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Matter in Our Surroundings

CBSE Class 9 Science • Chapter 1 • Detailed Master Notes

Chapter Overview:

Everything in this universe is made up of material which scientists call "matter". This chapter deals with the physical nature of matter, its particles, states of matter (Solid, Liquid, Gas), and their inter-conversion through Temperature and Pressure effects, and the phenomenon of Evaporation.

1. Physical Nature of Matter

Definition: Anything that occupies space (Volume) and has Mass is called Matter. E.g., Air, Food, Stones, Clouds, Stars.

Important SI Units:

Mass: Kilogram ($kg$)
Volume: Cubic metre ($m^3$). (Common unit: Litre ($L$)). $1 L = 1 dm^3$, $1 L = 1000 mL$, $1 mL = 1 cm^3$.
Density: Mass per unit volume. Density = $\frac{Mass}{Volume}$. SI Unit: $kg/m^3$.
Pressure: Pascal ($Pa$). Atmosphere ($atm$) is a common unit. $1 atm = 1.01 \times 10^5 Pa$.

Characteristics of Particles of Matter

Made of tiny particles: Matter is particulate, not continuous (like a block of wood). Dissolving KMnO₄ crystals in water shows that just a few crystals can color a large volume of water (Millions of particles in one crystal).
Have space between them: When we make tea, coffee, or lemonade, particles of one type of matter get into the spaces between particles of the other.
Continuously moving: Particles possess Kinetic Energy. As temperature rises, particles move faster.
Example: Smell of hot sizzling food reaches you several meters away, but for cold food, you have to go close (Diffusion).
Attract each other: Force of attraction keeps particles together. Strength of force varies (e.g., easy to break a stream of water with hand, but impossible to break an iron nail).

2. States of Matter

Matter around us exists in three states based on particle arrangement.

Property	Solid	Liquid	Gas
Shape & Volume	Definite shape, Fixed volume.	No fixed shape (takes shape of container), Fixed volume.	No fixed shape, No fixed volume.
Compressibility	Negligible.	Low.	High (e.g., LPG, CNG cylinders).
Particle Packing	Very close (Ordered).	Less close (Can slide over each other).	Free to move (Far apart).
Force of Attraction	Maximum.	Medium.	Minimum.
Kinetic Energy	Minimum (Vibrate at position).	More than solids.	Maximum (Move randomly).

3. Change of State of Matter

Matter can change from one state to another by changing Temperature or Pressure.

(a) Effect of Change of Temperature

Melting Point (Solid to Liquid): The temperature at which a solid melts to become a liquid at atmospheric pressure.
Ice melts at $0^\circ C$ (273.15 K).
Boiling Point (Liquid to Gas): The temperature at which a liquid starts boiling at atmospheric pressure.
Water boils at $100^\circ C$ (373 K).
Sublimation: Change of state directly from solid to gas without changing into liquid state (e.g., Camphor, Ammonium Chloride).
Deposition: Direct change of gas to solid without changing into liquid.

Latent Heat (Hidden Heat): The heat energy supplied is used up in overcoming the forces of attraction between the particles. Temperature does not rise during the phase change.

Latent Heat of Fusion: Amount of heat energy required to change $1 kg$ of solid into liquid at atmospheric pressure at its melting point.
Latent Heat of Vaporization: Amount of heat energy required to change $1 kg$ of liquid into gas at atmospheric pressure at its boiling point.

(b) Effect of Change of Pressure

Applying pressure and reducing temperature can liquefy gases.

Solid Carbon Dioxide ($CO_2$): Also known as Dry Ice. It is stored under high pressure. If pressure decreases to 1 atmosphere, it converts directly to gas (Sublimation) without becoming liquid. This is why it's called 'dry' ice.

4. Temperature Conversion

$$ T(K) = T(^\circ C) + 273 $$

Kelvin (K) is the SI unit of temperature. $0^\circ C = 273.15 K$ (Round off to 273).

To go from Celsius to Kelvin: Add 273.
To go from Kelvin to Celsius: Subtract 273.

5. Evaporation

Definition: The phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation. Unlike boiling (a bulk phenomenon), evaporation is a surface phenomenon.

Factors affecting Evaporation:

Surface Area: Increased surface area increases evaporation (e.g., Spreading out wet clothes to dry).
Temperature: Higher temp $\rightarrow$ higher kinetic energy $\rightarrow$ faster evaporation.
Humidity: Amount of water vapour in air. Higher humidity $\rightarrow$ lower evaporation.
Wind Speed: Wind moves water vapour away $\rightarrow$ faster evaporation.

Evaporation causes Cooling:

Particles of liquid absorb energy from the surroundings to regain the energy lost during evaporation, making the surroundings cool.

Acetone/Perfume: When poured on palm, particles gain energy from palm/surroundings and evaporate, causing cooling sensation.
Cotton Clothes in Summer: Cotton is a good absorber of water. It absorbs sweat and exposes it to the atmosphere for easy evaporation, cooling the body.
Earthen Pot (Matka): Water seeps through tiny pores, evaporates from the surface, and takes latent heat from the remaining water, cooling it down.

6. Two More States of Matter (Bonus)

Plasma: Super energetic and super excited particles. These particles are in the form of ionized gases.
Example: Fluorescent tube (helium gas), Neon sign bulbs (neon gas). The sun and stars glow because of plasma created by high temperature.
Bose-Einstein Condensate (BEC): Formed by cooling a gas of extremely low density (one hundred-thousandth of normal air) to super-low temperatures. Predicted by Albert Einstein and Satyendra Nath Bose.

Practice Zone

Q1: Convert 573 K to Celsius scale.

Ans: $573 - 273 = 300^\circ C$.

Q2: Why is ice at 273 K more effective in cooling than water at the same temperature?

Ans: Ice at 273 K ($0^\circ C$) has less energy than water at the same temperature because water particles have absorbed extra energy in the form of latent heat of fusion during melting.