NCERT Class 9 Science Exploration: Chapter 9 — 9. Atomic Foundations of Matter
This chapter, '9. Atomic Foundations of Matter,' builds upon the atomic structure concepts from Chapter 8, 'Journey Inside the Atom.' It delves into the fundamental principles governing mass during physical and chemical changes. Through activities, students investigate that mass remains constant during physical changes like dissolving salt in water or tearing paper. The chapter then poses the question of whether this holds true for chemical changes, setting the stage to explore concepts like the Law of Conservation of Mass. It highlights how properties of elements change when they form compounds, using water as an example. This foundational understanding is crucial for comprehending stoichiometry and quantitative aspects of chemical reactions in further studies.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 9 |
| Subject | Science |
| Book | Exploration |
| Chapter | Chapter 9 — 9. Atomic Foundations of Matter |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 4 minutes |
| Word count | 650 |
Learning outcomes
- Understand that mass is conserved during physical changes.
- Investigate mass changes during physical processes like dissolving and tearing.
- Explore the concept of mass conservation in chemical reactions.
- Recognize that properties of elements change upon compound formation.
- Grasp the foundational principles for quantitative chemical analysis.
Vocabulary
| Word | Meaning |
|---|---|
| Atom | The basic unit of a chemical element. |
| Electrons | Negatively charged subatomic particles. |
| Protons | Positively charged subatomic particles. |
| Neutrons | Subatomic particles with no net electric charge. |
| Valence shell | The outermost electron shell of an atom. |
| Compound | A substance formed when two or more chemical elements are chemically bonded together. |
| Physical change | A change in a substance that does not change its chemical identity. |
| Chemical change | A change that results in the formation of new chemical substances. |
| Combustible | Able to catch fire and burn easily. |
| Combustion | The process of burning something. |
| Solution | A homogeneous mixture composed of two or more substances. |
| Baking soda | Sodium hydrogencarbonate (NaHCO3). |
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Practice questions
- What is a physical change? Give an example. Answer: A physical change is a change in a substance that does not alter its chemical composition. Example: Tearing a piece of paper.
- What is a chemical change? Give an example. Answer: A chemical change results in the formation of new chemical substances with different properties. Example: Reaction of baking soda with vinegar.
- According to the activity, does the mass change when salt dissolves in water? Answer: No, the mass of the solution is equal to the sum of the masses of water and salt, indicating no significant change in mass.
- Why is water different from hydrogen and oxygen? Answer: Water is a compound formed from hydrogen and oxygen, and it has entirely different properties (like extinguishing fire) than its constituent elements (hydrogen being combustible and oxygen supporting combustion).
Frequently asked questions
What is the main idea of Chapter 8, '9. Atomic Foundations of Matter'?
The chapter explores how mass behaves during physical and chemical changes, introducing the concept of mass conservation.
Are all samples of water chemically identical?
Yes, pure water (H2O) is chemically identical regardless of its source, though impurities can be present.
What is the difference between O and O2?
O represents a single oxygen atom, while O2 represents an oxygen molecule, consisting of two oxygen atoms bonded together.
Why does dissolved salt conduct electricity but sugar does not?
Dissolved salt (an ionic compound) dissociates into ions that can carry electric charge, while sugar (a molecular compound) does not form ions in solution.
Does mass change during a physical change like dissolving salt?
No, the activity shows that the mass of the solution is equal to the sum of the masses of the solute and solvent, indicating no significant mass change.
What is the significance of the water example in this chapter?
It illustrates that compounds have properties different from their constituent elements (hydrogen and oxygen), and mass is conserved when they combine.
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NCERT Class 9 Science — Exploration — Chapter 9 — 9. Atomic Foundations of Matter. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.