82 Reasoning-Based Questions & Answers-Matter In Our Surroundings  -Class 9 Science

Chapter 1- Matter In Our Surroundings  – Class 9 Science -easoning-Based Questions & Answers

Table of Contents

Q1. If matter is made up of particles, why does a wooden block appear continuous rather than made of separate particles?

Answer: The particles in a wooden block are extremely small and tightly packed. Their size is beyond the resolving power of the human eye, making the block appear as a continuous, solid object. However, using a microscope, or by breaking the wood into smaller pieces, one can observe that it is composed of smaller units (fibers, molecules, atoms).

Q2. How can we prove that matter is made up of particles through an experiment using water and salt/sugar?

Answer:

  • Take a beaker filled halfway with water and mark its level.
  • Add some salt or sugar and stir until it dissolves.
  • Observe that the water level does not rise significantly.
    Inference: The dissolved salt/sugar does not occupy extra space but fits into the gaps between the water particles, proving that matter is made up of tiny particles with spaces between them.

Also Check – Rapid Revision –Chapter 1- Matter In Our Surroundings – Class 9 Science

Q3. Why can matter be classified based on both physical properties and chemical nature? How do these classifications differ?

Answer: Matter is classified based on:

  • Physical properties: These include state (solid, liquid, gas), density, conductivity, and solubility. This classification is based on how matter behaves under different physical conditions.
  • Chemical nature: This refers to the composition and reactivity of a substance (elements, compounds, mixtures). It tells how matter reacts with other substances.
    The main difference is that physical properties can often be reversed (e.g., melting and freezing water), whereas chemical properties involve a change in composition (e.g., burning paper turns it into carbon dioxide and ash, which cannot be reversed).

Also Check – Chapter 1 -MATTER IN OUR SURROUNDINGS -Class 9 – Simplified detailed Notes

Q4. A student claims that particles of matter are so small that they cannot be divided further. Why is this incorrect?

Answer: While particles of matter are very small, they can still be divided into atoms, molecules, and subatomic particles (protons, neutrons, electrons). The divisibility of particles depends on the level of observation:

  • A grain of salt appears indivisible to the naked eye.
  • Under a microscope, it appears as many tiny crystals.
  • On an atomic scale, salt consists of sodium and chloride ions.
  • On a subatomic scale, sodium and chlorine atoms contain electrons, protons, and neutrons.

Q5. How can the concept of diffusion be used to prove that matter is made up of particles?

Answer: Diffusion is the spontaneous mixing of particles without external force. It proves that matter is made up of particles because:

  • If matter were continuous, different substances would not mix without stirring.
  • When perfume is sprayed in a room, it spreads out without any stirring.
  • When a drop of ink is placed in water, it gradually spreads throughout.
    This proves that matter consists of individual, moving particles that occupy space and interact with each other.

Also Check – Chapter 1- Matter In Our Surroundings – Class 9 Science – Very Short Question and Answers

Q6. The particles of matter are extremely small, but how can we estimate their size using potassium permanganate crystals?

Answer: By performing a dilution experiment:

  • Dissolve one crystal of potassium permanganate in 100 mL of water.
  • Take 10 mL of this solution and mix it with another 90 mL of water.
  • Repeat this process 5–8 times.
    Observation: Even after several dilutions, the solution remains colored.
    Inference: This shows that each crystal contains millions of tiny particles, which are small enough to be divided repeatedly while still retaining color.

Q7. If a tiny drop of Dettol can still be smelled from a distance after repeated dilution, what does this indicate about the size of its particles?

Answer: This indicates that the particles of Dettol are extremely small and can mix with air or water even in very small concentrations. Since the smell persists after dilution, it suggests that the particles remain undetectably small yet retain their properties.

Q8. In an experiment, a single drop of ink is added to a beaker of water. Even without stirring, the entire water turns blue over time. How does this prove that particles of matter are extremely small and are always moving?

Answer:

  • The movement of ink in water without stirring shows that particles move on their own (diffusion).
  • The spreading of color suggests that particles are very small and can fit between the spaces in water molecules.
  • The continuous movement of ink proves that particles of matter are always in motion.

Q9. If the particles of matter are so small, why do they not escape a closed container?

Answer: Although particles are tiny and constantly moving, they are also held together by forces of attraction. In a closed container, these forces prevent them from escaping, unless external energy (like heat) is supplied to overcome these attractive forces.

Also Check – Chapter 1- A Detailed Guide to “Matter and Its Properties” Activities for Class 9 Students

Q10. If we keep two containers of water—one at room temperature and the other heated—why does diffusion happen faster in the heated one?

Answer:

  • Heat provides more kinetic energy to the particles.
  • Higher kinetic energy makes particles move faster, increasing the rate of diffusion.
    In hot water, ink or any other substance spreads out faster than in cold water due to increased molecular motion.

Q11. Why can we move our hands easily through air but not through water or a solid block of wood?

Answer:

  • Air: Particles are widely spaced with very weak attraction, allowing free movement.
  • Water: Particles are closer than in air, providing some resistance but still allowing movement.
  • Wood: Particles are tightly packed with strong attraction, preventing movement through it.

Q12. Why does diffusion occur faster in gases than in liquids and in liquids faster than in solids?

Answer:

  • Gas: Particles have high kinetic energy and very weak attraction, allowing rapid movement.
  • Liquid: Particles have moderate attraction, so diffusion occurs but slower than in gases.
  • Solid: Particles are rigid and tightly packed, making diffusion extremely slow.

Q13. Why do we see water droplets on the outer surface of a glass containing ice-cold water?

Answer:
Air contains water vapor as a gas. When it comes in contact with the cold glass, it loses energy and condenses into liquid water. This shows that particles of matter attract each other and change states when energy is removed.

Q14. A diver jumps into a swimming pool and cuts through the water. What does this tell us about the nature of water’s particles?

Answer:
The diver displaces water molecules, meaning they can move apart. This suggests liquid particles have moderate attraction, allowing movement but offering resistance.

Q15. Why is it easier to compress a gas than a liquid or a solid?

Answer:
In gases, particles are widely spaced with very weak attraction, making compression easy. In liquids, particles have less space, making them harder to compress. In solids, particles are fixed in place, making compression nearly impossible.

Also Check – Chapter 1- Matter In Our Surroundings – Class 9 Science – Long Question and Answers

Q16. If solids have a fixed shape, why can some solids like rubber bands be stretched?

Answer:
A rubber band is a solid because it has a definite shape and volume under normal conditions. However, it is made of elastic materials (polymers) that allow its particles to be rearranged temporarily under force. Once the force is removed, intermolecular forces pull the particles back, restoring the shape. If excessive force is applied, the bonds break and the rubber band snaps.

Q17. Sugar and salt take the shape of their containers. Should they be classified as solids or fluids?

Answer:
Even though sugar and salt take the shape of their containers, they are still solids because:

  • Each individual crystal has a fixed shape and does not flow like a liquid.
  • The ability to take the container’s shape is due to small-sized, loose particles, not because of fluidity.

Q18. Why is iron more rigid than chalk, even though both are solids?

Answer:
The rigidity of a solid depends on the strength of intermolecular forces:

  • Iron: Has strong metallic bonds, where atoms are tightly held in a lattice structure.
  • Chalk: Made of weaker ionic bonds and has gaps between particles, making it brittle.
    Thus, iron requires greater force to deform, while chalk easily crumbles under force.

Q19. If solids are incompressible, how can materials like foam and sponge be compressed easily?

Answer:

  • Sponge and foam contain tiny air pockets within their structure.
  • When pressed, the air escapes, making the material appear compressed.
  • The actual solid framework remains unchanged, so it’s the air—not the solid particles—that allows compressibility.

Q20. Why do some solids like ice float on their own liquid?

Answer:

  • Ice floats on water due to anomalous expansion.
  • Water expands when frozen, which reduces its density (density = mass/volume).
  • The open hexagonal structure of ice molecules creates more empty spaces, making ice less dense than liquid water.

Q21. Why do liquids take the shape of their containers but not have a fixed shape like solids?

Answer:

  • In liquids, particles experience weaker forces of attraction than in solids.
  • This allows particles to move freely, sliding past one another.
  • Although liquids adjust to the shape of their container, their volume remains constant due to intermolecular forces.

Q22. Why is diffusion slower in liquids than in gases?

Answer:

  • Diffusion depends on particle movement and spacing.
  • In gases, particles move freely with high kinetic energy, leading to rapid diffusion.
  • In liquids, stronger intermolecular forces and closer particle spacing slow down the rate of diffusion.

Also Check – Chapter 1- Matter In Our Surroundings – Class 9 Science – Solved MCQs

Q23. Why do liquids flow more easily when heated?

Answer:

  • Heat provides additional kinetic energy to liquid particles.
  • Increased kinetic energy weakens intermolecular forces, allowing particles to move more freely.
  • This reduction in viscosity causes the liquid to flow more easily.

Q24. How does the shape of a liquid change when poured into different containers, but the volume remains the same?

Answer:

  • Liquids have no fixed shape because their particles can move past one another.
  • Despite the change in shape, the volume remains constant as intermolecular forces hold the particles closely together, preventing compression.

Q25. If water and honey are both liquids, why does honey flow more slowly?

Answer:

  • Honey has stronger intermolecular forces due to its complex sugar molecules, which increases its viscosity.
  • In contrast, water’s weaker forces allow it to flow more freely.

Q26. Why do gases have neither a fixed shape nor a fixed volume?

Answer:

  • Gas particles have high kinetic energy and weak intermolecular forces.
  • They spread out to fill any container and can be compressed easily.
  • Their constant motion prevents them from maintaining a fixed shape.

Q27. Why does the smell of perfume spread quickly across a room but honey takes a long time to mix in water?

Answer:

  • Perfume is composed of volatile gas particles that move rapidly and have large spaces between them, leading to fast diffusion.
  • Honey, being a viscous liquid with stronger intermolecular forces, diffuses much more slowly.

Q28. Why do gases exert pressure on the walls of a container?

Answer:

  • Gas particles move randomly at high speeds and collide with the container walls.
  • These collisions transfer momentum, creating pressure that depends on temperature and volume.

Q29. If a gas is highly compressible, why don’t we feel air compressing around us?

Answer:

  • Air is already compressed by atmospheric pressure (approximately 1 atmosphere at sea level).
  • Our bodies are in equilibrium with this pressure, and noticeable compression requires an external force (like a piston or pressurized gas tank).

Q30. Why does heating a gas increase its pressure in a closed container?

Answer:

  • Heating increases the kinetic energy of gas particles, causing them to move faster.
  • Faster movement results in more frequent and forceful collisions with the container walls, thereby increasing the pressure.

Q31. Why does a balloon expand when air is blown into it?

Answer:

  • As more air molecules enter the balloon, they spread out and occupy more space.
  • The increased number of collisions against the inner walls stretches the balloon, causing it to expand.

Q32. If gases are highly compressible, why do liquids and solids not compress easily?

Answer:

  • Liquids and solids have much stronger intermolecular forces and significantly less empty space between particles.
  • In gases, the large gaps between particles allow for easy compression.

Q33. Why does an LPG cylinder remain in a liquid state but become a gas when released?

Answer:

  • Inside the cylinder, high pressure compresses the LPG into a liquid state.
  • When released, the pressure drops, allowing the liquid to evaporate and become a gas.

Q34. Why do deep-sea divers need to use compressed air tanks?

Answer:

  • At great depths, the high water pressure compresses gases in the body.
  • Compressed air tanks provide a breathable mixture at a pressure that matches the surrounding environment, preventing issues like decompression sickness.

Q35. If a helium balloon is left in sunlight, why does it burst?

Answer:

  • Sunlight heats the gas inside the balloon, increasing the kinetic energy of the helium atoms.
  • This causes the gas to expand and increases the pressure on the balloon’s walls.
  • If the pressure exceeds the elasticity of the balloon, it bursts.

Q36. Why does the temperature of a solid remain constant during melting, even though heat is being supplied continuously?

Answer:

  • During melting, the heat energy supplied is used to overcome the strong intermolecular forces between solid particles rather than increasing their temperature.
  • This absorbed energy, called the latent heat of fusion, allows the solid to transition into a liquid without a temperature change.

Q37. Why does ice melt faster when salt is added to it?

Answer:

  • Salt lowers the freezing point of water (freezing point depression).
  • With salt present, water molecules require less energy to remain in the liquid phase, thereby speeding up the melting process.

Q38. Why does increasing the temperature increase the rate of diffusion in liquids and gases?

Answer:

  • Higher temperatures provide more kinetic energy to particles, making them move faster.
  • Increased particle speed leads to quicker spreading (diffusion) throughout the medium.

Q39. Why do we feel more comfortable wearing light-colored clothes in summer?

Answer:

  • Light-colored clothes reflect most of the sun’s heat radiation, reducing heat absorption.
  • In contrast, dark-colored clothes absorb more heat, increasing the body’s temperature.

Q40. Why does the melting point of a solid indicate the strength of its intermolecular forces?

Answer:

  • A higher melting point indicates stronger intermolecular forces, which require more energy to break.
  • For example, iron (with strong metallic bonds) has a high melting point, whereas wax (with weaker forces) has a low melting point.

Q41. Why is ice at 0°C more effective at cooling than water at 0°C?

Answer:

  • Ice at 0°C absorbs latent heat of fusion from its surroundings when it melts.
  • Water at 0°C has already absorbed that energy, making it less effective as a cooling agent.

Q42. A block of ice melts into a puddle of water. Why is this considered a physical change rather than a chemical change?

Answer:

  • The chemical composition of water (H₂O) remains unchanged during the melting process.
  • Only the arrangement and energy of the particles change, and the process is reversible (water can be refrozen).

Q43. Why does water boil at a lower temperature on a mountain than at sea level?

Answer:

  • At higher altitudes, the atmospheric pressure is lower.
  • Boiling occurs when the vapor pressure of water equals the atmospheric pressure, so less energy (lower temperature) is required to reach that point.

Q44. Why does sweating cool down our body?

Answer:

  • Sweat evaporates from the skin, absorbing latent heat of vaporization in the process.
  • This heat absorption lowers the skin temperature, producing a cooling effect on the body.

Q45. Why does evaporation take place even below the boiling point?

Answer:

  • At any given temperature, some liquid particles possess higher kinetic energy than others.
  • These high-energy particles can escape from the surface, resulting in evaporation even before the boiling point is reached.

Q46. Why do wet clothes dry faster on a windy day?

Answer:

  • Wind increases air circulation, which removes the humid air surrounding the clothes.
  • This replacement with drier air speeds up the evaporation process by lowering the local water vapor concentration.

Q47. Why is more heat required to convert water into steam than to melt ice into water?

Answer:

  • The latent heat of vaporization (approximately 2260 J/g) is much higher than the latent heat of fusion (approximately 334 J/g).
  • Converting liquid water into steam requires overcoming the strong intermolecular forces in the liquid state, which demands more energy than the solid-to-liquid transition.

Q48. Why do burns from steam cause more damage than burns from boiling water at the same temperature?

Answer:

  • Steam contains extra energy due to its latent heat of vaporization.
  • When steam condenses on the skin, it releases this extra heat, leading to more severe burns.

Q49. How does an increase in humidity slow down the rate of evaporation?

Answer:

  • High humidity means the air already contains a large amount of water vapor.
  • With less capacity to hold additional moisture, the rate of evaporation decreases.

Q50. Why can gases be liquefied by increasing pressure and lowering temperature?

Answer:

  • Increasing pressure forces gas particles closer together, strengthening intermolecular attractions.
  • Lowering temperature reduces the kinetic energy of the particles, preventing them from escaping into the gas phase.
  • Together, these conditions allow gases to condense into a liquid.

Q51. Why do deep-sea divers use a mixture of gases instead of pure oxygen?

Answer:

  • At high water pressure, nitrogen can dissolve in the blood, and rapid changes in pressure may cause nitrogen bubbles (decompression sickness).
  • Divers use gas mixtures (often helium-oxygen) to minimize nitrogen absorption and reduce the risk of decompression sickness.

Q52. Why does a sealed bottle of soda fizz when opened?

Answer:

  • Soda contains dissolved carbon dioxide (CO₂) under high pressure.
  • When the bottle is opened, the pressure drops, and the dissolved CO₂ escapes as bubbles, causing the fizz.

Q53. Why does solid carbon dioxide (dry ice) change directly into gas without becoming a liquid?

Answer:

  • Dry ice undergoes sublimation because, at normal atmospheric pressure, CO₂ cannot exist as a liquid.
  • Instead of melting, it directly transitions from the solid state to the gaseous state when heated.

Q54. Why do naphthalene balls shrink and disappear over time without leaving a liquid?

Answer:

  • Naphthalene undergoes sublimation—it directly converts from solid to gas.
  • Since it does not pass through the liquid phase, it gradually disappears.

Q55. How can iodine crystals be turned into gas and back to solid without forming a liquid?

Answer:

  • Heating iodine turns it into a purple gas (sublimation).
  • Cooling the gas causes it to deposit back as solid iodine (deposition).

Q56. Why do some substances undergo sublimation while others do not?

Answer:

  • Sublimation occurs in substances that have weak intermolecular forces, allowing easy conversion to gas.
  • Additionally, these substances exhibit a vapor pressure higher than atmospheric pressure at room temperature.

Q57. Why does camphor disappear from a plate after a few days without melting?

Answer:

  • Camphor undergoes sublimation.
  • The solid camphor particles gain enough energy to escape directly as gas, so no liquid phase is observed.

Q58. Why does evaporation occur even at room temperature, while boiling occurs at a fixed temperature?

Answer:

  • Evaporation: Occurs at all temperatures because, at any given moment, some liquid particles have enough kinetic energy to escape as gas (a surface phenomenon).
  • Boiling: Occurs at a specific temperature when the vapor pressure equals atmospheric pressure, allowing bubbles to form throughout the liquid (a bulk phenomenon).

Q59. Why does evaporation occur only at the surface of a liquid?

Answer:

  • Surface molecules are less tightly bound than those in the interior.
  • They gain energy from their surroundings and escape into the air, leaving behind cooler molecules.

Q60. Why does water spread out in a puddle dry faster than the same volume of water in a narrow glass?

Answer:

  • A greater surface area exposes more water molecules to air, increasing the rate of evaporation.

Q61. Why do we dry wet clothes by spreading them out rather than piling them up?

Answer:

  • Spreading out clothes increases the surface area available for evaporation.
  • Piling clothes up limits exposure, so only the outer layer evaporates efficiently.

Q62. Why does evaporation occur faster on a hot day than on a cold day?

Answer:

  • Higher temperatures increase the kinetic energy of liquid particles, enabling more molecules to escape as gas.

Q63. Why does petrol evaporate faster than water, even at the same temperature?

Answer:

  • Petrol has weaker intermolecular forces than water, allowing its molecules to escape more quickly.

Q64. Why do water droplets form on the outside of a cold glass on a hot day?

Answer:

  • The surrounding air contains water vapor from evaporation.
  • When this vapor contacts the cold surface, it loses energy and condenses into liquid droplets—a process known as condensation.

Q65. Why do wet clothes take longer to dry on a humid day?

Answer:

  • High humidity means the air is already saturated with water vapor, reducing the rate at which additional water molecules can evaporate.

Q66. Why do deserts experience faster evaporation than coastal areas?

Answer:

  • Deserts have low humidity, which allows water to evaporate quickly.
  • Coastal areas typically have higher humidity, which slows down the evaporation process.

Q67. Why do wet clothes dry faster on a windy day?

Answer:

  • Wind removes the humid air from the vicinity of the wet clothes, enhancing evaporation by allowing more water molecules to escape.

Q68. Why does sweat evaporate faster when we stand under a fan?

Answer:

  • The moving air increases evaporation by removing the moisture-laden air from the skin, thereby speeding up heat removal.

Q69. If you spill water inside a closed room with no airflow, why does it take longer to evaporate?

Answer:

  • In a closed space, evaporated water accumulates, increasing the humidity and slowing further evaporation due to the reduced concentration gradient.

Q70. Why do we feel cold when sweat evaporates from our skin?

Answer:

  • Evaporation requires heat energy (latent heat), which is drawn from the skin, thereby reducing its temperature and making us feel cooler.

Q71. Why does alcohol feel cooler on the skin than water?

Answer:

  • Alcohol evaporates faster due to its weaker intermolecular forces, removing heat more quickly from the skin.

Q72. Why do earthen pots keep water cooler than plastic or metal bottles?

Answer:

  • The porous nature of earthen pots allows slow evaporation from the surface, which absorbs heat and cools the water inside.

Q73. Why do people in hot regions sprinkle water on the ground or rooftops?

Answer:

  • As water evaporates, it absorbs heat from the surroundings, reducing surface temperature and providing a cooling effect.

Q74. Why do we feel cooler when we wear damp clothes on a hot day?

Answer:

  • The evaporation of water from damp clothes draws heat away from the body, creating a cooling effect.

Q75. Why is a cotton cloth wrapped around a water bottle on a hot day?

Answer:

  • The wet cloth enhances evaporation from the bottle’s surface, which absorbs heat and cools the water inside—an effect used in desert coolers.

Q76. Why do we store vegetables in refrigerators to keep them fresh?

Answer:

  • Lower temperatures slow down the rate of evaporation and other metabolic processes, helping vegetables retain moisture and remain fresh longer.

Q77. Why do coolers work better in dry regions than in humid areas?

Answer:

  • Coolers rely on evaporation to remove heat.
  • In dry regions, low humidity allows rapid evaporation, while in humid areas, the reduced evaporation rate diminishes the cooling effect.

Q78. Why do desert animals have large ears?

Answer:

  • Large ears provide a greater surface area for heat dissipation through evaporation, helping the animal remain cool in extreme heat.

Q79. Why do doctors rub alcohol on a patient’s skin before giving an injection?

Answer:

  • Alcohol evaporates quickly, absorbing heat from the skin and creating a temporary cooling effect, which can reduce pain sensitivity.

Q80. Why do we use a wet handkerchief over the nose during a heatwave?

Answer:

  • A wet handkerchief cools the air before inhalation by promoting evaporation, thereby helping to reduce body temperature and prevent heat exhaustion.

Q81. In some places, water is stored in clay pots rather than steel containers. Why?

Answer:

  • Clay pots allow gradual evaporation, which cools the water inside naturally.
  • Steel containers do not permit evaporation, so the water remains warmer.

Q82. Why do we get goosebumps when we feel extremely cold?

Answer:

  • Goosebumps trap a layer of air close to the skin, reducing heat loss through evaporation and helping to retain body warmth.

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