NCERT Class 9 Science Chapter 1 Notes

NCERT Class 9 Science - Chapter 1: Matter in Our Surroundings

Introduction

Everything in our surroundings is made up of matter. Matter is anything that has mass and occupies space. This chapter discusses the physical nature, characteristics, and states of matter, along with concepts like diffusion, evaporation, and interconversion of states.

Physical Nature of Matter

1. Matter is Made of Particles

   • Matter is composed of small particles that are continuously moving.
   • The particles of matter are extremely small and cannot be seen with the naked eye.

2. Characteristics of Particles of Matter

   • Particles have spaces between them: This property allows substances like salt or sugar to dissolve in water.
   • Particles are continuously moving: This movement is called the kinetic energy of the particles.
   • Particles attract each other: The force of attraction varies in different types of matter.

States of Matter

Matter exists in three main states:

1. Solid State

   • Definite shape and volume.
   • High density and incompressible.
   • Strong intermolecular forces of attraction.
   • Particles vibrate in fixed positions.

2. Liquid State

   • No definite shape but definite volume.
   • Lesser density than solids.
   • Weaker intermolecular forces compared to solids.
   • Particles can move freely.

3. Gaseous State

   • Neither definite shape nor definite volume.
   • Least density among the three states.
   • Very weak intermolecular forces.
   • Particles move randomly at high speeds.

Change of State of Matter

Matter can change from one state to another by altering temperature or pressure.

1. Effect of Temperature

   • Melting (Solid to Liquid): The temperature at which a solid changes into a liquid is called its melting point (e.g., ice melts at 0°C).
   • Boiling (Liquid to Gas): The temperature at which a liquid changes into a gas is called its boiling point (e.g., water boils at 100°C).
   • Sublimation: Some substances directly change from solid to gas (e.g., camphor, dry ice).

2. Effect of Pressure

   • Increasing pressure can turn gases into liquids (e.g., LPG and CNG are stored under high pressure).

Evaporation

Evaporation is the process by which a liquid changes into a gas at a temperature below its boiling point.

Factors Affecting Evaporation

• Surface Area: Greater the surface area, faster the evaporation.
• Temperature: Higher temperature increases evaporation rate.
• Humidity: Higher humidity reduces evaporation.
• Wind Speed: Higher wind speed enhances evaporation.

Cooling Effect of Evaporation

• Evaporation causes cooling as it absorbs heat from the surroundings (e.g., sweating cools the body).

Conclusion

This chapter explains the fundamental nature of matter and its properties. The concepts of state change, evaporation, and the behavior of matter under different conditions are crucial to understanding chemistry and everyday phenomena.

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