Important Points: Matter in Our Surroundings - Science Class 9 PDF Download

What is Matter?

• Matter is anything that occupies space and has mass.
• Matter consists of very small particles which may be atoms, ions or molecules.
• Matter commonly exists in three primary states: solids, liquids and gases.

Matter

Characteristics of Particles of Matter

Motion and Kinetic Energy

Particles of matter are always in motion; this motion gives them kinetic energy.

• The kinetic energy of particles is lowest in solids, greater in liquids and highest in gases.

Kinetic Energy & Intermolecular Forces

Intermolecular Forces

Particles attract one another by forces called intermolecular forces.

• Intermolecular forces are strongest in solids, weaker in liquids and weakest in gases.

Intermolecular Spaces

Intermolecular Spaces

• The space between particles is called the intermolecular space.
• Intermolecular spaces are smallest in solids, larger in liquids and largest in gases.
• Generally, the smaller the intermolecular spaces, the stronger the intermolecular forces between particles.

States of Matter

Solids

• Particles are closely packed in a fixed, regular arrangement and can only vibrate about fixed positions.
• Solids have strong intermolecular forces, very small intermolecular spaces and low kinetic energy.
• Solids possess a definite shape and a definite volume.

Properties of Solid, Liquid & Gas

Liquids

• Particles are loosely packed compared to solids and can move/slide past one another.
• Liquids have moderate intermolecular forces, larger intermolecular spaces than solids and higher kinetic energy than solids.
• Liquids have a definite volume but take the shape of the container that holds them.

Gases

• Particles are far apart and move randomly with high speeds.
• Gases have very weak intermolecular forces, the largest intermolecular spaces and the highest kinetic energy among the three states.
• Gases do not have a definite shape or a definite volume; they expand to fill the available space.

Change in States of Matter

The states of matter are interconvertible and can be changed by changing the temperature and/or pressure.

Interconversion of States of Matter 

• Sublimation: A solid changes directly into a gas without passing through the liquid state (example: naphthalene, camphor, dry ice). The reverse process (gas to solid) is called deposition or exothermic sublimation.
• Fusion (Melting): A solid turns into a liquid on heating. The temperature at which this occurs is called the melting point. The reverse process (liquid to solid) is called freezing.
• Vaporisation: A liquid turns into a gas. When this happens throughout the liquid at a particular temperature it is called boiling; the temperature is the boiling point. The reverse process (gas to liquid) is called condensation.

Interconversion of States of Matter - Evaporation

Evaporation

Evaporation is a surface phenomenon in which particles at the surface of a liquid gain enough energy to overcome intermolecular forces and escape as vapour.

Evaporation

• Factors affecting the rate of evaporation:
• Surface area exposed - larger surface area increases evaporation.
• Temperature - higher temperature increases particle energy and speeds up evaporation.
• Humidity - lower humidity in the surrounding air increases evaporation; high humidity slows it down.
• Wind speed or air movement - moving air carries away vapour and increases evaporation.
• Nature of the liquid (vapour pressure) - liquids with higher vapour pressure evaporate more readily.
• Cooling effect: Evaporation causes cooling because the particles that escape carry away heat (energy), lowering the temperature of the remaining liquid.

Latent Heat

• Latent heat is the heat required to change the state of a unit mass of a substance without a change in temperature.
• Latent heat of fusion (Lf): Heat required to convert 1 kg of a solid into liquid at its melting point. Unit: joule per kilogram (J kg^-1).
• Latent heat of vaporisation (Lv): Heat required to convert 1 kg of a liquid into gas at its boiling point. Unit: J kg^-1.
• During a phase change (melting, boiling, sublimation) the temperature of the substance remains constant while latent heat is absorbed or released.

Density and Volume

Density is defined as the mass of a substance per unit volume.

• Mathematical expression: density = mass / volume.
• SI unit of density is kg m^-3. Common alternative unit for liquids and solids is g cm^-3.
• Example: Density of water is approximately 1000 kg m^-3 or 1 g cm^-3.
• Volume is the space occupied by a substance. SI unit of volume is m³, but common units are litre (L) and millilitre (mL).
• Unit conversions:
  • 1 m³ = 1000 dm³ = 1000 L
  • 1 L = 1 dm³
  • 1 L = 1000 mL = 1000 cm³
  • 1 mL = 1 cm³

Summary

The essential points are: matter is made of particles with definite properties (motion, intermolecular forces, spaces) that determine whether a substance is solid, liquid or gas; states can change by adding or removing heat; evaporation is a surface phenomenon that causes cooling; latent heat is the energy for state change without temperature change; and density relates mass to volume with standard unit kg m^-3. These concepts explain many everyday observations and form the foundation for further study of thermal properties and behaviour of matter.