Matter in Our Surroundings — Complete Class 9 Science Notes
Comprehensive & exam-oriented notes aligned with NCERT — definitions, diagrams, examples, solved numericals, practice questions, and FAQs for quick revision.
Author: Semester Exam Point • Subject: Class 9 Science • Last updated:
Introduction
Matter is anything that occupies space and has mass. Everything around you — air, water, books, chairs, and even light from a flame — can be discussed in terms of matter (note: light is energy; we often discuss sources of light rather than light itself as matter).
This chapter explains particle nature of matter, the three common states (solid, liquid, gas), how matter changes state, and real-life examples that help in understanding the concepts for Class 9 exams.
Definition & Fundamental Properties of Matter
Definition
Matter is anything that has mass and occupies space.
Key Properties
- Mass: Amount of matter in a body (measured in kg or g).
- Volume: Space occupied by an object (m³, cm³, L).
- Inertia: Tendency to resist change in motion (related to mass).
Particle Nature of Matter (Postulates)
- Matter is made of very small particles (atoms/molecules).
- Particles have spaces between them; the amount of space depends on the state.
- Particles are in constant motion — vibration in solids, translation in gases, intermediate in liquids.
- Particles attract each other; strength depends on the state.
States of Matter
The three classical states are Solid, Liquid, and Gas. Below is a concise comparison.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Definite | Takes shape of container | Fills whole container |
| Volume | Definite | Definite | Variable |
| Interparticle Space | Very small | Moderate | Large |
| Interparticle Force | Strong | Moderate | Weak |
Examples: Ice (solid), Water (liquid), Oxygen (gas).
Change of State (Phase Changes)
Change of state occurs when matter gains or loses heat energy. Common phase changes:
- Melting: Solid → Liquid
- Freezing: Liquid → Solid
- Evaporation: Liquid → Gas (surface phenomenon)
- Boiling: Liquid → Gas (bulk phenomenon at boiling point)
- Condensation: Gas → Liquid
- Sublimation: Solid → Gas (skips liquid)
Melting and Boiling Points
Each substance has a characteristic melting point and boiling point (e.g., water: melting 0°C, boiling 100°C at 1 atm).
Heating Curve (Qualitative)
When heating a substance, temperature rises until a phase change starts. During the phase change, temperature remains constant until the change completes (energy used to break bonds — latent heat).
Evaporation
Evaporation is the process where molecules at the surface of a liquid escape into the gas phase at temperatures below the boiling point.
Factors Affecting Evaporation
- Surface area (larger area → faster evaporation)
- Temperature (higher temperature → faster)
- Humidity of air (lower humidity → faster)
- Wind or air movement (increases rate)
- Nature of liquid (volatile liquids evaporate faster)
Cooling Effect of Evaporation
Evaporation causes cooling because high-energy molecules escape, lowering the average kinetic energy of the remaining liquid (for example, sweating cools the body).
Latent Heat & Simple Calculations
Latent heat is the heat required for a substance to change its state without change in temperature.
Formula: Q = m × L, where
Q= heat absorbed or released (J)m= mass (kg or g)L= latent heat (J/kg or J/g)
Example values: Latent heat of fusion for water = 334 J/g (approx.), latent heat of vaporization for water = 2260 J/g (approx.).
Important Diagrams & Figures
Particle Diagrams (Solid / Liquid / Gas)
Heating Curve (Sketch)
Labelled sketch: temperature vs time when heating a solid through melting to boiling. (Teachers: consider adding a plotted image if you have time.)
Solved Examples & Numericals
Example 1 — Latent Heat
Q: How much heat is required to convert 100 g of ice at 0°C to water at 0°C? (latent heat of fusion of water = 334 J/g)
A: Use Q = m × L = 100 g × 334 J/g = 33400 J.
Example 2 — Evaporation (qualitative)
Q: Explain why clothes dry faster on a windy day.
A: Wind removes saturated air near the surface and replaces it with drier air, increasing evaporation rate. Also, moving air carries away escaped vapour molecules.
Practice Questions
Objective (MCQ)
- Which of the following is not a property of matter? (a) Mass (b) Volume (c) Light (d) Inertia
- Evaporation is a ___ phenomenon. (a) Bulk (b) Surface (c) Nuclear (d) Chemical
Short Answer
- Define sublimation and give one example.
- Why does the temperature remain constant during melting?
Long Answer
- Explain the particle model of matter with diagrams and examples.
- Describe the heating curve of water from -10°C to 110°C and explain why the curve has flat sections.
Quick Summary Table
- Matter = mass + volume.
- Solid: definite shape, definite volume; strong forces.
- Liquid: definite volume, shape of container; moderate forces.
- Gas: no definite shape or volume; weak forces.
- Evaporation is surface phenomenon; boiling is bulk phenomenon.
- Latent heat formula: Q = mL.
Frequently Asked Questions
Q: Is air matter?
A: Yes. Air has mass and occupies space, so it is matter.
Q: Why does temperature not change during melting?
A: The heat energy is used to break intermolecular forces (latent heat) rather than increase kinetic energy, so temperature remains constant during the phase change.
Q: Difference between evaporation and boiling?
A: Evaporation occurs at the surface and at temperatures below boiling point; boiling occurs throughout the liquid at its boiling point and forms bubbles.