electrochemistry in higher classes.

Students preparing for exams should refer to the official NCERT website for authentic textbooks and syllabus updates.

Continue reading NCERT Class 10 Science Chapter 2 – Acids, Bases and Salts to strengthen your chemistry fundamentals further.

FAQs

Q1. What is NCERT Class 10 Science Chapter 1 about?
It explains chemical reactions, balanced equations, types of reactions, corrosion and rancidity.

Q2. Why is balancing chemical equations important?
It follows the Law of Conservation of Mass and ensures equal number of atoms on both sides.

Q3. What are the main types of chemical reactions in Chapter 1?
Combination, decomposition, displacement, double displacement and redox reactions.

Q4. What is corrosion?
Corrosion is the gradual destruction of metals due to reaction with air and moisture, such as rusting of iron.

Q5. Why is Chapter 1 important for exams?
It builds the conceptual base for all chemistry chapters in Class 10 and higher studies.

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