Class 8 Science NCERT Notes – Chapter 7: Particulate Nature of Matter (PDF, MindMap, Q&A, Quizzes)

Student Notes: The Particulate Nature of Matter

1. What is Matter Made Of?

• All matter is composed of a large number of extremely small particles called constituent particles. These particles are the basic building blocks of any substance.

• These particles are so tiny they cannot be seen even with an ordinary microscope.

• For example, a single piece of chalk is made up of many smaller units. Even if you grind chalk into a fine powder, each tiny speck is still chalk, just smaller. If you could keep breaking it down, you would eventually reach its basic constituent particles.

• Similarly, when sugar dissolves in water, it breaks down into its constituent particles, which are too small to see but can be tasted.

Historical Insight: The idea that matter is made of tiny, indivisible particles is not new. An ancient Indian philosopher, Acharya Kanad, called these particles Parmanu (atoms).

2. Key Properties of Matter’s Particles

• Interparticle Spaces: There are spaces between the particles of matter.
  • When you dissolve sugar in water, the sugar particles fit into the spaces between the water particles. This is why the final volume of the solution can be less than the combined volumes of the water and sugar before mixing.
  • Gases have a lot of space between their particles, which is why you can compress air in a syringe. Liquids, like water, are practically incompressible.

• Interparticle Attractions: Particles of matter are held together by forces of attraction.
  • The strength of these forces determines the physical state of the substance (solid, liquid, or gas).
  • These forces are strongest in solids, weaker in liquids, and weakest (negligible) in gases.

• Constant Motion: The constituent particles of matter are in constant motion.
  • This movement is why the color of a potassium permanganate crystal spreads through water without stirring.
  • It’s also why the fragrance of an incense stick spreads across a room; moving air particles hit the fragrance particles and help them spread.
  • Heating increases the movement of particles. For example, potassium permanganate spreads faster in hot water than in cold water because the water particles are moving more vigorously.

3. The Three States of Matter

The differences in interparticle spacing, attraction, and movement determine whether a substance is a solid, liquid, or gas.

A. Solids

• Shape and Volume: Solids have a definite shape and a definite volume.

• Particle Arrangement:
  • Particles are tightly and closely packed.
  • Interparticle attractions are very strong, holding the particles in fixed positions.
  • Interparticle spacing is minimal.

• Particle Movement: Particles cannot move freely past each other. They can only vibrate or oscillate in their fixed positions.

• Melting: When a solid is heated, its particles vibrate more vigorously. At a certain temperature, called the melting point, the vibrations become so strong that the particles break free from their fixed positions, and the solid turns into a liquid.

B. Liquids

• Shape and Volume: Liquids have a definite volume but no fixed shape. They take the shape of their container.

• Particle Arrangement:
  • Particles are slightly looser than in solids.
  • Interparticle attractions are weaker than in solids but strong enough to keep the particles close together.
  • Interparticle spacing is greater than in solids.

• Particle Movement: Particles are free to move past each other but only within the limited space of the liquid.

• Boiling: When a liquid is heated, its particles move more vigorously. At the boiling point, particles gain enough energy to escape the liquid and turn into a gas (vapour). Vapour formation that occurs slowly at the surface at any temperature is called evaporation.

C. Gases

• Shape and Volume: Gases have no fixed shape and no fixed volume. They spread out to fill the entire space available to them.

• Particle Arrangement:
  • Particles are far apart from each other.
  • Interparticle attractions are negligible (minimal).
  • Interparticle spacing is at a maximum.

• Particle Movement: Particles move freely and randomly in all directions.

What are Fluids? 

Both liquids and gases can flow and do not have a fixed shape. Because of these shared properties, they are classified as fluids.

4. Summary Table of States of Matter

Property	Solid	Liquid	Gas
Shape & Volume	Fixed shape, fixed volume	No fixed shape, fixed volume	No fixed shape, no fixed volume
Interparticle Spacing	Minimum	More than solids	Maximum
Interparticle Attraction	Maximum (very strong)	Weaker than solids	Minimum (negligible)
Particle Movement	Vibrate in fixed positions	Can move past each other	Move freely in all directions

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5. Glossary of Key Terms

Term	Definition
Atom	The tiny particles that make up all matter. Atoms of many elements can combine to form molecules.
Boiling Point	The temperature at which a liquid boils and turns into vapor at atmospheric pressure. At this point, vapor formation is very fast and occurs throughout the liquid.
Constituent Particle	The basic unit that makes up a larger piece of a substance or material.
Evaporation	The process of vapor formation that occurs at all temperatures, even below the boiling point, though slowly and only at the surface of a liquid.
Fluids	A classification for substances that can flow and do not retain a fixed shape. Both liquids and gases are classified as fluids.
Interparticle Attractions	The attractive forces that hold the constituent particles of matter together. The strength of these forces decides the physical state of a substance.
Interparticle Spaces	The available spaces between the particles of matter.
Matter	A substance composed of a large number of extremely small particles. The text explores matter in its three states: solid, liquid, and gas.
Melting Point	The minimum temperature at which a solid melts to become a liquid at the atmospheric pressure.
Molecule	A stable particle formed when a certain number of atoms of the same or different elements combine. For example, a water molecule is made of two hydrogen atoms and one oxygen atom.
Parmanu	An ancient Indian philosophical concept, proposed by Acharya Kanad, referring to tiny, indivisible, eternal particles that make up matter. This is an early conception of the atom.

Review Quiz

Answer the following questions in 2-3 sentences each.

1. According to Activity 7.1 (grinding chalk), what is matter ultimately composed of?

2. What happens to sugar particles when they are dissolved in water, and where do they go?

3. What are interparticle attractions and what role do they play in determining the state of matter?

4. Describe the arrangement and movement of particles in the solid state.

5. Define the term “melting point” and explain what happens to particle motion when a solid reaches this temperature.

6. Why do liquids, like water, have a fixed volume but not a fixed shape?

7. What is the “boiling point” of a liquid, and how does it differ from evaporation?

8. Explain why gases do not have a fixed shape or a fixed volume, using the concept of interparticle forces.

9. Based on Activity 7.6 (the syringe experiment), why are gases compressible while liquids are practically incompressible?

10. How does temperature affect the speed at which particles spread through a liquid, as demonstrated by the potassium permanganate experiment?

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Answer Key

1. Matter is composed of a large number of extremely small constituent particles. As demonstrated with the chalk, a substance can be broken down repeatedly into smaller pieces until you reach these basic building blocks, which cannot be broken down any further by physical means.

2. When sugar dissolves in water, it breaks up into its constituent particles which are too small to see. These tiny sugar particles then occupy the interparticle spaces available between the water particles.

3. Interparticle attractions are the attractive forces that hold the constituent particles of matter together. The strength of these attractions, which depends on the substance and the distance between particles, ultimately decides the physical state of the substance (solid, liquid, or gas).

4. In solids, particles are tightly packed in fixed positions due to very strong interparticle attractions. Their movement is restricted to vibrating or oscillating about their fixed positions; they cannot move past each other.

5. The melting point is the minimum temperature at which a solid melts to become a liquid at atmospheric pressure. When heated to its melting point, a solid’s particles vibrate so vigorously that they start to leave their fixed positions as the interparticle forces weaken.

6. Liquids have a fixed volume because their particles are still held close together by interparticle attractions, although these forces are weaker than in solids. They lack a fixed shape because the particles are free to move past each other within the limited space of the container.

7. The boiling point is the specific temperature at which a liquid rapidly turns into vapor throughout its bulk, not just at the surface. Evaporation is a slower process of vapor formation that occurs only at the surface of a liquid and can happen at any temperature below the boiling point.

8. Gases lack a fixed shape and volume because the interparticle attractions between their particles are negligible. This allows the particles to be completely free, moving in all directions and spreading out to occupy the entire available space of any container they are in.

9. Gases are compressible because their particles have a lot of space between them in their natural state. Pushing the plunger of the syringe forces these particles closer together, reducing the gas’s volume. Liquids are practically incompressible because their particles are already closely packed with very little interparticle space to reduce.

10. The potassium permanganate experiment shows that higher temperatures increase the movement of particles. The coloring spreads fastest in hot water and slowest in ice-cold water because the water particles move more vigorously at higher temperatures, hitting and spreading the permanganate particles more quickly.

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Essay Questions

Consider the following questions and formulate detailed, essay-style answers that synthesize information from across the chapter.

1. Compare and contrast the three states of matter—solid, liquid, and gas. Your answer should detail the differences and similarities in interparticle spacing, interparticle attraction, packing of particles, and particle movement for each state.

2. Describe the journey of a substance like ice as it is heated, melts into water, and then boils into water vapor. Explain the changes occurring at the particle level at each stage, focusing on thermal energy, interparticle forces, and particle arrangement.

3. Using the results from Activity 7.2 (dissolving sugar), Activity 7.7 (change in water level), and Activity 7.9 (spreading incense smoke), build an argument for the particulate nature of matter and the constant motion of these particles.

4. The text identifies both liquids and gases as “fluids.” Explain what properties these two states of matter share that lead to this classification and what key properties related to volume and compressibility distinguish them from one another.

5. Explain the ancient Indian philosophical concept of Parmanu as introduced by Acharya Kanad. How does this historical idea relate to the modern scientific understanding of the “constituent particles” that is presented throughout the chapter?

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Detailed briefing of Chapter 7: Particulate Nature of Matter

Introduction

This document provides a comprehensive analysis of the particulate nature of matter, outlining the fundamental principles that govern the properties of solids, liquids, and gases. The core concept is that all matter is composed of extremely small, constantly moving constituent particles. The physical state of a substance is determined by three key factors: the strength of the attractive forces between these particles (interparticle attractions), the distance between them (interparticle spacing), and their thermal energy, which dictates their movement.

Solids exhibit a definite shape and volume due to strong interparticle attractions that lock particles into a tightly packed, fixed structure, allowing only for vibrations. Liquids have a definite volume but no fixed shape because their weaker interparticle forces permit particles to move past one another within a limited space. Gases have neither a fixed shape nor a fixed volume, as their interparticle attractions are negligible, allowing particles to move freely and expand to fill any available space. The transition between these states is driven by changes in thermal energy, which alters the balance between particle motion and interparticle forces.

I. The Fundamental Particulate Theory of Matter

The foundational principle is that all substances are composed of a vast number of extremely small basic units known as constituent particles. These particles are too small to be observed even with an ordinary microscope.

A. Core Postulate and Evidence

• Matter is Divisible: Any piece of a substance, such as a chalk stick, can be physically broken down into progressively smaller specks. This process of physical change reduces the size of the pieces but does not alter the substance itself. Ultimately, this process would lead to the indivisible constituent particles that are the basic building blocks of the substance.

• Dissolution as Proof: The dissolution of sugar in water provides compelling evidence for this theory. When sugar dissolves, its grains break down into millions of constituent particles. These particles become imperceptible but their presence is confirmed by the sweet taste of the water. They distribute themselves by occupying the available spaces between the water particles, known as interparticle spaces.

B. Historical Context: The Concept of Parmanu

The idea that matter is composed of tiny, indivisible particles is not a modern one. Acharya Kanad, an ancient Indian philosopher, articulated this concept in his work Vaisheshika Sutras. He proposed that matter is made up of eternal, indivisible particles called Parmanu (atoms).

II. Defining Properties of Matter: Forces, Spacing, and Energy

The distinct characteristics of solids, liquids, and gases are a direct result of the interplay between the forces holding their particles together, the space between them, and their inherent energy.

A. Interparticle Attractions

The constituent particles of matter are held together by forces that are attractive in nature. The strength of these interparticle attractions is a critical factor in determining a substance’s physical state.

• The force’s strength is dependent on the nature of the substance and the distance between particles.
• The attractive force decreases drastically with even a slight increase in interparticle distance.

B. The Role of Thermal Energy

Thermal (heat) energy is the primary determinant of the physical state of matter. It directly influences the motion and spacing of particles.

• Low Thermal Energy (Solids): Particles have low energy, remain close, and experience strong attractive forces, restricting their movement to small vibrations.

• Increased Thermal Energy (Liquids): At the melting point, thermal energy overcomes the attractive forces enough to allow particles to move away from their fixed positions, transforming the solid into a liquid.

• High Thermal Energy (Gases): In the gaseous state, particles possess sufficient energy to completely overcome interparticle attractions, allowing them to move freely and independently in all directions.

III. Analysis of the Three States of Matter

The properties of each state of matter can be explained through the particulate model.

A. The Solid State

• Macroscopic Properties: Solids have a definite shape and a definite volume.

• Particulate Explanation:
  • Particles are tightly and closely packed.
  • Interparticle attractions are very strong, holding particles in fixed positions.Particle movement is restricted to vibrations or oscillations around their fixed positions; they cannot move past each other.

• Transition to Liquid (Melting): When heated, particles in a solid vibrate more vigorously. At a specific temperature, the vibrations become so intense that particles break free from their fixed positions, and the solid melts into a liquid. This temperature is the melting point, defined as the minimum temperature at which a solid becomes a liquid at atmospheric pressure.

• Melting Point Data: The strength of interparticle forces is reflected in the melting point.

S.No.	Material	Melting Point
1.	Ice	0 °C
2.	Urea	133 °C
3.	Iron	1538 °C

B. The Liquid State

• Macroscopic Properties: Liquids have a definite volume but no fixed shape; they take the shape of their container.

• Particulate Explanation:
  • Interparticle attractions are weaker than in solids but still strong enough to keep the particles close together.
  • Particles are free to move past one another but are confined within a limited space.
  • Interparticle spacing is slightly greater than in solids (with exceptions like water, where ice particles are farther apart).

• Transition to Gas (Vaporization):
  • Boiling: When a liquid is heated to its boiling point, particle movement becomes so vigorous that particles can escape the liquid state entirely, forming a gas (vapor). This process occurs rapidly throughout the bulk of the liquid.
  • Evaporation: Vapor formation can also occur slowly at any temperature, but only from the surface of the liquid.

C. The Gaseous State

• Macroscopic Properties: Gases have no fixed shape and no fixed volume; they expand to fill the entire available space.

• Particulate Explanation:
  • Interparticle attractions are negligible.
  • Particles move freely, rapidly, and randomly in all directions.
  • Interparticle spacing is maximal, leading to high compressibility. This is demonstrated by pushing the plunger of a syringe filled with air; the volume of the gas decreases as particles are forced closer together.

• Classification as Fluids: Both liquids and gases are classified as fluids because they have the ability to flow and do not retain a fixed shape.

D. Comparative Summary of States

Property	Solid	Liquid	Gas
Interparticle Spacing	Minimum	Little more than in solids	Maximum
Packing of Particles	Closely packed	A little loosely packed than in solids	Particles are free
Interparticle Attraction	Maximum	Slightly weaker than in solids	Minimum (negligible)
Movement of Particles	Negligible (only vibrations)	Restricted to limited space	In all the available space
Shape & Volume	Fixed shape, fixed volume	No fixed shape, fixed volume	No fixed shape, no fixed volume

IV. Evidence of Particle Motion

The theory that particles are in constant, random motion is supported by several observable phenomena.

• Diffusion in Liquids: When a grain of potassium permanganate is placed in water, pink streaks are initially observed. Over time, the entire volume of water becomes uniformly pink. This occurs because the constantly moving water particles collide with and pull away particles of potassium permanganate, distributing them throughout the liquid.

• Diffusion in Gases: The fragrance from a burning incense stick in one corner of a room will eventually be smelled throughout the entire room. This happens as the moving particles of air collide with the fragrance particles, helping them to spread and fill the available space.

• Influence of Temperature on Motion: The rate of diffusion increases with temperature. Potassium permanganate spreads fastest in hot water, slower in room-temperature water, and slowest in ice-cold water. This demonstrates that providing heat increases the kinetic energy and speed of particle movement.

V. Advanced Concepts and Applications

A. Identification of Constituent Particles: Atoms and Molecules

The document clarifies that the “constituent particles” that make up matter are atoms and molecules.

• Atoms: An element like iron is made up of atoms of iron. Some atoms (e.g., hydrogen, oxygen) cannot exist independently.

• Molecules: A stable particle formed when atoms combine. For example, two hydrogen atoms combine to form a hydrogen molecule. A water molecule is composed of two hydrogen atoms and one oxygen atom.

B. Practical Application: How Soap Works

The particulate nature of matter explains everyday processes like cleaning. When washing oily clothes with soap, numerous soap particles surround the oil particles. One end of each soap particle attaches to the oil, while the other end mixes with water, effectively lifting the oil from the fabric so it can be washed away.