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📥 Download PDF (Free)Think about this: you strike a matchstick, and it bursts into flame. You leave your bicycle out in the rain, and it slowly turns reddish-brown with rust. You drop a fizzy tablet into water, and bubbles shoot up everywhere.
What do all these have in common? They’re all chemical reactions - and they’re happening around you every single day. In this chapter, you’ll learn what chemical reactions really are, how scientists write them in a shorthand code (chemical equations), and the different types of reactions you need to know for your board exams.
Board exam tip: This chapter carries 5β8 marks every year. The good news? It’s one of the most scoring chapters once you understand the patterns.
What You’ll Learn in This Chapter
- How to tell when a chemical reaction has happened
- How to write and balance chemical equations
- The 5 types of chemical reactions (with easy tricks to remember each)
- What oxidation and reduction mean in everyday life
- Why food goes stale and iron gets rusty - and how to prevent it
Key Concepts
1. What is a Chemical Reaction?
Imagine you’re cooking an egg. The runny, transparent egg white turns solid and white when heated. Can you turn it back into a raw egg? Nope. That’s because a chemical reaction has happened - the substances have permanently changed into something new.
In chemistry terms: a chemical reaction is when one or more substances (called reactants) transform into completely new substances (called products).
How do you know a chemical reaction has happened?
Look for these 5 clues:
- π¨ Change in colour - iron turns reddish-brown when it rusts
- π¨ Gas is released - vinegar + baking soda = fizzy COβ bubbles
- π‘οΈ Temperature changes - burning paper releases heat
- βͺ A solid appears in a liquid - this is called a precipitate
- π Change in smell - milk turning sour
2. Chemical Equations - The Shorthand Code
Scientists are lazy (in a good way). Instead of writing “magnesium reacts with oxygen to form magnesium oxide,” they use a short code called a chemical equation.
Word equation:
Magnesium + Oxygen β Magnesium Oxide
Chemical equation:
2Mg + Oβ β 2MgO
The arrow (β) means “produces” or “gives.” Reactants go on the left, products go on the right.
Symbols you’ll see in equations
| Symbol | Meaning |
|---|---|
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
| (aq) | Dissolved in water (aqueous) |
| β | Gas escapes |
| β | Precipitate (solid) formed |
| Ξ | Heat applied |
3. How to Balance a Chemical Equation
Here’s a rule that nature never breaks: atoms can’t be created or destroyed. This is called the Law of Conservation of Mass. So whatever atoms you start with, you must end with - no more, no less.
That’s why we balance equations - to make sure the count of each type of atom is the same on both sides.
Step-by-step method (with example):
Unbalanced: Fe + HβO β FeβOβ + Hβ
Step 1: Count atoms on each side.
| Element | Left side | Right side |
|---|---|---|
| Fe | 1 | 3 β |
| H | 2 | 2 β |
| O | 1 | 4 β |
Step 2: Start with the element that appears in the fewest formulas. Balance Fe first β put 3 before Fe on the left.
3Fe + HβO β FeβOβ + Hβ
Step 3: Balance O β put 4 before HβO.
3Fe + 4HβO β FeβOβ + Hβ
Step 4: Now H is off (8 on left, 2 on right) β put 4 before Hβ.
3Fe + 4HβO β FeβOβ + 4Hβ
Final check: Fe: 3=3 β | H: 8=8 β | O: 4=4 β
Balanced: 3Fe + 4HβO β FeβOβ + 4Hβ
Golden rule: Only add numbers before formulas (coefficients). Never change the formula itself - that changes the substance!
4. Types of Chemical Reactions
There are 5 types you need to know. Here’s an easy way to remember them all:
a) Combination Reaction - “Two become one”
Two or more substances join together to form a single new substance.
Pattern: A + B β AB
Real-life example: When quicklime (CaO) is added to water, it gets really hot and forms slaked lime. This is why construction workers feel heat when they mix cement!
CaO(s) + HβO(l) β Ca(OH)β(aq) + Heat
b) Decomposition Reaction - “One breaks into many”
A single substance breaks apart into two or more simpler substances. It’s the opposite of combination.
Pattern: AB β A + B
What causes it to break apart? Three things:
- Heat (thermal decomposition) - 2Pb(NOβ)β Ξβ 2PbO + 4NOβ + Oβ
- Electricity (electrolytic decomposition) - 2HβO electricityβ 2Hβ + Oβ
- Sunlight (photolytic decomposition) - 2AgBr sunlightβ 2Ag + Brβ
Fun fact: The silver bromide reaction is why old-school camera film works - light breaks the chemical and leaves silver behind, creating your photo!
c) Displacement Reaction - “The bully kicks someone out”
A more reactive element pushes out a less reactive element from its compound. Think of it like the class bully taking someone’s seat.
Pattern: A + BC β AC + B (A is more reactive than B)
Real-life example: Drop an iron nail into blue copper sulphate solution. Wait an hour. The solution turns green and the nail gets coated in reddish-brown copper.
Fe(s) + CuSOβ(aq) β FeSOβ(aq) + Cu(s)
Iron is more reactive than copper, so it kicks copper out.
d) Double Displacement Reaction - “Partner swap”
Two compounds swap their partners (ions) to form two completely new compounds. Like a dance where pairs exchange partners.
Pattern: AB + CD β AD + CB
Example:
NaβSOβ(aq) + BaClβ(aq) β BaSOββ + 2NaCl(aq)
A white solid (BaSOβ) suddenly appears in the liquid - that’s the precipitate. So this is also called a precipitation reaction.
e) Oxidation and Reduction (Redox Reactions)
This is less about “what reacts” and more about “what happens to the atoms.”
- Oxidation = Gain of oxygen OR loss of hydrogen OR loss of electrons
- Reduction = Loss of oxygen OR gain of hydrogen OR gain of electrons
Memory trick: OIL RIG - Oxidation Is Loss, Reduction Is Gain (of electrons)
They always happen together. If one substance gets oxidised, another gets reduced - that’s why it’s called a redox reaction.
Example:
CuO + Hβ β Cu + HβO
- CuO loses oxygen β it’s reduced
- Hβ gains oxygen β it’s oxidised
5. Oxidation in Daily Life - Corrosion and Rancidity
Corrosion - When metals slowly “rot”
Leave an iron gate in the rain and it slowly turns reddish-brown. That’s rusting - a type of corrosion.
4Fe + 3Oβ + xHβO β 2FeβOβΒ·xHβO (rust)
It’s not just iron - silver turns black (tarnishing), copper turns green (patina on old statues).
How to prevent corrosion:
- Painting or oiling - blocks air and moisture
- Galvanising - coating iron with a layer of zinc
- Alloying - mixing with other metals (stainless steel = iron + chromium + nickel)
Rancidity - When food goes “off”
Ever opened a packet of chips and it smelled weird and tasted terrible? The fats and oils in the food got oxidised. This is rancidity.
How to prevent rancidity:
- Store food in airtight containers (no oxygen = no oxidation)
- Add antioxidants (like BHA, BHT in packaged food)
- Refrigerate - cold slows down oxidation
- Nitrogen flushing - chip packets are filled with nitrogen gas instead of air
Important Definitions
| Term | What It Means (Plain English) |
|---|---|
| Chemical reaction | A process where substances change into completely new substances |
| Reactant | The starting substance (left side of the equation) |
| Product | The new substance formed (right side of the equation) |
| Chemical equation | A shorthand way to write a reaction using symbols and formulas |
| Balanced equation | An equation where atom counts are equal on both sides |
| Catalyst | A substance that speeds up a reaction without getting used up itself |
| Exothermic reaction | A reaction that releases heat (feels hot) |
| Endothermic reaction | A reaction that absorbs heat (feels cold) |
| Oxidation | Gain of oxygen / loss of hydrogen / loss of electrons |
| Reduction | Loss of oxygen / gain of hydrogen / gain of electrons |
| Redox reaction | A reaction where oxidation and reduction happen at the same time |
| Precipitate | A solid that appears when two liquids react together |
| Corrosion | Slow destruction of metal by air and moisture (e.g., rusting) |
| Rancidity | Food fats getting oxidised, causing bad smell and taste |
Solved Examples (Step-by-Step)
Example 1: Balance this equation
Hβ + Oβ β HβO
Let’s work through it:
Step 1: Count atoms.
Left: H=2, O=2 | Right: H=2, O=1
Oxygen doesn’t match.
Step 2: Put 2 before HβO β Hβ + Oβ β 2HβO
Now: Left: H=2, O=2 | Right: H=4, O=2
Oxygen matches now, but hydrogen doesn’t.
Step 3: Put 2 before Hβ β 2Hβ + Oβ β 2HβO
Left: H=4, O=2 | Right: H=4, O=2 β Balanced!
Answer: 2Hβ + Oβ β 2HβO
Example 2: Identify the reaction type
Zn + HβSOβ β ZnSOβ + Hβ
Think: Zinc is alone on the left. It reacts with a compound and pushes hydrogen out. One element replacing another = displacement reaction.
(Zinc is more reactive than hydrogen in the reactivity series.)
Example 3: Identify oxidation and reduction
MnOβ + 4HCl β MnClβ + 2HβO + Clβ
Look at what happened:
- HCl lost its hydrogen β HCl is oxidised
- MnOβ lost its oxygen β MnOβ is reduced
Since both happened in one reaction β this is a redox reaction.
Important Questions for Board Exams
1-Mark Questions
- What is a chemical equation?
- Define a combination reaction with one example.
- What is a displacement reaction?
- What type of reaction happens when silver chloride turns grey in sunlight?
- Define rancidity.
2-Mark Questions
- Explain the difference between exothermic and endothermic reactions with one example each.
- Why is it necessary to balance a chemical equation?
- What happens when iron nails are placed in copper sulphate solution? Write the equation.
- What is corrosion? Give two ways to prevent it.
- Why are chip packets flushed with nitrogen gas?
3-Mark Questions
- Balance: FeβOβ + Al β AlβOβ + Fe. Name the reaction type.
- Explain the thermite reaction. What type of reaction is it?
- What is a double displacement reaction? Give one example with a balanced equation.
- Distinguish between oxidation and reduction with examples.
- List five observations that indicate a chemical reaction has taken place.
5-Mark Questions
- Write and balance the equations for: (a) burning of magnesium ribbon (b) electrolysis of water (c) reaction of zinc with dilute HβSOβ. Identify each reaction type.
- Describe all five types of chemical reactions with one balanced equation for each.
Quick Revision - Read This Before Your Exam
Scan this in 2 minutes to refresh everything:
- Chemical reactions create new substances - look for colour change, gas, temperature change, precipitate, or smell change
- Chemical equations: reactants on left β products on right
- Balancing is based on the Law of Conservation of Mass - atoms in = atoms out
- Combination: A + B β AB (two become one)
- Decomposition: AB β A + B (one breaks into many) - caused by heat, electricity, or light
- Displacement: More reactive element kicks out less reactive one
- Double Displacement: Two compounds swap ions - often forms a precipitate
- Redox: Oxidation (gain Oβ / lose Hβ / lose eβ») + Reduction (opposite) happen together
- Memory trick: OIL RIG - Oxidation Is Loss, Reduction Is Gain (of electrons)
- Corrosion = slow oxidation of metals (prevent: paint, galvanise, alloy)
- Rancidity = oxidation of food fats (prevent: airtight, antioxidants, refrigerate, nitrogen flush)
Test Your Understanding
Let’s check how well you understood this chapter. Pick your answer for each question, then check the answer to see if you’re right!
Q1. Which of the following is NOT a sign that a chemical reaction has occurred?
a) Change in colour
b) Change in shape of the container
c) Evolution of a gas
d) Change in temperature
Click to see answer
Answer: b) Change in shape of the container
The container’s shape is a physical change, not a chemical one. Chemical reactions change the substances themselves - look for colour, gas, temperature, precipitate, or smell changes.
Q2. In the equation 2Mg + Oβ β 2MgO, the reactants are:
a) MgO
b) Mg and Oβ
c) Mg and MgO
d) Oβ and MgO
Click to see answer
Answer: b) Mg and Oβ
Reactants are always on the left side of the arrow. Products (MgO) are on the right.
Q3. The Law of Conservation of Mass states that:
a) Energy cannot be created or destroyed
b) Mass is always gained in a reaction
c) Atoms cannot be created or destroyed in a chemical reaction
d) Products always weigh more than reactants
Click to see answer
Answer: c) Atoms cannot be created or destroyed in a chemical reaction
This is exactly why we balance equations - the same atoms on the left must appear on the right. Option (a) is about energy conservation, which is a different law.
Q4. CaO + HβO β Ca(OH)β is an example of:
a) Decomposition reaction
b) Displacement reaction
c) Combination reaction
d) Double displacement reaction
Click to see answer
Answer: c) Combination reaction
Two substances (CaO and HβO) combine to form a single product - Ca(OH)β. That’s the A + B β AB pattern.
Q5. When silver bromide is exposed to sunlight, it decomposes. This is called:
a) Thermal decomposition
b) Electrolytic decomposition
c) Photolytic decomposition
d) Displacement reaction
Click to see answer
Answer: c) Photolytic decomposition
“Photo” = light. Decomposition caused by sunlight is photolytic. Thermal = heat, Electrolytic = electricity.
Q6. In the reaction Fe + CuSOβ β FeSOβ + Cu, iron displaces copper because:
a) Iron is cheaper than copper
b) Iron is more reactive than copper
c) Iron is less reactive than copper
d) Copper is a gas
Click to see answer
Answer: b) Iron is more reactive than copper
In displacement reactions, the more reactive element always pushes out the less reactive one. Iron sits above copper in the reactivity series.
Q7. A white precipitate of BaSOβ forms when NaβSOβ reacts with BaClβ. This is a:
a) Combination reaction
b) Decomposition reaction
c) Displacement reaction
d) Double displacement reaction
Click to see answer
Answer: d) Double displacement reaction
The two compounds swap their ions: Na goes with Cl, Ba goes with SOβ. Two compounds exchange partners = double displacement.
Q8. In the reaction CuO + Hβ β Cu + HβO, which substance is oxidised?
a) CuO
b) Hβ
c) Cu
d) HβO
Click to see answer
Answer: b) Hβ
Hβ gains oxygen (becomes HβO) β it’s oxidised. CuO loses oxygen (becomes Cu) β it’s reduced. Remember: gain of oxygen = oxidation.
Q9. What is OIL RIG a memory trick for?
a) Types of chemical reactions
b) How to balance equations
c) Oxidation Is Loss, Reduction Is Gain (of electrons)
d) Order of reactivity of metals
Click to see answer
Answer: c) Oxidation Is Loss, Reduction Is Gain (of electrons)
OIL = Oxidation Is Loss of electrons. RIG = Reduction Is Gain of electrons. This trick helps you remember redox concepts quickly.
Q10. Chip packets are filled with nitrogen gas to:
a) Make the packet look bigger
b) Add flavour to the chips
c) Prevent oxidation of fats (rancidity)
d) Keep the chips warm
Click to see answer
Answer: c) Prevent oxidation of fats (rancidity)
Nitrogen is an unreactive gas. By replacing air (which contains oxygen) with nitrogen, the fats in chips don’t get oxidised, keeping them fresh longer.
Q11. The green coating that forms on copper over time is due to:
a) Rancidity
b) Rusting
c) Corrosion
d) Decomposition
Click to see answer
Answer: c) Corrosion
Corrosion happens to many metals, not just iron. Copper reacts with air and moisture to form a green layer (copper carbonate). Rusting specifically refers to iron only.
Q12. Which method can prevent rusting of iron?
a) Keeping it in water
b) Galvanising (coating with zinc)
c) Exposing it to moist air
d) Adding salt water to it
Click to see answer
Answer: b) Galvanising (coating with zinc)
Galvanising coats iron with zinc, which blocks air and moisture. Options a, c, and d would all speed up rusting because they add moisture/oxygen.
How did you do? Scored 10+ out of 12? You’ve nailed this chapter! Below 10? Go back and re-read the sections you got wrong - you’ll get there.
Next Chapter: Chapter 2 - Acids, Bases and Salts
Chapter Navigation
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Next: Acids, Bases and Salts Class 10 Notes
Related Chapters in Class 10 Science
- Acids, Bases and Salts Class 10 Notes
- Metals and Non-metals Class 10 Notes
- Carbon and its Compounds Class 10 Notes
Practice What You Learned
Test yourself with our Class 10 Science Mock Papers to see how well you’ve mastered the concepts.
Explore More
Weightage in Board Exams
Chemical Reactions and Equations is Chapter 1 of the CBSE Class 10 Science syllabus and sits in Unit I, Chemical Substances – Nature and Behaviour, which carries 25 marks of the 80 mark theory paper. That unit is shared with Acids, Bases and Salts, Metals and Non-metals, and Carbon and its Compounds, so the marks are split across those chapters and CBSE does not publish a fixed per chapter figure.
Balancing equations and identifying reaction types are near-certain marks every year, and oxidation and reduction shows up in both short-answer and case-study formats.
Frequently Asked Questions
What is a balanced chemical equation and why must equations be balanced?
A balanced chemical equation has the same number of atoms of each element on both sides of the arrow. Equations must be balanced because of the law of conservation of mass: matter is neither created nor destroyed in a chemical reaction, so the total mass of the reactants equals the total mass of the products.
What are the five main types of chemical reactions in Class 10?
Combination, in which two or more substances form a single product. Decomposition, in which one substance breaks into two or more. Displacement, in which a more reactive element displaces a less reactive one. Double displacement, in which two compounds exchange ions. Oxidation and reduction, in which oxygen or hydrogen or electrons are gained or lost.
What is the difference between oxidation and reduction?
Oxidation is the gain of oxygen or the loss of hydrogen or the loss of electrons. Reduction is the loss of oxygen or the gain of hydrogen or the gain of electrons. The two always happen together in a redox reaction, so the substance that is oxidised acts as the reducing agent and the one that is reduced acts as the oxidising agent.
Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The blue colour of copper sulphate fades to pale green as iron sulphate forms, and a reddish brown deposit of copper collects on the nail. This is a displacement reaction.
What is a redox reaction? Give one everyday example.
A redox reaction is one in which oxidation and reduction occur at the same time. Respiration is an everyday example: glucose is oxidised to carbon dioxide and water while the oxygen we breathe in is reduced. Rusting of iron and the burning of fuels are also redox processes.
What are corrosion and rancidity?
Corrosion is the slow damage of a metal by air, moisture or chemicals around it, as when iron rusts or silver tarnishes black. Rancidity is the oxidation of fats and oils in food, which produces an unpleasant smell and taste. Rancidity is slowed by adding antioxidants, packing food in nitrogen, or refrigeration.
