Class 9 Chapter 1 Matter in Our Surroundings

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 Matter: Anything that has mass and occupies space.

 Key Points:

  • Matter is made up of particles
  • Particles are in continuous motion
  • There is space between particles
  • Particles attract each other

 Examples:

  • Air
  • Water
  • Stone
  • Book

 Characteristics of Particles of Matter:

  1. Very small in size
  2. In constant motion
  3. Have space between them
  4. Show force of attraction

 States of Matter:

  1. Solid
    • Fixed shape and volume
    • Particles are tightly packed
  2. Liquid
    • Fixed volume, no fixed shape
    • Particles are less tightly packed
  3. Gas
    • No fixed shape and volume
    • Particles are far apart

Physical Nature of Matter :- 

Matter is made of particles:-
Matter consists of very small particles.

Characteristics of particles:-
1.Have space between them
    Example: Sugar dissolves in water

2.Are continuously moving:-
   Brownian motion

3.Attract each other:-
   Strong in solids
   Weak in gases

States of Matter

  • Solid: Fixed shape & volume (strong attraction)
  • Liquid: Fixed volume, no fixed shape
  • Gas: No fixed shape & volume (very weak attraction)

 Diffusion

  • Particles mix on their own
    • Fastest in gases, slowest in solids

Characteristics of particles of matter :

  1. Very small in size
    Particles of matter are extremely tiny and cannot be seen with naked eyes.
  2. Have spaces between them
    There are empty spaces between particles, which allow them to move and mix with each other (this explains Diffusion).
  3. Are constantly moving
    Particles are always in motion (this is related to Brownian Motion).
  4. Attract each other
    Particles have forces of attraction between them, which keep them together.

These four points explain the basic behavior of matter in solids, liquids, and gases.

Properties of solids, liquids, and gases :

1. Solids

  • Have a fixed shape and fixed volume
  • Particles are closely packed
  • Very strong force of attraction between particles
  • Do not flow
  • Almost no compressibility

2. Liquids

  • Have fixed volume but no fixed shape (take the shape of container)
  • Particles are less tightly packed than solids
  • Moderate force of attraction
  • Can flow easily
  • Slightly compressible

3. Gases

  • Have no fixed shape and no fixed volume
  • Particles are far apart
  • Very weak force of attraction
  • Flow very easily
  • Highly compressible
 

Melting Point

  • The temperature at which a solid changes into a liquid.
  • At this point, particles gain enough energy to overcome their attraction.
  • Example: Ice melts into water at 0°C (normal conditions).

Boiling Point

  • The temperature at which a liquid changes into a gas.
  • At this point, particles move very fast and escape into the air.
  • Example: Water boils into steam at 100°C (normal conditions).
Latent Heat :
   Latent heat is the heat energy absorbed or released by a                       substance without any change in temperature, during a change       of state.

  Types of Latent Heat

    1. Latent Heat of Fusion

  •   Heat required to change a solid into a liquid without changing    temperature.
  •   Example: Ice → Water at 0°C
    

   2. Latent Heat of Vaporization

  • Heat required to change a liquid into a gas without changing temperature.
  • Example: Water → Steam at 100°C

Sublimation: Sublimation is the process in which a solid changes directly into a gas without passing through the liquid state.

Example: Dry ice (solid carbon dioxide) turning into gas.


Effect of Change of Temperature and Pressure (on States of Matter)

1. Effect of Temperature

  • When temperature increases, particles gain energy and move faster.
    → Solid → Liquid → Gas
    (Example: Ice melts into water, water becomes steam)
  • When temperature decreases, particles lose energy and move slower.
    → Gas → Liquid → Solid
    (Example: Steam condenses into water, water freezes into ice)

2. Effect of Pressure

  • When pressure increases, particles come closer together.
    → Gas can change into liquid
    (Example: Gas cylinder – gas is stored in liquid form under high pressure)
  • When pressure decreases, particles move farther apart.
    → Liquid can change into gas easily 

Evaporation

Evaporation is the process by which a liquid changes into vapour (gas) from its surface at any temperature below its boiling point.

Example: Drying of wet clothes, water turning into vapour from an open bowl.

Factors Affecting Evaporation

  1. Temperature
    • Higher temperature → faster evaporation
    • Because particles gain more energy and escape easily
  2. Surface Area
    • Larger surface area → faster evaporation
    • Example: Water spreads and dries faster on a plate than in a glass
  3. Wind Speed (Air Flow)
    • Higher wind speed → faster evaporation
    • Air removes water vapour, allowing more liquid to evaporate
  4. Humidity
    • Higher humidity → slower evaporation
    • Because air already contains more water vapour
    • Low humidity → faster evaporation 

Deposition

Deposition is the process in which a gas changes directly into a solid without passing through the liquid state.

Example:

  • Formation of frost (water vapour in air directly turns into ice on cold surfaces)
  • Formation of snow from water vapour in clouds. 

Opposite Process:

  • Deposition is the opposite of sublimation.

Evaporation Causing Cooling

Evaporation causes cooling because when a liquid evaporates, it takes heat (energy) from its surroundings.

  • The particles with higher energy escape from the surface of the liquid.
  • This removes heat from the remaining liquid.
  • As a result, the liquid (and surroundings) become cooler.

Examples:

  • Sweating: Sweat evaporates from your skin and takes heat from your body, making you feel cool.
  • Water in an earthen pot (matka): Water stays cool because it slowly evaporates through tiny pores.
  • Perfume or sanitizer: Feels cold when applied because it evaporates quickly. 

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