Grade 8 Ch 7 Particulate Nature of Matter

 

CH 7 PARTICULATE NATURE OF MATTER

Introduction

• Matter exists in different forms: solids, liquids, gases.
• Questions to think: Why can we pile sand but not water? How does air give weight to a balloon?

7.1 What is Matter Composed of?

• Chalk experiment: breaking into smaller pieces → shows matter is made of small constituent particles.
• A Constituent particle is the basic unit that makes up a larger piece of a substance or material.
• Sugar in water: dissolves, not seen but tasted → proves particles exist and mix.
• The spaces between the particles are known as Interparticle space.
• Matter is composed of extremely small particles.
• Particles cannot be seen even with ordinary microscope.
• Interparticle spaces: sugar particles fill gaps between water particles.

7.2 What Decides Different States of Matter?

• The constituent particles of matter are held together through forces which are attractive in nature, these forces are called interparticle attractions.
• Strength depends on the nature of substance & the interparticle distance.
• The Strength of these forces (interparticle attractions ) decides the physical state of the substances (solid, liquid, gas).

Our Scientific Heritage: 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 tiny, indivisible eternal particles called Parmanu (atoms).

Analysis of the Three States of Matter

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

7.2.1  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

 7.2.2 . 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.

7.2.3 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.

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

7.4 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.

7.5 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.

 Thermal Energy and Change of State

• Thermal energy decides state of matter.
• Solids: low energy, strong attractions, only vibrations.
• Liquids: more energy, particles move within space.
• Gases: high energy, particles free, negligible attractions.
• Heating increases energy → changes state (melting, boiling, evaporation).

Snapshots (Key Points)

• Matter is made of very small particles.
• Particles have interparticle attractions.
• Solids: strongest forces, fixed shape & volume.
• Liquids: weaker forces, fixed volume, no fixed shape.
• Gases: negligible forces, no fixed shape & volume.
• Heating increases particle motion and changes state.

Keep the curiosity alive

1. Choose the correct option.

The primary difference between solids and liquids is that the constituent particles are:

(i) closely packed in solids, while they are stationary in liquids.

(ii) far apart in solids and have fixed position in liquids.

(iii) always moving in solids and have fixed position in liquids.

(iv) closely packed in solids and move past each other in liquids.

Answer:- (iv) closely packed in solids and move past each other in liquids.

2. Which of the following statements are true? Correct the false statements.

(i) Melting ice into water is an example of the transformation of a solid into a liquid.

Answer: True

(ii) Melting process involves a decrease in interparticle attractions during the transformation.

Answer: True

(iii) Solids have a fixed shape and a fixed volume.

Answer: True

(iv) The interparticle interactions in solids are very strong, and the interparticle spaces are very small.

Answer: True

(v) When we heat camphor in one corner of a room, the fragrance reaches all corners of the room.

Answer: True

(vi) On heating, we are adding energy to the camphor, and the energy is released as a smell.

Answer: False

Correction for (vi): On heating, we add energy to the camphor, which causes its particles to move faster and turn into a gas, spreading the fragrance throughout the room.

3. Choose the correct answer with justification. If we could remove all the constituent particles from a chair, what would happen?

(i) Nothing will change.

(ii) The chair will weigh less due to lost particles.

(iii) Nothing of the chair will remain.

Answer: (iii) Nothing of the chair will remain.

JUSTIFICATION:- Everything is made up of tiny particles. If all the particles that make up the chair are removed, then there will be nothing left. The chair would completely vanish because its entire structure is formed by those particles.

4. Why do gases mix easily, while solids do not?

Answer: Gases mix easily because their particles are far apart, have very weak interparticle attractions, and move freely in all directions. This allows gas particles to spread and mix with other gases quickly, like when smoke or fragrance fills a room. Solids, however, do not mix easily because their particles are closely packed, held together by strong interparticle attractions, and can only vibrate in fixed positions. This prevents solid particles from moving and mixing with other substances.

5. When spilled on the table, milk in a glass tumbler, flows and spreads out, but the glass tumbler stays in the same shape. Justify this statement.

Answer: Milk, a liquid, flows and spreads out when spilled on the table because its particles are loosely packed and have weaker interparticle attractions, allowing them to move past each other and take the shape of the surface they are on. The document explains that liquids have no fixed shape but a fixed volume, which is why milk spreads out. The glass tumbler, a solid, stays in the same shape because its particles are tightly packed with strong interparticle attractions, keeping them in fixed positions. This gives solids a definite shape and volume, so the tumbler does not change shape when milk is spilled.

6. Represent diagrammatically the changes in the arrangement of particles as ice melts and transforms into water vapour.

Answer:

7. Draw a picture representing particles present in the following:

(i) Aluminium foil    (ii) Glycerin   (iii) Methane gas

Answer:     

                  (i) Aluminium foil           (ii) Glycerin                     (iii) Methane gas

8. Observe Fig. 7.16a which shows the image of a candle that was just extinguished after burning for some time. Identify the different states of wax in the figure and match them with Fig. 7.16b showing the arrangement of particles.

                                             

Answer:

1. Solid wax (at base): Rigid, unmelted portion—matches tightly packed particles.

2.Liquid wax (melted pool): Flowing around wick—matches loosely arranged particles with movement.

3. Gaseous wax (vapour/smoke): Rising as fumes—matches widely spaced, freely moving particles.
The figure shows transitions: solid to liquid (melting) and liquid to gas (evaporation), with particle arrangements changing from fixed to mobile.

9. Why does the water in the ocean taste salty, even though the salt is not visible? Explain.

Answer: Ocean water tastes salty because it contains dissolved salts like sodium chloride. The salt is broken into tiny particles and mixes completely with water. These particles are too small to be seen with our eyes, but they are still present and give the salty taste.

10. Grains of rice and rice flour take the shape of the container when placed in different jars. Are they solids or liquids? Explain.

Answer: Grains of rice and rice flour are solids. They take the shape of the container when placed in different jars because, as solids, they consist of closely packed particles with strong interparticle attractions. However, unlike typical solids with a fixed shape, loose particles like rice grains and flour can shift and settle to fill the container’s shape due to gravity and the space between the particles. Their volume remains definite, and they do not flow like liquids, confirming they are solids.