NCERT Solutions for Class 10 Science Chapter 1 – Chemical Reactions and Equations
Welcome to our comprehensive NCERT Solutions for Class 10 Science Chapter 1 – Chemical Reactions and Equations. This chapter forms the foundation of chemistry and is crucial for both board exams and competitive exams like JEE and NEET.
Key Concepts Overview
Chemical reactions are processes where substances (reactants) are transformed into new substances (products) through the breaking and forming of chemical bonds. Understanding these reactions is fundamental to chemistry.
What is a Chemical Equation?
A chemical equation is a symbolic representation of a chemical reaction. It shows:
- The reactants (substances that react)
- The products (substances that are formed)
- The physical states (solid, liquid, gas, aqueous)
- The conditions required for the reaction
Writing and Balancing Chemical Equations
A balanced chemical equation has an equal number of atoms of each element on both sides. This follows the Law of Conservation of Mass.
Example – Burning of Magnesium:
Word equation: Magnesium + Oxygen to Magnesium Oxide
Unbalanced: Mg + O2 to MgO
Balanced: 2Mg + O2 to 2MgO
Types of Chemical Reactions
1. Combination Reaction
Two or more substances combine to form a single product.
General form: A + B to AB
Example: CaO + H2O to Ca(OH)2 (Quick lime reacts with water to form slaked lime)
2. Decomposition Reaction
A single compound breaks down into two or more simpler substances.
Types of Decomposition:
- Thermal decomposition: Breaking down by heat
- Electrolytic decomposition: Breaking down by electricity
- Photolytic decomposition: Breaking down by light
3. Displacement Reaction
A more reactive element displaces a less reactive element from its compound.
Example: Fe + CuSO4 to FeSO4 + Cu (Iron displaces copper)
4. Double Displacement Reaction
Exchange of ions between two compounds to form new compounds.
Example: Na2SO4 + BaCl2 to BaSO4 + 2NaCl
5. Oxidation and Reduction Reactions (Redox)
Oxidation: Addition of oxygen or removal of hydrogen
Reduction: Addition of hydrogen or removal of oxygen
Both reactions occur simultaneously in redox reactions.
NCERT Exercise Solutions
Page 6 – Intext Questions
Q1. Why should a magnesium ribbon be cleaned before burning in air?
Answer: Magnesium ribbon should be cleaned before burning to remove the layer of magnesium oxide that forms on its surface when exposed to air. This oxide layer prevents the burning of magnesium ribbon. Cleaning with sandpaper removes this layer, allowing magnesium to burn properly with a bright white flame.
Q2. Write the balanced equation for: Hydrogen + Chlorine to Hydrogen chloride
Answer: H2 + Cl2 to 2HCl
Page 10 – Intext Questions
Q1. A solution of substance X is used for whitewashing. Name the substance X.
Answer: The substance X is Calcium hydroxide (slaked lime). Its chemical formula is Ca(OH)2. When slaked lime reacts with carbon dioxide in air, it forms calcium carbonate which gives a shiny finish to walls.
Important Questions for Board Exams
1 Mark Questions
- What is a chemical equation?
- Define oxidation reaction with example.
- What type of reaction is respiration?
3 Mark Questions
- Explain the different types of decomposition reactions with examples.
- What is a redox reaction? Identify the oxidizing and reducing agents.
- Why do we apply paint on iron articles?
Key Points to Remember
- Always balance chemical equations before solving numerical problems
- Remember the reactivity series for displacement reactions
- Oxidation and reduction always occur together
- Corrosion is an oxidation reaction
- Rancidity can be prevented by adding antioxidants
Continue Learning: Explore Chapter 2 – Acids, Bases and Salts for more NCERT Solutions.
Key Concepts: Chemical Reactions and Equations
This is one of the most important and high-weightage chapters of Class 10 Science. Chemical Reactions and Equations introduces the language of chemistry — how to write, balance, and classify chemical reactions.
What is a Chemical Reaction?
A chemical reaction is a process in which reactants are converted into products, involving the breaking and formation of chemical bonds. The atoms are rearranged but are never created or destroyed (Law of Conservation of Mass).
Signs that a chemical reaction has occurred:
- Change in colour (e.g., iron rusting turns reddish-brown)
- Evolution of a gas (e.g., CO₂ released when acid reacts with a carbonate)
- Change in temperature (heat produced or absorbed)
- Formation of a precipitate (insoluble solid formed in solution)
- Change in state
Balancing Chemical Equations
A balanced chemical equation has equal numbers of each type of atom on both sides. Steps to balance:
- Write the unbalanced skeleton equation
- Count atoms of each element on both sides
- Add coefficients (whole numbers) to balance — do NOT change the formulae of substances
- Verify by counting atoms again
Example: Fe + H₂O → Fe₃O₄ + H₂
Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
Types of Chemical Reactions
- Combination Reaction: Two or more reactants combine to form a single product. A + B → AB. Example: 2Mg + O₂ → 2MgO (Magnesium burning)
- Decomposition Reaction: A single compound breaks into two or more simpler substances. AB → A + B. Example: 2H₂O → 2H₂ + O₂ (electrolysis of water)
- Displacement Reaction: A more reactive element displaces a less reactive element from its compound. A + BC → AC + B. Example: Fe + CuSO₄ → FeSO₄ + Cu
- Double Displacement Reaction: Exchange of ions between two compounds (usually in solution) forming a precipitate or gas. AB + CD → AD + CB. Example: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
- Oxidation-Reduction (Redox) Reaction: Oxidation (loss of electrons / gain of oxygen) and reduction (gain of electrons / loss of oxygen) occur simultaneously.
Exothermic and Endothermic Reactions
- Exothermic Reactions: Release heat energy. Example: Burning of fuels, respiration, neutralisation. Represented as: A + B → C + D + Heat
- Endothermic Reactions: Absorb heat energy from surroundings. Example: Photosynthesis, decomposition of calcium carbonate. Represented as: A + B + Heat → C + D
Effects of Oxidation in Everyday Life
- Corrosion: Slow oxidation of metals. Example: Iron rusting (Fe₂O₃·xH₂O), silver tarnishing (Ag₂S)
- Rancidity: Oxidation of fats and oils in food, causing bad smell and taste. Prevented by: using antioxidants (e.g., nitrogen gas flushing in chips packets), storing food in refrigerators, vacuum packing
Important Board Exam Questions
- Balance the equation: Pb(NO₃)₂ → PbO + NO₂ + O₂
- What is rancidity? How is it prevented?
- What is corrosion? Give two methods to prevent it.
- Identify the type of reaction: 2KClO₃ → 2KCl + 3O₂
- Why does the colour of copper sulphate solution change when iron is placed in it?
- Define a combination reaction. Give one example from daily life.
- Distinguish between exothermic and endothermic reactions with examples.
Common Mistakes in Board Exams
- Forgetting to include state symbols ↑ (gas), ↓ (precipitate), (s), (l), (aq), (g) when asked
- Changing chemical formulae while balancing (you can only change coefficients, not subscripts)
- Confusing displacement with double displacement — in displacement, only ONE element replaces another
- Forgetting that corrosion and rancidity are both caused by oxidation
