Properties of Matter: Why Objects Behave the Way They Do

Everything around us has a shape, size, and presence.
But physics does not stop at what we see. It asks deeper questions.
Why does a stone sink while wood floats?
Why do gases spread in all directions?
Why does weight change but mass does not?
All these questions are answered by understanding the properties of matter.

Physical Properties and States of Matter

A physical property is a characteristic of matter that can be observed or measured without changing the substance itself.
Examples of physical properties include:

• Mass
• Volume
• Density
• State (solid, liquid, gas)

States of Matter

Matter exists in different physical states depending on:

• Arrangement of particles
• Intermolecular force
• Kinetic energy of particles

Solid State

In solids:

• Particles are closely packed
• Intermolecular force is very strong

Properties:

• Fixed shape
• Fixed volume
• Very low compressibility

Examples: stone, iron, wood

Liquid State

In liquids:

• Particles are less closely packed
• Intermolecular force is moderate

Properties:

• Fixed volume
• No fixed shape
• Can flow

Examples: water, oil, milk

Gaseous State

In gases:

• Particles are far apart
• Intermolecular force is very weak

Properties:

• No fixed shape
• No fixed volume
• Highly compressible

Examples: air, oxygen, carbon dioxide

Mass

Mass is the amount of matter present in a body.
Important points:

• Mass remains constant everywhere
• Mass does not depend on gravity
• SI unit of mass is kilogram (kg)
  Example:
A body has the same mass on Earth and on the Moon.

Weight

Weight is the force with which Earth attracts a body towards its centre.
Weight depends on:

• Mass of the body
• Acceleration due to gravity
  Formula:

Weight = Mass × g

Important points:

• Weight changes from place to place
• Weight is maximum at poles and minimum at equator
• Weight becomes zero in space

Difference Between Mass and Weight

Mass is a scalar quantity and remains constant.
Weight is a vector quantity and depends on gravity.
This difference is a very common exam trap.

Volume

Volume is the space occupied by a body.
SI unit of volume is cubic metre (m³).
Volume can be measured by:

• Measuring dimensions (regular solids)
• Water displacement method (irregular solids)

Density

Density is the mass per unit volume of a substance.
Formula:

Density = Mass / Volume

SI unit of density is kg m⁻³.
Density helps explain:

• Why some objects float
• Why some objects sink

Specific Gravity (Relative Density)

Specific gravity is the ratio of density of a substance to the density of water.

Specific Gravity = Density of substance / Density of water

Important points:

• It has no unit
• It is a pure number
• If specific gravity > 1 → substance sinks
• If specific gravity < 1 → substance floats

Principle of Archimedes

When a body is immersed partially or completely in a fluid, it experiences an upward force called buoyant force.
Archimedes’ principle states:
The loss of weight of a body immersed in a fluid is equal to the weight of the fluid displaced.
This principle explains floating and sinking.

Applications of Archimedes’ Principle

• Designing ships and submarines
• Determining density of substances
• Hydrometers and lactometers

Pressure

Pressure is defined as force acting per unit area.
Formula:

Pressure = Force / Area

SI unit of pressure is pascal (Pa).
Pressure increases when:

• Force increases
• Area decreases

Example:
A sharp knife cuts better than a blunt knife.

Pressure in Fluids

Pressure in liquids:

• Increases with depth
• Acts in all directions

This is why:

• Dams are thicker at the bottom
• Divers experience more pressure at greater depths

Atmospheric Pressure

The air around us exerts pressure on everything.
Atmospheric pressure:

• Decreases with altitude
• Is maximum at sea level

Barometer

A barometer is an instrument used to measure atmospheric pressure.
Mercury barometer works on the principle of:

• Pressure exerted by air balancing a column of mercury

Atmospheric pressure at sea level is equal to:

• 76 cm of mercury column

Importance of Barometer

• Weather prediction
• Measuring altitude
• Studying atmospheric changes

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FAQs – PROPERTIES OF MATTER (Exam-Focused)