Class 8 chapter 7

 What Is Matter Composed of? — Constituent particles

Matter is composed of tiny fundamental building blocks that combine in stages to form everything in the physical universe. 

To understand what matter is made of, you can break it down level by level, starting from the macroscopic scale down to the subatomic world:

1. Molecules and Atoms (Chemical Level)

At the base of everyday physical matter are atoms and molecules:

- Molecules: Groups of two or more atoms bound together chemically (e.g., H2O or CO2).

- Atoms: The basic unit of a chemical element. An atom consists of a dense central nucleus surrounded by a cloud of negatively charged electrons.

2. Subatomic Particles (Atomic Level)

Inside every atom, there are three primary subatomic particles:

- Proton:

  * Charge: Positive (+1e)

  * Location: Atomic Nucleus

  * Role/Nature: Defines the element (atomic number Z).

- Neutron:

  * Charge: Neutral (0)

  * Location: Atomic Nucleus

  * Role/Nature: Adds mass and provides nuclear binding force.

- Electron:

  * Charge: Negative (-1e)

  * Location: Electron Cloud

  * Role/Nature: Bound by electromagnetic force; determines chemical reactivity.

3. Elementary Particles (Fundamental Level)

According to the Standard Model of Particle Physics, subatomic particles are split into two categories: fundamental (elementary) particles that cannot be divided further, and composite particles.

Quarks (Composite Builders):

- Protons and neutrons are composite particles called hadrons (specifically baryons).

- They are made of fundamental particles called quarks, held together by the strong force (carried by massless force-particles called gluons):

  * Proton: Composed of 2 Up Quarks (+2/3e) + 1 Down Quark (-1/3e) -> Net charge = +1.

  * Neutron: Composed of 1 Up Quark (+2/3e) + 2 Down Quarks (-1/3e) -> Net charge = 0.

Leptons (Truly Elementary):

- Electrons are leptons—they are truly fundamental particles and are not made of smaller quarks. 

- Other fundamental leptons include muons, taus, and their corresponding neutrinos.

Interparticle Spaces and Interparticle Attractions

1. Interparticle Spaces

Interparticle spaces are the spaces or gaps present between the particles of matter (atoms, molecules, or ions).

Interparticle Spaces in Different States of Matter

1. Solids:

• Particles are closely packed together.
• Interparticle spaces are very small.
• Therefore, solids have a definite shape and definite volume.
• Example: Iron, stone, and ice.

2. Liquids:

Particles are less closely packed than in solids.

Interparticle spaces are larger than in solids.

Therefore, liquids have a definite volume but no definite shape.

Example: Water, milk, and oil.

3. Gases:

Particles are very far apart from one another.

Interparticle spaces are very large.

Therefore, gases have neither a definite shape nor a definite volume.

Example: Oxygen, nitrogen, and carbon dioxide.

2. Interparticle Attractions

Interparticle attractions are the forces of attraction between the particles of matter that tend to hold them together.

Interparticle Attractions in Different States of Matter:-

1. Solids:

Particles have strong attractive interactions.

They remain close to fixed positions and vibrate about those positions.

Therefore, solids are rigid and difficult to compress.

2. Liquids:

Particles have sufficient attractive interactions to remain close together, but they can move past one another.

Therefore, liquids can flow and take the shape of their containers.

3. Gases:

At ordinary conditions, attractive interactions between gas particles are usually weak compared with their kinetic energy.

Particles move freely and randomly in all directions.

Therefore, gases spread out and can be compressed easily.

How Particles Move in Different States of Matter — Effect of Temperature on Particle Motion

A. Movement of Particles in Different States of Matter

All matter is made up of tiny particles. These particles are in continuous motion, but their movement differs in solids, liquids and gases because of differences in interparticle spaces and forces of attraction.

In Solids

• Particles are very closely packed.
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• The force of attraction between particles is generally very strong.
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• Particles cannot move freely from one place to another.
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• They vibrate about their fixed positions.
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• Solids have a definite shape and definite volume.
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• Examples: Ice, iron, stone and wood.

In Liquids

• Particles are less closely packed than in solids.
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• The force of attraction between particles is weaker than in solids.
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• Particles can move and slide past one another.
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• Liquids can flow and take the shape of their container.
• 
  
• Liquids have a definite volume but no definite shape.
• 
  
• Examples: Water, milk and oil.

3. In Gases

• Particles are very far apart compared with those in solids and liquids.
• The force of attraction between particles is generally very weak.
• Particles move rapidly and randomly in all directions.
• They collide with one another and with the walls of their container.
• Gases have neither a definite shape nor a definite volume. They spread to fill the available space.
• Examples: Oxygen, nitrogen and carbon dioxide.

Effect of Temperature on Particle Motion

Temperature is a measure of how hot or cold a body is. At the microscopic level, the temperature of a substance is related to the average kinetic energy of its particles.

When temperature increases:

• Particles gain average kinetic energy.

• Their motion becomes more vigorous.

• Diffusion generally occurs faster.

• Solids may expand as their particles vibrate more vigorously.

• With sufficient heating, a solid may melt into a liquid and a liquid may boil or evaporate into a gas.

When temperature decreases:

• Particles lose average kinetic energy.

• Their motion becomes less vigorous.

• Diffusion generally occurs more slowly.

• With sufficient cooling, a gas may condense into a liquid, and a liquid may freeze into a solid.

Example: When ice is heated, its particles vibrate more vigorously. At its melting point, the particles gain enough energy to move past one another, and the ice changes into liquid water.

Important point: Particles do not stop moving completely at ordinary temperatures, even in solids. They continue to vibrate about their positions.

Diffusion in Liquids and Gases

Diffusion is the process by which particles of one substance spread into another substance due to their random motion, resulting in the mixing of the substances.

Diffusion generally occurs from a region of higher concentration to a region of lower concentration until the particles become more evenly distributed.

 Diffusion in Gases

• Gas particles move rapidly and randomly in all directions.
• There are large spaces between gas particles.
• Therefore, gas particles can spread and mix quickly.
• Diffusion is generally faster in gases than in liquids.
• Example 1: When perfume is sprayed in one corner of a room, its smell gradually spreads throughout the room.
• Example 2: The smell of cooking food spreads from the kitchen to other parts of a house.
• Example 3: A small amount of ammonia gas released in a suitable laboratory demonstration can spread through the surrounding air.

Diffusion in Liquids

• Liquid particles are in continuous random motion.
• They can move past one another.
• Particles of one liquid can spread among the particles of another liquid.
• Diffusion occurs more slowly in liquids than in gases under comparable conditions.
• Example 1: When a drop of ink is added to water, the ink gradually spreads throughout the water.
• Example 2: A crystal of potassium permanganate placed in water gradually colours the water purple, even without stirring.

Factors Affecting the Rate of Diffusion

1. Temperature: Higher temperature generally increases the rate of diffusion because particles move faster.

2. State of Matter: Diffusion is generally fastest in gases, slower in liquids and extremely slow in solids under ordinary conditions.

3. Concentration Difference: A larger difference in concentration generally produces a greater net movement of particles from the region of higher concentration to the region of lower concentration.

4. Particle Size and Nature of the Substance: The rate of diffusion also depends on the properties of the diffusing particles and the medium through which they move.