Class 9 Science Chapter 9 Atomic Foundations of Matter – NCERT Solutions & Easy Notes Part 1 | PJ Sir

 

Class 9 Science Chapter 9 Atomic Foundations of Matter – NCERT Solutions & Easy Notes Part 1

Exploration – Grade 9 Science | Chapter 9
Part 1: Laws of Chemical Combination & Dalton’s Atomic Theory

By Priyank Joshi (PJ Sir)
Chemistry Educator & Mentor | 18+ Years of Teaching Experience

Atomic Foundations of Matter – Let's Understand It Simply

Have you ever wondered what happens to matter during a chemical reaction?

Does some mass disappear?
Can new mass suddenly be created?
And why do elements in a compound always combine in a particular proportion?

In Chapter 9 – Atomic Foundations of Matter, we begin answering these questions.

In Part 1, we will understand three important concepts:

1. Law of Conservation of Mass
2. Law of Constant Proportions
3. Dalton’s Atomic Theory

1. Law of Conservation of Mass

Suppose two substances react with each other.

Will their total mass change after the reaction?

No — if we consider the complete system.

The basic idea is:

Matter can neither be created nor destroyed during a chemical reaction.

Therefore:

Mass of Reactants = Mass of Products

This is called the Law of Conservation of Mass.

The chapter attributes this law to Antoine Lavoisier in 1789 and describes him as the Father of Modern Chemistry. CHAPTER 9 ATOMIC FOUNDATION OF …

Simple Example

Suppose:

Reactant A = 4 g
Reactant B = 6 g

Total mass before reaction:

4 + 6 = 10 g

Therefore, total mass of all the products after the reaction will also be:

10 g

🧠 PJ Sir Trick

Remember just one line:

“Before = After”

or

Total Mass of Reactants = Total Mass of Products

Why Can Mass Sometimes Appear to Decrease?

This is an important concept.

In the chapter, baking soda reacts with vinegar and produces carbon dioxide gas.

When the gas is allowed to escape, the final measured mass appears different.

But when the reaction is performed so that the gas remains within the system, the initial and final masses match. CHAPTER 9 ATOMIC FOUNDATION OF …

So remember:

If gas escapes from an open system, measured mass may appear to decrease.

That does not mean the Law of Conservation of Mass has been violated.

Quick Question

Q. 20 g of hydrogen reacts completely with 160 g of oxygen. According to the Law of Conservation of Mass, what mass of product will be formed?

Solution:

Total mass of reactants

= 20 g + 160 g

= 180 g

Therefore, mass of product = 180 g

This question is based directly on the chapter's Pause and Ponder section. CHAPTER 9 ATOMIC FOUNDATION OF …

2. Law of Constant Proportions

Now think about water.

Can hydrogen and oxygen combine in any random mass ratio and still form the same compound, water?

No.

Joseph Proust proposed that:

In a compound, elements are always present in a fixed ratio by mass.

This is called the Law of Constant Proportions.

It is also known as:

Law of Definite Proportions
or
Proust’s Law. CHAPTER 9 ATOMIC FOUNDATION OF …

Example: Water

According to the chapter, purified water contains hydrogen and oxygen in the mass ratio:

Hydrogen : Oxygen = 1 : 8

So if we decompose 9 g of water:

Hydrogen = 1 g

Oxygen = 8 g

Whether the purified water originally came from a river, borewell or ocean, this mass proportion remains the same. CHAPTER 9 ATOMIC FOUNDATION OF …

🧠 PJ Sir Trick

Remember:

Compound = Fixed Ratio

The source can change.

The proportion does not.

Quick Numerical

Sodium chloride contains sodium and chlorine in the mass ratio:

23 : 35.5

If 46 g sodium reacts completely, how much chlorine is required?

Since:

23 g Na requires 35.5 g Cl

Therefore:

46 g Na requires

(35.5 ÷ 23) × 46

= 71 g

So,

Answer = 71 g chlorine

The same example is given in the chapter. CHAPTER 9 ATOMIC FOUNDATION OF …

3. Dalton’s Atomic Theory

Now an important question:

Why is mass conserved?

And why do elements combine in fixed proportions?

John Dalton tried to explain such observations using his Atomic Theory.

The chapter presents the following main ideas:

Dalton proposed that:

1. All matter is made of extremely small particles called atoms.

2. Atoms participate in chemical reactions.

3. Atoms cannot be created or destroyed during a chemical reaction.

4. Atoms of a given element are identical in mass and chemical properties.

5. Atoms of different elements have different masses and chemical properties.

6. Atoms combine in simple whole-number ratios to form compounds.

7. The relative number and kinds of atoms remain constant in a given compound. CHAPTER 9 ATOMIC FOUNDATION OF …

Understand Dalton With One Example

When hydrogen and oxygen combine to form water, their atoms are not simply destroyed.

Instead, the atoms combine and become arranged as part of the new substance.

Similarly, the chapter explains that when magnesium burns in air, magnesium atoms combine with oxygen atoms to form magnesium oxide. CHAPTER 9 ATOMIC FOUNDATION OF …

🧠 PJ Sir Concept

Think of atoms like LEGO blocks.

During an ordinary chemical reaction:

Blocks are rearranged.

They are not simply made to disappear.

This makes the idea of conservation of mass easier to understand.

Law of Conservation of Mass vs Law of Constant Proportions

Law	What to Remember
Law of Conservation of Mass	Total mass remains conserved during a chemical reaction
Law of Constant Proportions	Elements in a compound occur in a fixed ratio by mass
Scientist	Antoine Lavoisier → Conservation of Mass
Scientist	Joseph Proust → Constant Proportions

30-Second Revision 🚀

Before leaving this page, remember these 6 points:

① Mass of reactants = Mass of products

② Matter is not created or destroyed in a chemical reaction

③ Antoine Lavoisier → Law of Conservation of Mass

④ A compound has a fixed proportion of its elements by mass

⑤ Joseph Proust → Law of Constant Proportions

⑥ Dalton explained matter in terms of atoms and their combinations

Check Yourself 🎯

Try these without looking above:

Q1. What is the Law of Conservation of Mass?

Q2. Why may the measured mass appear to decrease when a gas-producing reaction is carried out in an open container?

Q3. Who proposed the Law of Constant Proportions?

Q4. Hydrogen and oxygen occur in purified water in what mass ratio?

Q5. State any three postulates of Dalton’s Atomic Theory.

Q6. If 12 g of one reactant combines completely with 28 g of another reactant, what will be the total mass of products according to the Law of Conservation of Mass?

Answer to Q6: 40 g

What Will We Learn in Part 2?

Now we know that matter consists of atoms.

But this leads to the next question:

Why do atoms combine with each other?

And when they combine:

Why do some atoms share electrons while others transfer them?

In Part 2 – Chemical Bonding Made Easy, we will understand:

Molecules → Chemical Bonds → Covalent Bond → H₂ → Cl₂ → O₂ → HCl → H₂O → Ionic Bond → NaCl → Cations & Anions

The textbook itself moves from Dalton’s theory into how atoms combine and then introduces electron sharing and transfer. CHAPTER 9 ATOMIC FOUNDATION OF … CHAPTER 9 ATOMIC FOUNDATION OF …

A Message from PJ Sir

Don't try to memorise Chemistry before you understand it.

If the concept is clear, formulas and reactions become much easier to remember.

Build your fundamentals now—because the same ideas will become much more important as you move towards higher-level Chemistry.

— Priyank Joshi (PJ Sir)
Chemistry Educator & Mentor

Reference

NCERT Exploration – Grade 9 Science, Chapter 9: Atomic Foundations of Matter.

This page contains independently written explanations, examples and learning support prepared by Priyank Joshi (PJ Sir) for students.

Continue Chapter 9 – Atomic Foundations of Matter

Part 1 – Laws of Chemical Combination & Dalton’s Atomic Theory
You are reading Part 1.

Part 2 – Chemical Bonding Made Easy: Covalent & Ionic Bonds
PART 2

Part 3 – Chemical Formulae Made Easy: Ions, Valency & Criss-Cross Method
PART 3

Part 4 – Properties, Molecular Mass & Formula Unit Mass
PART 4