Class 10 Science Chapter 1 Chemical Reactions and Equations NCERT Solutions 2026 PDF Download
📝 Introduction
Welcome to the first chapter of Class 10 Science! Chemical Reactions and Equations forms the foundation of Chemistry. In this chapter, students will explore how substances interact to form new substances with entirely different properties. You will learn how to represent these changes using chemical equations and the crucial skill of balancing them to satisfy the Law of Conservation of Mass. Furthermore, the chapter categorizes reactions into distinct types such as Combination, Decomposition, Displacement, and Redox reactions, giving you the tools to predict the outcome of various chemical changes in daily life.
[Image of types of chemical reactions]🔑 Key Concepts & Formulas
- Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction. Total mass of reactants = Total mass of products.
- Combination Reaction: Two or more reactants combine to form a single product.
Formula: A + B → AB - Decomposition Reaction: A single compound breaks down into two or more simpler substances (requires heat, light, or electricity).
Formula: AB → A + B - Displacement Reaction: A more reactive element displaces a less reactive element from its compound.
Formula: A + BC → AC + B - Double Displacement Reaction: Exchange of ions between two compounds.
Formula: AB + CD → AD + CB - Redox Reaction: Reactions where Oxidation (gain of oxygen/loss of hydrogen) and Reduction (loss of oxygen/gain of hydrogen) happen simultaneously.
- Exothermic vs Endothermic: Exothermic releases heat; Endothermic absorbs energy.
📚 Part 1: NCERT Solutions (In-Text & Exercises)
Q1: Why should a magnesium ribbon be cleaned before burning in air?
Ans: Magnesium ribbon is often coated with a layer of magnesium oxide (MgO) due to reaction with moist air. This layer hinders burning. It must be cleaned with sandpaper to remove this oxide layer so that the magnesium metal can burn smoothly.
Q2: Write the balanced equation for: Hydrogen + Chlorine → Hydrogen chloride.
Ans: H₂ + Cl₂ → 2HCl
Q3: What is a balanced chemical equation? Why should chemical equations be balanced?
Ans: A balanced chemical equation is one where the number of atoms of each element remains the same on both the reactant and product sides. Equations must be balanced to satisfy the Law of Conservation of Mass, which states that mass is neither created nor destroyed during a chemical reaction.
Q4: Translate and balance: Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
Ans: 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄↓
Q5: What does one mean by exothermic and endothermic reactions? Give examples.
Ans:
- Exothermic: Reactions where heat is released along with the formation of products.
Example: Burning of natural gas (CH₄ + 2O₂ → CO₂ + 2H₂O + Heat). - Endothermic: Reactions that require energy (heat, light, or electricity) to break reactants.
Example: Decomposition of calcium carbonate (CaCO₃ + Heat → CaO + CO₂).
Q6: Why is respiration considered an exothermic reaction?
Ans: During respiration, glucose breaks down in our cells to combine with oxygen, producing carbon dioxide, water, and energy. Since energy is released to power our bodily functions, it is an exothermic reaction.
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Q7: Why are decomposition reactions called the opposite of combination reactions?
Ans: In a combination reaction, two or more substances combine to form a single new substance (A + B → AB). In a decomposition reaction, a single substance breaks down into two or more simpler substances (AB → A + B). Since the processes are reverse to each other, they are called opposites.
Q8: Explain the terms oxidation and reduction with examples.
Ans:
- Oxidation: Gain of Oxygen or loss of Hydrogen.
Ex: 2Cu + O₂ → 2CuO (Copper gains oxygen). - Reduction: Loss of Oxygen or gain of Hydrogen.
Ex: CuO + H₂ → Cu + H₂O (Copper oxide loses oxygen).
Q9: Why do we apply paint on iron articles?
Ans: Iron articles are painted to prevent rusting. Painting cuts off the contact of iron with air (oxygen) and moisture, which are the essential conditions for rusting to occur.
⚡ Part 2: 15 Extra Practice Questions (PYQ Style)
Short Answer Type Questions
Q1: Identify the type of reaction: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe.
Ans: This is a Displacement Reaction because Aluminum (more reactive) displaces Iron (less reactive) from its oxide. It is also a Thermite reaction used to join railway tracks.
Q2: Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Ans: Iron is more reactive than Copper. It displaces copper from copper sulphate solution. The blue colour of copper sulphate fades to light green due to the formation of Ferrous Sulphate (FeSO₄), and a reddish-brown coating of copper deposits on the iron nail.
Q3: Define Rancidity. How can it be prevented?
Ans: Rancidity is the oxidation of fats and oils in food resulting in a bad smell and taste. It can be prevented by adding antioxidants, flushing bags with Nitrogen gas (like in chips packets), or keeping food in airtight containers.
Q4: What happens when lead nitrate is heated? Write the equation.
Ans: It undergoes thermal decomposition to produce yellow lead monoxide, reddish-brown Nitrogen Dioxide (NO₂) gas, and Oxygen.
2Pb(NO₃)₂ (heat) → 2PbO + 4NO₂ + O₂
Q5: Balance the equation: HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O.
Ans: 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
Long Answer Type Questions
Q6: Explain the electrolysis of water with a chemical equation. Why is the volume of gas collected at one electrode double that of the other?
Ans: Decomposition of water by electricity is called electrolysis.
2H₂O (electricity) → 2H₂ + O₂
Water consists of 2 parts Hydrogen and 1 part Oxygen. Therefore, the volume of Hydrogen gas collected (at cathode) is double the volume of Oxygen gas collected (at anode).
Q7: A shiny brown element 'X' on heating in air becomes black in colour. Name the element 'X' and the black compound formed. Write the equation.
Ans: The shiny brown element 'X' is Copper (Cu). On heating in air, it reacts with oxygen to form a black coating of Copper (II) Oxide (CuO).
Equation: 2Cu + O₂ (heat) → 2CuO (Black)
Q8: What is a precipitation reaction? Give an example of a Double Displacement reaction that forms a precipitate.
Ans: A reaction in which an insoluble solid (precipitate) is formed is called a precipitation reaction. Example: Mixing Lead Nitrate and Potassium Iodide solutions.
Pb(NO₃)₂ (aq) + 2KI (aq) → PbI₂ (Yellow Precipitate) + 2KNO₃ (aq)
Competency Based Questions
Q9: Why are chips packets flushed with Nitrogen gas?
Ans: Nitrogen is an inert (unreactive) gas. Flushing chips packets with nitrogen prevents the oil and fats in the chips from coming into contact with Oxygen. This prevents oxidation (rancidity), keeping the chips fresh and crispy.
Q10: Identify the oxidizing and reducing agent: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂.
Ans:
- Oxidized: HCl (loses Hydrogen to become Cl₂).
- Reduced: MnO₂ (loses Oxygen to become MnCl₂).
- Oxidizing Agent: MnO₂
- Reducing Agent: HCl
Q11: Write the chemical equation for the reaction of Quick lime with water. Is it exothermic?
Ans: CaO (s) + H₂O (l) → Ca(OH)₂ (aq) + Heat.
Yes, it is highly exothermic (releases a large amount of heat) and forms Slaked Lime.
Q12: Why is silver chloride kept in dark coloured bottles?
Ans: Silver chloride (AgCl) is light-sensitive. In the presence of sunlight, it decomposes into Silver (grey) and Chlorine gas (photolytic decomposition). Dark bottles cut off the light exposure to prevent this reaction.
Q13: Name the type of reaction used in black and white photography.
Ans: Photolytic Decomposition reaction involving silver salts (Silver Chloride or Silver Bromide).
2AgBr (Sunlight) → 2Ag + Br₂
Q14: Give an example of a decomposition reaction carried out by electricity.
Ans: Electrolysis of water is a common example. Another is the decomposition of molten Sodium Chloride.
2NaCl (molten) → 2Na + Cl₂
Q15: What is the colour of Ferrous Sulphate crystals? What happens when they are heated?
Ans: Ferrous Sulphate crystals are pale green. On heating, they first lose water molecules and turn white. On further heating, they decompose to form Ferric Oxide (Reddish brown), SO₂ and SO₃ gases (smell of burning sulphur).