Class 10 Science Chapter 1 – Chemical Reactions and Equations is one of the most important chapters for understanding basic chemistry. This chapter explains how substances change during chemical reactions(Physical and chemical changes), how to represent reactions using chemical equations, how to balance equations, and different types of chemical reactions.
These notes are designed for easy learning, quick revision, and exam preparation, with important definitions, examples, equations, observations, and key points.
1. What Is a Chemical Reaction?
A chemical reaction is a process in which one or more substances undergo a chemical change to form one or more new substances with different properties.
The substances that take part in a chemical reaction are called reactants, while the new substances formed are called products.
Example
When magnesium ribbon burns in oxygen, it forms magnesium oxide.
Magnesium + Oxygen → Magnesium Oxide
2Mg + O₂ → 2MgO
Here:
- Magnesium (Mg) and Oxygen (O₂) are reactants.
- Magnesium oxide (MgO) is the product.
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2. How Do We Know That a Chemical Reaction Has Taken Place?
A chemical reaction may be identified by one or more observable changes.
Common Signs of a Chemical Reaction
- Change in state
- Change in colour
- Evolution of a gas
- Change in temperature
- Formation of a precipitate
- Change in smell
Example: Magnesium Burning in Air
When magnesium ribbon burns, it produces a bright white flame and forms a white powder of magnesium oxide.
2Mg + O₂ → 2MgO
3. Chemical Equations
A chemical equation is a short and systematic way of representing a chemical reaction using symbols and chemical formulae.
Word Equation
Magnesium + Oxygen → Magnesium Oxide
Chemical Equation
Mg + O₂ → MgO
The chemical equation gives more information than a word equation because it uses chemical symbols and formulae.
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4. Balanced Chemical Equation
A chemical equation in which the number of atoms of each element is equal on both sides of the equation is called a balanced chemical equation.
Why Do We Balance Chemical Equations?
Chemical equations are balanced according to the law of conservation of mass.
According to this law:
Matter can neither be created nor destroyed in a chemical reaction.
Therefore, the total number of atoms of each element must remain the same before and after a reaction.
Example
Unbalanced equation:
H₂ + O₂ → H₂O
Balanced equation:
2H₂ + O₂ → 2H₂O
Number of atoms:
| Element | Reactants | Products |
|---|---|---|
| H | 4 | 4 |
| O | 2 | 2 |
Therefore, the equation is balanced.
5. How to Balance a Chemical Equation
Balancing should be done by changing the coefficients placed before chemical formulae. The chemical formula itself should never be changed.
Example
Fe + H₂O → Fe₃O₄ + H₂
Step 1: Balance iron.
3Fe + H₂O → Fe₃O₄ + H₂
Step 2: Balance oxygen.
There are four oxygen atoms in Fe₃O₄, so use 4H₂O.
3Fe + 4H₂O → Fe₃O₄ + H₂
Step 3: Balance hydrogen.
There are eight hydrogen atoms on the left, so use 4H₂.
3Fe + 4H₂O → Fe₃O₄ + 4H₂
The equation is now balanced.
6. State Symbols in Chemical Equations
State symbols tell us the physical state of the substances involved in a reaction.
| Symbol | Meaning |
|---|---|
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
| (aq) | Aqueous solution |
Example
CaCO₃(s) → CaO(s) + CO₂(g)
Here, calcium carbonate and calcium oxide are solids, while carbon dioxide is a gas.
Other Conditions
Conditions such as heat, sunlight, pressure, or a catalyst may also be written above or below the reaction arrow.
Example:
7. Types of Chemical Reactions
Chemical reactions can be classified into different types based on how reactants combine or break apart.
The major types covered in Class 10 are:
- Combination Reaction
- Decomposition Reaction
- Displacement Reaction
- Double Displacement Reaction
- Oxidation Reaction
- Reduction Reaction
- Redox Reaction
8. Combination Reaction
A reaction in which two or more substances combine to form a single product is called a combination reaction.
General Form
A + B → AB
Example 1: Formation of Calcium Oxide
2Ca + O₂ → 2CaO
Calcium combines with oxygen to form calcium oxide.
Example 2: Formation of Water
2H₂ + O₂ → 2H₂O
Hydrogen and oxygen combine to form water.
Key Point
Two or more reactants → One product
Want to know More ? Read this : What is a Combination Reaction? | Definition, Examples & Explanation
9. Decomposition Reaction
A reaction in which one compound breaks down into two or more simpler substances is called a decomposition reaction.
General Form
AB → A + B
Decomposition reactions usually require energy in the form of heat, light, or electricity.
9.1 Thermal Decomposition
A decomposition reaction caused by heat is called thermal decomposition.
Example:
Calcium carbonate decomposes into calcium oxide and carbon dioxide.
9.2 Electrolytic Decomposition
A decomposition reaction caused by electricity is called electrolytic decomposition.
Example:
Water decomposes into hydrogen and oxygen.
9.3 Photochemical Decomposition
A decomposition reaction caused by light is called photochemical decomposition.
Example:
Silver chloride decomposes in sunlight.
Key Point
One reactant → Two or more products
Read More : What is a Decomposition Reaction? | Definition, Types, Examples & Explanation
10. Displacement Reaction
A reaction in which a more reactive element displaces a less reactive element from its compound is called a displacement reaction.
Example
Fe + CuSO₄ → FeSO₄ + Cu
Iron displaces copper from copper sulphate solution.
Observation
The blue colour of copper sulphate solution changes because iron sulphate is formed, and copper is deposited.
Key Point
A + BC → AC + B
11. Double Displacement Reaction
A reaction in which two compounds exchange their ions to form two new compounds is called a double displacement reaction.
Example
Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
Barium sulphate is formed as a white insoluble solid.
Precipitation Reaction
A reaction in which an insoluble solid called a precipitate is formed is called a precipitation reaction.
In the above reaction:
BaSO₄ is the white precipitate.
Key Point
AB + CD → AD + CB
12. Oxidation and Reduction
Oxidation and reduction are important concepts in chemistry.
Oxidation
Oxidation can be understood as:
- Addition of oxygen
- Removal of hydrogen
- Loss of electrons
Example
2Cu + O₂ → 2CuO
Copper combines with oxygen and is oxidised.
Reduction
Reduction can be understood as:
- Removal of oxygen
- Addition of hydrogen
- Gain of electrons
Example
CuO + H₂ → Cu + H₂O
Copper oxide loses oxygen and is reduced to copper.
You may explore in detail about : Difference Between Oxidation and Reduction | Definition, Examples & Explanation
13. Redox Reaction
A reaction in which oxidation and reduction occur simultaneously is called a redox reaction.
Example
CuO + H₂ → Cu + H₂O
Here:
- CuO is reduced to Cu.
- H₂ is oxidised to H₂O.
Therefore, it is a redox reaction.
Easy Trick
Oxidation = Loss of electrons
Reduction = Gain of electrons
Remember: OIL RIG
Oxidation Is Loss
Reduction Is Gain
14. Exothermic Reactions
A reaction that releases heat energy is called an exothermic reaction.
Example: Burning of Natural Gas
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
Combustion reactions are generally exothermic.
Respiration Is Also Exothermic
During respiration, glucose reacts with oxygen and releases energy.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Read More : Difference Between Exothermic and Endothermic Reactions
15. Endothermic Reactions
A reaction that absorbs energy from its surroundings is called an endothermic reaction.
Some decomposition reactions are endothermic because they require heat, light, or electricity to occur.
Example:
Still want to explore more ? Read This : Types of Chemical Reactions | Definition, Examples & Explanation
16. Effects of Oxidation in Everyday Life
Oxidation is not limited to laboratory reactions. It also occurs in many everyday processes.
Two important examples are:
- Corrosion
- Rancidity
17. Corrosion
The slow deterioration of metals due to their reaction with substances present in the environment, such as oxygen and moisture, is called corrosion.
Example: Rusting of Iron
Iron reacts with oxygen and moisture to form rust.
Rust is mainly hydrated iron(III) oxide.
Conditions Required for Rusting
Rusting generally requires:
- Oxygen
- Water or moisture
How to Prevent Corrosion
Corrosion can be prevented by:
- Painting
- Oiling or greasing
- Galvanisation
- Alloying
- Electroplating
Learn More : What is Corrosion? | Definition, Examples & Prevention Methods
18. Rancidity
The oxidation of fats and oils present in food, resulting in an unpleasant smell and taste, is called rancidity.
How Can Rancidity Be Prevented?
- Store food in airtight containers.
- Keep food refrigerated when appropriate.
- Add suitable antioxidants.
- Pack some food products using nitrogen gas.
Example
Chips and other oily foods are often packed with nitrogen gas to reduce oxidation.
Read More About Rancidity : What is Rancidity? | Definition, Examples, Prevention & Explanation
19. Important Chemical Reactions to Remember
Formation of Magnesium Oxide
2Mg + O₂ → 2MgO
Formation of Water
2H₂ + O₂ → 2H₂O
Thermal Decomposition of Calcium Carbonate
Decomposition of Water
Decomposition of Silver Chloride
Displacement Reaction
Fe + CuSO₄ → FeSO₄ + Cu
Double Displacement Reaction
Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
Reduction of Copper Oxide
CuO + H₂ → Cu + H₂O
20. Important Definitions for Exams
Chemical Reaction
A process in which one or more substances undergo chemical changes to form new substances.
Chemical Equation
A symbolic representation of a chemical reaction using chemical formulae and symbols.
Balanced Chemical Equation
An equation in which the number of atoms of every element is equal on both sides.
Combination Reaction
A reaction in which two or more substances combine to form a single product.
Decomposition Reaction
A reaction in which one compound breaks down into two or more simpler substances.
Displacement Reaction
A reaction in which a more reactive element displaces a less reactive element from its compound.
Double Displacement Reaction
A reaction in which two compounds exchange ions to form two new compounds.
Oxidation
A process involving addition of oxygen, removal of hydrogen, or loss of electrons.
Reduction
A process involving removal of oxygen, addition of hydrogen, or gain of electrons.
Corrosion
The slow deterioration of a metal due to chemical reactions with its surroundings.
Rancidity
The oxidation of fats and oils that causes undesirable changes in their smell and taste.
21. Important Differences
Combination Reaction vs Decomposition Reaction
| Combination Reaction | Decomposition Reaction |
|---|---|
| Two or more substances combine. | One compound breaks down. |
| Usually one product is formed. | Two or more products are formed. |
| A + B → AB | AB → A + B |
Oxidation vs Reduction
| Oxidation | Reduction |
|---|---|
| Addition of oxygen | Removal of oxygen |
| Removal of hydrogen | Addition of hydrogen |
| Loss of electrons | Gain of electrons |
22. Most Important Exam Points
- Chemical reactions produce new substances.
- A chemical equation represents a chemical reaction using symbols and formulae.
- Chemical equations must be balanced according to the law of conservation of mass.
- Never change the chemical formula while balancing an equation.
- Combination reactions form one main product.
- Decomposition reactions involve the breakdown of one compound.
- Displacement depends on the reactivity of elements.
- Double displacement reactions involve exchange of ions.
- Formation of an insoluble solid is called a precipitation reaction.
- Oxidation and reduction occur together in a redox reaction.
- Corrosion damages metals.
- Rancidity affects food containing fats and oils.
- Oxidation can be prevented or slowed using appropriate methods such as airtight packaging and antioxidants.
23. Quick Revision: Chapter in One Look
Chemical Reaction
↓
Chemical Equation
↓
Balanced Equation
↓
Types of Reactions
→ Combination
→ Decomposition
→ Displacement
→ Double Displacement
→ Redox
Oxidation → Addition of oxygen / Loss of electrons
Reduction → Removal of oxygen / Gain of electrons
Corrosion → Damage to metals
Rancidity → Oxidation of fats and oils
24. How to Prepare This Chapter for the Exam
For scoring well in Class 10 Science, focus especially on:
- Learning all important definitions.
- Understanding the different types of chemical reactions.
- Practising balancing chemical equations.
- Memorising important reactions and their conditions.
- Understanding oxidation and reduction.
- Learning corrosion and rancidity with their prevention methods.
- Practising NCERT questions and previous-year questions.
⭐ Final Tip
Don’t try to memorize the entire chapter at once. Understand the concept, practise the equations, revise regularly, and test yourself.
Learn → Understand → Practice → Revise → Score High!