CBSE Class 9 Science — Matter in Our Surroundings

Chapter Overview & Weightage

Matter in Our Surroundings is Chapter 1 of Class 9 Science. It covers the physical nature of matter, its states, and the interconversion between states. This is a foundational chapter that carries 6–8 marks in CBSE Class 9 exams and provides the conceptual basis for understanding chemistry throughout secondary school.

Key Concepts You Must Know

Matter: Anything that has mass and occupies space.

Kinetic theory: Particles of matter are in constant random motion. The speed of motion increases with temperature.

States of matter: Solid, liquid, gas. (Plasma and Bose-Einstein condensate are two additional states — mentioned in NCERT for awareness.)

Properties of the three states:

Diffusion: The spreading of particles from higher concentration to lower concentration. Gases diffuse faster than liquids; liquids diffuse faster than solids (at the same temperature). Diffusion rate increases with temperature.

States and State Changes

Melting: Solid → Liquid (at melting point) Freezing: Liquid → Solid (at freezing point; same temperature as melting point) Evaporation: Liquid → Gas (at any temperature, from the surface) Boiling: Liquid → Gas (throughout the bulk, at boiling point) Condensation: Gas → Liquid Sublimation: Solid → Gas directly (dry ice, iodine, camphor, ammonium chloride) Deposition: Gas → Solid directly

Important Concepts

Latent Heat

The heat absorbed or released during a change of state at constant temperature is called latent heat. “Latent” means hidden — the temperature doesn’t change during the state transition, so the heat is “hidden.”

Latent heat of fusion: Heat absorbed to convert 1 kg of solid to liquid at its melting point. For ice: 3.34 × 10⁵ J/kg.

Latent heat of vaporisation: Heat absorbed to convert 1 kg of liquid to gas at its boiling point. For water: 22.6 × 10⁵ J/kg (much higher than fusion — why steam burns are more severe than hot water burns).

Evaporation vs Boiling

Factors Affecting Evaporation Rate

1. Temperature: Higher temperature → more molecules have enough energy to escape → faster evaporation
2. Surface area: More surface exposed → more molecules can escape → faster evaporation
3. Humidity: Higher humidity (more water vapour in air) → less evaporation (air is already “full”)
4. Wind speed: Wind removes vapour from above the surface, maintaining low humidity above → faster evaporation

Cooling effect of evaporation: When molecules with highest kinetic energy escape (evaporate), the average kinetic energy of the remaining molecules decreases → temperature of remaining liquid falls. This is why sweating cools the body.

Solved Previous Year Questions

PYQ 1 — Why steam causes more burns than boiling water (3 marks)

Q: Why does steam at 100°C cause more severe burns than water at 100°C?

Solution: Both are at the same temperature (100°C), but steam has additional energy — the latent heat of vaporisation (~22.6 × 10⁵ J/kg). When steam condenses to water on your skin, it releases this extra energy in addition to cooling from 100°C to body temperature. The extra energy from condensation causes the more severe burn.

Boiling water at 100°C only releases heat as it cools — it doesn’t release latent heat of vaporisation when it contacts skin.

PYQ 2 — Sublimation (2 marks)

Q: What is sublimation? Give two examples of substances that sublime.

Solution: Sublimation is the process by which a solid directly converts to gas without passing through the liquid state. The reverse process (gas → solid) is called deposition.

Examples: (1) Dry ice (solid CO₂) sublimes at −78.5°C at atmospheric pressure. (2) Camphor (used in religious ceremonies) slowly sublimes at room temperature. (3) Iodine — purple gas appears when iodine crystals are gently heated.

PYQ 3 — Why wet clothes dry faster on a windy day (2 marks)

Q: Explain why wet clothes dry faster on a windy day.

Solution: Drying of clothes is evaporation — water molecules leave the wet cloth surface and enter the air. Wind continuously removes water vapour from around the cloth, keeping the humidity above the cloth low. This maintains a large concentration gradient of water vapour (high at cloth surface, low in moving air), so evaporation continues rapidly. On a calm day, the air near the cloth becomes saturated, slowing evaporation.

PYQ 4 — Heating curve description (4 marks)

Q: Describe the temperature-time graph when ice at −10°C is heated until all ice becomes steam.

Solution:

1. A→B: Ice temperature rises from −10°C to 0°C (heat increases kinetic energy of ice molecules)
2. B→C: Temperature stays at 0°C while ice melts (latent heat of fusion absorbed — breaking intermolecular bonds in ice)
3. C→D: Water temperature rises from 0°C to 100°C
4. D→E: Temperature stays at 100°C while water boils to steam (latent heat of vaporisation absorbed)
5. E→F: Steam temperature rises above 100°C (superheated steam)

The flat regions (B→C and D→E) are the key features — temperature is constant during phase transitions.

Difficulty Distribution

Expert Strategy

Draw the heating/cooling curve if the question asks about interconversion of states — it organises your answer visually and earns diagram marks. Label all the phase transitions and the flat regions.

For evaporation questions, always give the mechanism first (“evaporation occurs because surface molecules with sufficient kinetic energy escape the liquid…”) before listing factors. This ensures you get method marks even if you miss a factor.

Common Traps