Matter in Our Surroundings — Science Class 9 Notes (CBSE & HBSE)
Free NCERT Science notes for Matter in Our Surroundings (Class 9) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Matter in Our Surroundings (CBSE & HBSE)
CBSE emphasises the particle nature of matter, change of state with energy concepts (latent heat) and quantitative reasoning, while HBSE focuses on definitions, properties of the three states and direct factual questions on evaporation and its applications.
Characteristics of Particles of Matter
What is Matter?
Matter is anything that has mass and occupies space (volume). Air, water, stones, plants, animals and even smell-causing gases are all matter.
Early Indian philosophers classified matter into the Panch Tatva — air, earth, fire, sky and water. Modern science classifies matter on the basis of physical state (solid, liquid, gas) and chemical composition (elements, compounds, mixtures).
Particle Nature of Matter
Matter is made up of tiny particles. These particles are extremely small — far beyond what we can see.
When potassium permanganate (a few crystals) is dissolved in water and diluted repeatedly, the colour still persists, proving that even a tiny crystal contains a huge number of particles.
Characteristics of Particles
| Characteristic | Meaning / Evidence |
|---|---|
| Particles are very small | A crystal of KMnO4 colours thousands of litres of water |
| Particles have space between them | Sugar/salt dissolves in water without raising the level much |
| Particles are continuously moving | They possess kinetic energy; diffusion proves this |
| Particles attract each other | A force of attraction holds particles together |
Diffusion
Diffusion is the intermixing of particles of two different types of matter on their own. It is faster in gases, slower in liquids and negligible in solids.
- Rate of diffusion increases with temperature because kinetic energy of particles increases.
- Smell of incense/perfume spreading across a room is diffusion in gases.
CBSE trap: Diffusion proves BOTH that particles have space between them AND that they are in continuous motion.
States of Matter and Their Properties
The Three States of Matter
Matter exists in three physical states — solid, liquid and gas — based on inter-particle space, force of attraction and arrangement.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Definite | No definite shape (takes container's shape) | No definite shape |
| Volume | Definite | Definite | No definite volume |
| Inter-particle space | Very small | More than solid | Very large |
| Force of attraction | Strongest | Moderate | Weakest |
| Compressibility | Negligible | Very low | High |
| Fluidity (can flow) | No | Yes | Yes |
| Rate of diffusion | Very slow | Slow | Fast |
Why Solids are Rigid
In solids, particles are tightly packed with strong forces of attraction, giving them rigidity and a fixed shape. Solids may break under force but generally keep their shape.
Liquids
Liquids have particles with weaker attraction than solids, so they flow and take the shape of the container while keeping a fixed volume. Gases and even solids (sugar, salt) can dissolve in liquids — important for life processes.
Gases
Gases are highly compressible (used in LPG cylinders, CNG, oxygen cylinders for hospitals). Gas particles move randomly at high speed and exert pressure on container walls by colliding with them.
HBSE/CBSE point: The states of matter are interconvertible; the state can be changed by changing temperature or pressure.
Change of State, Latent Heat, Evaporation and Cooling
Change of State
Matter changes from one state to another by changing temperature or pressure.
| Change | Name | Direction |
|---|---|---|
| Solid → Liquid | Melting (fusion) | Heat absorbed |
| Liquid → Solid | Freezing/Solidification | Heat released |
| Liquid → Gas | Vaporisation/Boiling | Heat absorbed |
| Gas → Liquid | Condensation | Heat released |
| Solid → Gas (direct) | Sublimation | Heat absorbed |
| Gas → Solid (direct) | Deposition | Heat released |
Key Temperatures
- Melting point of ice = 273.15 K (0°C). The melting point of a solid measures the strength of attraction between its particles.
- Boiling point of water = 373 K (100°C). Boiling is a bulk phenomenon (whole liquid).
- Conversion: K = °C + 273.
Latent Heat
Latent heat is the heat energy absorbed or released during a change of state without any change in temperature.
- Latent heat of fusion: heat needed to convert 1 kg of solid into liquid at its melting point. For ice = 3.34 × 10⁵ J/kg.
- Latent heat of vaporisation: heat needed to convert 1 kg of liquid into vapour at its boiling point. For water = 22.5 × 10⁵ J/kg.
Steam at 373 K causes more severe burns than boiling water at 373 K because steam carries extra latent heat of vaporisation.
Effect of Pressure
Applying high pressure and lowering temperature can liquefy gases. Solid CO₂ (dry ice) is stored under high pressure and sublimes directly to gas at 1 atmosphere — used as a refrigerant.
Evaporation
Evaporation is the change of a liquid into vapour at any temperature below its boiling point. It is a surface phenomenon.
Factors increasing evaporation:
- Increase in surface area (clothes spread to dry).
- Increase in temperature.
- Decrease in humidity.
- Increase in wind speed.
Evaporation Causes Cooling
During evaporation, the fast-moving particles leave the surface and absorb energy (latent heat) from the surroundings/liquid, lowering the temperature — this produces a cooling effect.
- Acetone/perfume on the palm feels cold as it evaporates.
- We sweat on a hot day; sweat evaporates absorbing body heat and cools us.
- Water kept in an earthen pot (matka) stays cool as water seeps and evaporates through pores.
CBSE trap: Boiling is a bulk phenomenon at a fixed temperature; evaporation is a surface phenomenon at all temperatures. Don't confuse them.
Frequently asked questions
Are these Matter in Our Surroundings notes free?
Yes — the Matter in Our Surroundings notes for Science (Class 9) on Siksha Sarovar are completely free to read, with no account required.
Do these notes follow CBSE and HBSE?
Yes. The Matter in Our Surroundings notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.
What does the Matter in Our Surroundings chapter cover?
Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Matter in Our Surroundings.