Chemical Reactions and Equations — Class 10 Science Notes | CBSE Chapter 1

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 5–8 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:

• 🎨 Change in colour — iron turns reddish-brown when it rusts
• 💨 Gas is released — vinegar + baking soda = fizzy CO₂ bubbles
• 🌡️ Temperature changes — burning paper releases heat
• ⚪ A solid appears in a liquid — this is called a precipitate
• 👃 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 → Magnesium Oxide

Chemical equation:
2Mg + O₂ → 2MgO

The arrow (→) means “produces” or “gives.” Reactants go on the left, products go on the right.

Symbols you’ll see in equations

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 + H₂O → Fe₃O₄ + H₂

Step 1: Count atoms on each side.

Step 2: Start with the element that appears in the fewest formulas. Balance Fe first → put 3 before Fe on the left.

3Fe + H₂O → Fe₃O₄ + H₂

Step 3: Balance O → put 4 before H₂O.

3Fe + 4H₂O → Fe₃O₄ + H₂

Step 4: Now H is off (8 on left, 2 on right) → put 4 before H₂.

3Fe + 4H₂O → Fe₃O₄ + 4H₂

Final check: Fe: 3=3 ✓ | H: 8=8 ✓ | O: 4=4 ✓

Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂

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 → 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) + H₂O(l) → Ca(OH)₂(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 → A + B

What causes it to break apart? Three things:

• Heat (thermal decomposition) — 2Pb(NO₃)₂ ^Δ→ 2PbO + 4NO₂ + O₂
• Electricity (electrolytic decomposition) — 2H₂O ^electricity→ 2H₂ + O₂
• Sunlight (photolytic decomposition) — 2AgBr ^sunlight→ 2Ag + Br₂

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 → 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) + CuSO₄(aq) → FeSO₄(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 → AD + CB

Example:

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄↓ + 2NaCl(aq)

A white solid (BaSO₄) 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 + H₂ → Cu + H₂O

• CuO loses oxygen → it’s reduced
• H₂ gains oxygen → 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 + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (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

Solved Examples (Step-by-Step)

Example 1: Balance this equation

H₂ + O₂ → H₂O

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 H₂O → H₂ + O₂ → 2H₂O
Now: Left: H=2, O=2 | Right: H=4, O=2
Oxygen matches now, but hydrogen doesn’t.

Step 3: Put 2 before H₂ → 2H₂ + O₂ → 2H₂O
Left: H=4, O=2 | Right: H=4, O=2 ✓ Balanced!

Answer: 2H₂ + O₂ → 2H₂O

Example 2: Identify the reaction type

Zn + H₂SO₄ → ZnSO₄ + H₂

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

MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂

Look at what happened:

• HCl lost its hydrogen → HCl is oxidised
• MnO₂ lost its oxygen → MnO₂ is reduced

Since both happened in one reaction → this is a redox reaction.

Important Questions for Board Exams

1-Mark Questions

1. What is a chemical equation?
2. Define a combination reaction with one example.
3. What is a displacement reaction?
4. What type of reaction happens when silver chloride turns grey in sunlight?
5. Define rancidity.

2-Mark Questions

1. Explain the difference between exothermic and endothermic reactions with one example each.
2. Why is it necessary to balance a chemical equation?
3. What happens when iron nails are placed in copper sulphate solution? Write the equation.
4. What is corrosion? Give two ways to prevent it.
5. Why are chip packets flushed with nitrogen gas?

3-Mark Questions

1. Balance: Fe₂O₃ + Al → Al₂O₃ + Fe. Name the reaction type.
2. Explain the thermite reaction. What type of reaction is it?
3. What is a double displacement reaction? Give one example with a balanced equation.
4. Distinguish between oxidation and reduction with examples.
5. List five observations that indicate a chemical reaction has taken place.

5-Mark Questions

1. Write and balance the equations for: (a) burning of magnesium ribbon (b) electrolysis of water (c) reaction of zinc with dilute H₂SO₄. Identify each reaction type.
2. 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 → products on right
• Balancing is based on the Law of Conservation of Mass — atoms in = atoms out
• Combination: A + B → AB (two become one)
• Decomposition: AB → 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 O₂ / lose H₂ / lose e⁻) + 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

Q2. In the equation 2Mg + O₂ → 2MgO, the reactants are:

a) MgO
b) Mg and O₂
c) Mg and MgO
d) O₂ and MgO

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

Q4. CaO + H₂O → Ca(OH)₂ is an example of:

a) Decomposition reaction
b) Displacement reaction
c) Combination reaction
d) Double displacement reaction

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

Q6. In the reaction Fe + CuSO₄ → FeSO₄ + 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

Q7. A white precipitate of BaSO₄ forms when Na₂SO₄ reacts with BaCl₂. This is a:

a) Combination reaction
b) Decomposition reaction
c) Displacement reaction
d) Double displacement reaction

Q8. In the reaction CuO + H₂ → Cu + H₂O, which substance is oxidised?

a) CuO
b) H₂
c) Cu
d) H₂O

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

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

Q11. The green coating that forms on copper over time is due to:

a) Rancidity
b) Rusting
c) Corrosion
d) Decomposition

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

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