In short: Chemical Reactions and Equations (Chapter 1, Class 10 Science) covers five reaction types - combination, decomposition, displacement, double displacement, and redox. Master balancing equations, memorize examples for each type, and learn daily-life applications like corrosion and rancidity to score full marks in board exams.
Chemical Reactions and Equations is the first chapter in CBSE Class 10 Science, and it is one of the most important chapters for board exams. This chapter forms the base for all of Chemistry in Class 10 and carries significant marks. The good news is that once you understand the logic, this chapter becomes very scoring.
This guide covers every concept from the chapter with clear explanations, examples, and board exam tips. For the full chapter-wise marks distribution, refer to the CBSE Class 10 Science chapter-wise marks weightage.
What Is a Chemical Reaction?
A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) with different properties. You can identify a chemical reaction by changes such as change in color, change in temperature, evolution of gas, formation of a precipitate, or change in state.
For example, when iron is left in moist air, it gradually turns brown (rust). This color change indicates a chemical reaction has occurred.
Writing Chemical Equations
A chemical equation is a shorthand way of representing a chemical reaction using symbols and formulas.
A word equation describes the reaction in words: Magnesium + Oxygen gives Magnesium Oxide
A chemical equation uses symbols: Mg + O2 gives MgO
A balanced chemical equation ensures that the number of atoms of each element is the same on both sides: 2Mg + O2 gives 2MgO
How to Balance Chemical Equations
Balancing equations is a fundamental skill for this chapter. Here is the step-by-step method:
Step 1: Write the unbalanced equation with correct formulas. Step 2: List the number of atoms of each element on both sides. Step 3: Start balancing with the element that appears in the fewest formulas (usually not hydrogen or oxygen). Step 4: Use coefficients (numbers placed before formulas) to balance. Never change the subscripts in a formula. Step 5: Check that all elements are balanced.
Solved Example: Balance the equation for the reaction between iron and water.
Unbalanced: Fe + H2O gives Fe3O4 + H2
Count atoms: Left side: Fe = 1, H = 2, O = 1 Right side: Fe = 3, O = 4, H = 2
Balance Fe: 3Fe + H2O gives Fe3O4 + H2 Balance O: 3Fe + 4H2O gives Fe3O4 + H2 Balance H: 3Fe + 4H2O gives Fe3O4 + 4H2
Final balanced equation: 3Fe + 4H2O gives Fe3O4 + 4H2
Verify: Fe = 3 on both sides. H = 8 on both sides. O = 4 on both sides. Balanced.
Board Exam Tip: Always verify your balanced equation by counting atoms on both sides. Show this verification in your answer for full marks.
Types of Chemical Reactions
This is the most important section of the chapter. Board exams frequently ask you to identify reaction types and give examples.
Combination Reactions
Two or more reactants combine to form a single product.
General form: A + B gives AB
Examples: CaO + H2O gives Ca(OH)2 (Quick lime reacts with water to form slaked lime. This is also exothermic.) 2H2 + O2 gives 2H2O (Hydrogen burns in oxygen to form water.) C + O2 gives CO2 (Carbon burns in oxygen to form carbon dioxide.)
Key point: When a single product is formed from two or more reactants, it is always a combination reaction.
Decomposition Reactions
A single reactant breaks down into two or more products.
General form: AB gives A + B
Decomposition reactions are classified based on the energy source used:
Thermal decomposition (by heat): 2FeSO4 heated gives Fe2O3 + SO2 + SO3 CaCO3 heated gives CaO + CO2
Electrolytic decomposition (by electricity): 2H2O electrolysis gives 2H2 + O2
Photolytic decomposition (by light): 2AgCl sunlight gives 2Ag + Cl2 2AgBr sunlight gives 2Ag + Br2
Board Exam Tip: The decomposition of silver chloride and silver bromide by sunlight is used in black-and-white photography. This is a frequently asked application question.
Displacement Reactions
A more reactive element displaces a less reactive element from its compound.
General form: A + BC gives AC + B (where A is more reactive than B)
Examples: Fe + CuSO4 gives FeSO4 + Cu (Iron displaces copper from copper sulphate solution. The blue solution turns green.) Zn + CuSO4 gives ZnSO4 + Cu (Zinc displaces copper.)
The reactivity series determines which element will displace which. More reactive metals (like zinc, iron) can displace less reactive metals (like copper, silver) from their salt solutions.
Double Displacement Reactions
Two compounds exchange their ions to form two new compounds. These often result in the formation of a precipitate.
General form: AB + CD gives AD + CB
Examples: Na2SO4 + BaCl2 gives BaSO4 (precipitate) + 2NaCl NaOH + HCl gives NaCl + H2O (This is also a neutralization reaction.)
Board Exam Tip: When a precipitate forms, write its state symbol as (s) and mark it with a downward arrow in the equation.
Oxidation and Reduction (Redox Reactions)
Oxidation is the gain of oxygen or loss of hydrogen by a substance. Reduction is the loss of oxygen or gain of hydrogen by a substance.
When oxidation and reduction happen simultaneously, it is called a redox reaction.
Example: CuO + H2 gives Cu + H2O
Here, CuO is reduced to Cu (lost oxygen). H2 is oxidized to H2O (gained oxygen). CuO is the oxidizing agent. H2 is the reducing agent.
Another example: ZnO + C gives Zn + CO
ZnO is reduced (lost oxygen). C is oxidized (gained oxygen).
Board Exam Tip: Always identify both the substance being oxidized and the substance being reduced. Also identify the oxidizing agent and reducing agent.
Exothermic and Endothermic Reactions
Exothermic reactions release heat energy. Examples include burning of natural gas (CH4 + 2O2 gives CO2 + 2H2O + heat), respiration, and decomposition of vegetable matter into compost.
Endothermic reactions absorb heat energy. Examples include decomposition of calcium carbonate by heating, and the decomposition of lead nitrate by heating.
Effects of Oxidation Reactions in Daily Life
Corrosion: When metals are attacked by substances like oxygen, water, and acids, they corrode. Rusting of iron (Fe reacting with O2 and H2O to form Fe2O3.xH2O) is the most common example. Prevention methods include painting, oiling, greasing, galvanization, chrome plating, and alloying.
Rancidity: When fats and oils are oxidized, they become rancid, changing the smell and taste of food. Prevention methods include storing food in airtight containers, adding antioxidants, flushing bags with nitrogen gas, and refrigeration.
Board Exam Tip: Questions on corrosion prevention methods and rancidity are frequently asked for 2-3 marks.
Activity-Based Questions from NCERT
NCERT includes several activities that are important for exams:
The burning of magnesium ribbon (bright white flame, white ash of MgO) demonstrates combination reaction.
Heating of lead nitrate in a test tube (yellow powder releases brown fumes of NO2) demonstrates thermal decomposition.
Adding iron nails to copper sulphate solution (blue solution turns green, reddish-brown copper deposits on nail) demonstrates displacement reaction.
Practice describing these activities clearly, including what you observe and why it happens. For a broader list of important questions from all chapters, see our CBSE Class 10 Science important questions guide.
Tips for Scoring Full Marks
Memorize the general forms of all five reaction types. Practice balancing at least 20 equations to build speed and accuracy. Learn at least three examples for each reaction type. For board exams, knowing how to write answers that score maximum marks makes a big difference. For exam answers, always write balanced equations with state symbols: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous.
Padhaao's chapter-wise practice for Class 10 Science covers all the question types from this chapter, including equation balancing practice and reaction identification quizzes.
Frequently Asked Questions
How many marks does Chemical Reactions and Equations carry in CBSE Class 10?
This chapter typically carries 5 to 8 marks in the board exam, including a mix of 1-mark MCQs, 2-mark short answers, and sometimes a 3-mark question on balancing equations or identifying reaction types. It is one of the most scoring chapters in the Chemistry section.
What is the easiest way to balance chemical equations?
Start by listing all elements in the equation. Balance the element that appears in the fewest compounds first, and leave hydrogen and oxygen for last. Use coefficients (numbers before formulas) to balance - never change the subscripts. Practice with at least 20 equations and the process becomes almost automatic.
What is the difference between displacement and double displacement reactions?
In a displacement reaction, a single more reactive element replaces a less reactive element from a compound (for example, iron displacing copper from CuSO4). In a double displacement reaction, two compounds exchange their ions to form two new compounds (for example, Na2SO4 + BaCl2 forming BaSO4 and NaCl). A precipitate often forms in double displacement reactions.
How do I remember all the reaction types and their examples?
Group your study by reaction type. For each type, learn the general form (A + B gives AB for combination), memorize two to three key examples, and associate each with a visual observation (color change, gas bubbles, precipitate). Writing these in a comparison table and reviewing it every few days helps retain them long-term.
Are NCERT activities important for board exams?
Yes, NCERT activities are frequently tested. Questions like "What do you observe when iron nails are placed in copper sulphate solution?" come directly from textbook activities. Learn the procedure, the observation, and the reason behind each result. These are easy marks if you prepare them.
This chapter rewards thoroughness. Learn every reaction type, practice every example, and you will find these questions among the easiest marks in your board exam.
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