Matter in Our Surroundings - Previous Year Questions Solved Examples (Class 9 Science)
Matter is anything that occupies space and has mass. Understanding states of matter, properties, and changes forms the foundation of science. These questio
TL;DR: Matter is anything that occupies space and has mass. Understanding states of matter, properties, and changes forms the foundation of science. These qu…
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Matter is anything that occupies space and has mass. Understanding states of matter, properties, and changes forms the foundation of science. These questio
Matter in Our Surroundings - Previous Year Questions — Solved Numerical Examples (Step by Step)
Example 1: Define matter. Classify matter based on physical state and explain characteristics of each state. [4 marks]
Solution: Matter is anything that occupies space and has mass. Classification by state: Solid: definite shape and volume, incompressible, particles closely packed and fixed in position, low kinetic energy. Liquid: definite volume but takes container shape, incompressible, particles close but can move, moderate kinetic energy. Gas: no definite shape or volume, highly compressible, particles far apart with random motion, high kinetic energy. All three states consist of same particles; difference lies in arrangement and kinetic energy.
Example 2: Explain evaporation and condensation. Why is evaporation a cooling process? [3 marks]
Solution: Evaporation is conversion of liquid to gas at any temperature. Surface molecules with high kinetic energy escape into air. Condensation is conversion of gas to liquid when cooled. Evaporation is cooling because: particles with highest kinetic energy escape, leaving behind molecules with lower average kinetic energy, thus temperature decreases. This is why sweating cools the body; water evaporates from skin taking heat energy. Rate of evaporation increases with temperature, surface area, and decreases with humidity.
Example 3: What is sublimation? Give two examples and explain the process. [3 marks]
Solution: Sublimation is direct conversion of solid to gas without passing through liquid state. It occurs when vapor pressure of solid exceeds atmospheric pressure. Examples: (1) Dry ice (solid CO2) converts to CO2 gas at room temperature, (2) Camphor tablets in wardrobes emit vapors directly. The process is reverse of deposition (gas to solid). Energy is required for sublimation. In high altitude areas with low atmospheric pressure, sublimation is more pronounced.
Example 4: Distinguish between physical and chemical changes with examples. [3 marks]
Solution: Physical change: No new substance forms, original composition remains unchanged, reversible, affects only physical properties (color, shape, state). Examples: melting ice, dissolving sugar, tearing paper, magnetizing iron. Chemical change: New substance with different properties forms, original substance no longer exists, generally irreversible, involves breaking and making of bonds. Examples: burning coal, rusting iron, cooking food, photosynthesis. Both can release or absorb energy.
Example 5: Explain the difference between mass and weight. [2 marks]
Solution: Mass is amount of matter in an object, constant everywhere, measured in kg, scalar quantity. Weight is gravitational force on mass, W = mg, varies with location (changes on Moon and Earth), measured in Newton, vector quantity. Mass of person on Earth = mass on Moon, but weight on Moon is 1/6 of weight on Earth because g = 1.6 m/s² on Moon vs 10 m/s² on Earth.
Example 6: Why are gases highly compressible while solids and liquids are nearly incompressible? [3 marks]
Solution: Gases have particles far apart with large intermolecular spaces. Applying pressure reduces these spaces, compressing the gas significantly. Solids have tightly packed particles in fixed positions; no large spaces exist to compress. Liquids have particles close but with some freedom to move; particles are already close, limiting compression. The distance between gas particles (compared to their size) is much larger than for solids and liquids, allowing easy compression.
Example 7: Explain the role of temperature in changing physical state of matter. [3 marks]
Solution: Temperature represents average kinetic energy of particles. As temperature increases: (1) Solid melts to liquid at melting point when particles gain energy to overcome fixed positions, (2) Liquid vaporizes to gas at boiling point when particles have energy to escape liquid state, (3) Sublimation occurs at high temperature and low pressure. Conversely, cooling reverses these changes. Each substance has characteristic melting and boiling points. Higher temperature indicates faster particle motion and greater kinetic energy, enabling state changes.
Tips
- Always relate state of matter to particle arrangement: closer spacing = solid, intermediate = liquid, far apart = gas.
- Distinguish between melting/freezing, vaporization/condensation, and sublimation/deposition.
- Remember that state change temperature is constant at standard pressure; only the rate of change varies.
- For calculations involving states, use appropriate formulas: W = mg for weight, Q = mL for latent heat.
Frequently Asked Questions
Why does ice float on water while most solids sink?
Solid ice is less dense than liquid water (water expands on freezing), so ice floats. This is anomalous expansion of water.
What happens during boiling and why is it different from evaporation?
Boiling: rapid vaporization throughout the liquid at fixed temperature (boiling point). Evaporation: slow conversion at surface at any temperature. Boiling occurs when vapor pressure equals atmospheric pressure.
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