30 Long Answer Type Questions — Model Answers

1. 1. Define 'matter'. Explain with two examples why air is considered matter.
   Model Answer: Matter is anything that occupies space and has mass. Air qualifies as matter because it occupies space — for example, when a balloon is inflated, the volume of the balloon increases as air fills it, demonstrating that air occupies space. Secondly, air has mass: an experiment where a closed container of known mass is filled with air and weighed shows that removing the air (by creating a vacuum) changes the measured mass. These observations confirm that air meets both required properties of matter.
2. 2. Describe the characteristics of solids, liquids and gases with suitable examples and explain how particle arrangement accounts for these properties.
   Model Answer: Solids have definite shape and volume because their particles are tightly packed in a regular arrangement and can only vibrate about fixed positions — examples include iron, wood and ice. Liquids have definite volume but no definite shape; they take the shape of the container because particles are close but not fixed, allowing them to slide past each other — examples are water and oil. Gases have neither definite shape nor volume; they expand to fill any container because particles are far apart and move freely at high speeds — examples include oxygen and nitrogen. The particle arrangement and strength of interparticle forces explain behavior: strong forces in solids keep particles locked, moderate forces in liquids allow flow, and negligible forces in gases allow high compressibility and rapid diffusion.
3. 3. Explain evaporation as a process. Discuss the factors that affect the rate of evaporation and give real-life applications of evaporation.
   Model Answer: Evaporation is the process where molecules at the surface of a liquid escape into the vapour phase at temperatures below the boiling point. It is a surface phenomenon because only surface molecules with sufficient kinetic energy overcome intermolecular attraction to leave. Factors affecting evaporation rate include temperature (higher temperature increases kinetic energy and rate), surface area (larger area exposes more molecules), humidity (lower humidity favors evaporation), and wind (moving air removes vapour and increases rate). Real-life applications include cooling by sweating (evaporation of sweat cools the body), drying clothes outdoors (wind and sunlight increase evaporation), and traditional cooling methods like earthen pot water coolers where porous material increases evaporative loss, cooling the water.
4. 4. Differentiate between evaporation and boiling, and explain why boiling point depends on external pressure.
   Model Answer: Evaporation occurs at the surface of a liquid at any temperature below the boiling point and causes cooling because higher-energy molecules escape. Boiling is a bulk phenomenon where vapour bubbles form within the liquid and rise to the surface; it occurs at a specific temperature — the boiling point. The boiling point depends on external pressure because boiling happens when vapour pressure of the liquid equals the external pressure; at higher external pressure (e.g., under a pressure cooker) vapour pressure must become larger and so the boiling point increases, whereas at lower pressure (e.g., on a mountain) boiling occurs at a lower temperature.
5. 5. What is diffusion? Explain with examples how diffusion provides evidence for the particle nature of matter.
   Model Answer: Diffusion is the spontaneous spreading of particles from regions of higher concentration to lower concentration due to their random motion. Examples include the spreading of a perfume scent in a room and the gradual mixing of a drop of dye in water. These processes occur without visible bulk movement, indicating that particles themselves are moving randomly. Diffusion in gases is fast because particles move quickly over large distances, while diffusion in liquids is slower due to closer packing and friction. The observable mixing despite the absence of any macroscopic stirring provides direct evidence that matter is made of tiny particles in constant motion.
6. 6. Describe Brownian motion and explain its significance in supporting the kinetic molecular theory of matter.
   Model Answer: Brownian motion is the erratic, zig-zag movement observed for tiny particles (such as pollen grains) suspended in a fluid, viewed under a microscope. First observed by Robert Brown, this motion arises because the suspended particles are continuously bombarded by the much smaller molecules of the fluid, which themselves are in constant random motion. Brownian motion provides macroscopic, visual evidence of molecular motion and supports the kinetic molecular theory that matter consists of moving particles; it bridges observable phenomena and microscopic particle behavior.
7. 7. Explain the particle model of matter and outline its key postulates. How does the model help explain changes of state?
   Model Answer: The particle model states that matter is composed of very small particles (atoms or molecules) which are in constant motion; attractive forces act between them; higher temperature increases their kinetic energy. Key postulates include: particles have space between them; different states correspond to different arrangements and motion; temperature relates to average kinetic energy. The model explains changes of state as changes in particle motion and separation: heating increases kinetic energy so solids melt to liquids and liquids vaporize to gases as particles overcome attractive forces; cooling reduces motion allowing gases to condense and liquids to freeze into solids.
8. 8. Describe an activity to demonstrate diffusion in liquids and explain the observations using particle theory.
   Model Answer: Place a beaker of still water and add a drop of potassium permanganate or a colored dye at the centre without stirring. Observe the slow spreading of color throughout the water. Using particle theory, the dye molecules move randomly and gradually spread into regions of lower concentration until uniform distribution is achieved. The absence of bulk currents confirms mixing is due to molecular motion; diffusion is slower in liquids compared to gases because particles in liquids experience greater intermolecular attraction and friction.
9. 9. Explain how temperature affects the rate of diffusion and provide two examples demonstrating this effect.
   Model Answer: Temperature increases the average kinetic energy of particles, causing them to move faster; thus diffusion rate increases with temperature. Examples: (1) A room heated will spread a perfume scent faster than a cold room because air molecules move more rapidly. (2) A hot cup of tea mixes sugar faster than cold tea because the increased molecular motion speeds up dissolution and diffusion of sugar molecules in the liquid.
10. 10. Discuss compressibility of solids, liquids and gases. Explain why gases are highly compressible while solids are nearly incompressible.
    Model Answer: Compressibility refers to how readily a substance's volume decreases under pressure. Gases are highly compressible because particles are far apart, leaving large void spaces that allow particles to be pushed closer. Liquids are slightly compressible because particles are close together with little free space. Solids are nearly incompressible because particles occupy fixed positions with minimal empty space and strong interparticle forces that resist compression. The particle model clarifies these differences: compressibility depends on initial spacing between particles and bond strength.
11. 11. Define latent heat and explain its role during melting and boiling with relevant examples.
    Model Answer: Latent heat is the energy absorbed or released during a change of state without a change in temperature. During melting, a solid absorbs latent heat (latent heat of fusion) which is used to break intermolecular attractions and convert solid into liquid at the melting point — for example, ice absorbing latent heat at 0°C to become water. During boiling, a liquid absorbs latent heat of vaporization at the boiling point to convert into vapour without temperature rise — e.g., water at 100°C absorbs energy to form steam. Latent heat explains why temperature remains constant during the actual phase change.
12. 12. Explain why steam can cause more severe burns than boiling water at the same temperature.
    Model Answer: Steam at 100°C contains the same sensible heat as boiling water plus additional latent heat (heat of vaporization). When steam comes into contact with skin it condenses into liquid water, releasing the latent heat in addition to transferring sensible heat, delivering more total energy to the skin and causing more severe burns. This additional energy release upon condensation is why steam injuries are often worse than scalds from boiling water.
13. 13. Describe sublimation and give three substances that sublime. Explain the industrial or household importance of sublimation.
    Model Answer: Sublimation is the direct transition of a substance between solid and gas phases without passing through the liquid phase. Examples include camphor, naphthalene (mothballs), and dry ice (solid CO₂). Industrially, sublimation is used for purification of heat-sensitive solids (sublimation purification in chemistry) and in freeze-drying of food and pharmaceuticals where removal of water by sublimation preserves structure. In households, naphthalene sublimes slowly releasing vapour that repels moths.
14. 14. Discuss the meaning of boiling point and explain how altitude (external pressure) affects the boiling point of water.
    Model Answer: Boiling point is the temperature at which the vapour pressure of a liquid equals the external pressure, causing bubbles of vapour to form throughout the liquid. At higher altitudes atmospheric pressure is lower, so vapour pressure requirement is met at a lower temperature and water boils at a temperature lower than 100°C. This affects cooking times and requires adjustments in recipes and pressure cooking to achieve proper cooking at high altitudes.
15. 15. Explain the difference between physical and chemical changes with appropriate examples and tests to distinguish them.
    Model Answer: A physical change alters physical properties such as shape or state without forming new substances — e.g., melting of ice or tearing paper. A chemical change results in formation of new substances with different chemical properties — e.g., burning of paper producing ash and gases, or rusting of iron forming iron oxide. Tests to distinguish include: (a) physical changes are often reversible, chemical changes are not easily reversible; (b) chemical changes are accompanied by energy change, color change, gas evolution or precipitate formation, whereas physical changes typically are not.
16. 16. Give a detailed explanation of how a measuring cylinder experiment can show that matter occupies space and has mass.
    Model Answer: To show matter occupies space, place a known volume of water in a measuring cylinder and record the level. Gently add a small solid object; the water level rises indicating displacement equal to the object's volume, demonstrating the object occupies space. To show mass, weigh an empty sealed container, then fill it with air (normal condition) and weigh again — using a vacuum pump to remove air or comparing with a container with and without gas (experimental setups) shows mass difference attributable to contained matter. These measurements demonstrate both space occupation and mass, the defining properties of matter.
17. 17. How does the Kinetic Molecular Theory explain pressure exerted by gases? Include the effect of temperature and volume on pressure in your answer.
    Model Answer: According to kinetic theory, gas pressure arises from collisions of moving gas molecules with the walls of the container; each collision exerts a tiny force, and the sum of many collisions per unit area produces pressure. Increasing temperature raises average molecular speed and collision frequency and force, thus increasing pressure if volume is constant. Reducing container volume increases collision frequency as molecules have less space to travel between collisions, so pressure rises if temperature remains constant (Boyle’s law qualitatively explained). These relationships are encapsulated in the ideal gas behaviour at moderate conditions.
18. 18. Describe an experiment to demonstrate that diffusion in gases is faster than in liquids, and explain the observations.
    Model Answer: For gases, place a small quantity of concentrated ammonia at one end of a long glass tube and hydrochloric acid at the other end — a white ring of ammonium chloride forms nearer to the HCl end quickly, showing rapid diffusion. For liquids, add a drop of dye to water in a beaker and observe slow spreading over several minutes. The faster formation of the ring in gases and quicker smell propagation demonstrate that gas molecules move faster and have larger mean free paths than molecules in liquids, where closer packing and greater intermolecular attraction slow diffusion.
19. 19. Explain why heating increases the rate of evaporation and how this principle is used in everyday cooling systems.
    Model Answer: Heating increases the average kinetic energy of liquid molecules; more molecules attain sufficient energy to escape the surface, increasing evaporation rate. This is used in cooling systems like evaporative coolers and perspiration — when sweat evaporates from skin it removes heat producing a cooling effect. Similarly, refrigerators and cooling towers use phases and evaporation/condensation cycles (with controlled pressures/temperatures) to transfer heat away from an environment.
20. 20. Discuss the role of intermolecular forces in determining the properties of matter and provide examples where these forces influence macroscopic behaviour.
    Model Answer: Intermolecular forces (IMFs) — such as dipole–dipole, hydrogen bonding and van der Waals forces — determine how tightly particles are held together and thus influence melting/boiling points, viscosity, surface tension and vapour pressure. For example, water’s relatively high boiling point and surface tension arise from hydrogen bonding; ethanol has lower boiling point due to weaker IMFs. IMFs affect viscosity — glycerol is viscous due to strong hydrogen bonding — and capillary action in plants relies on cohesive and adhesive forces. Thus microscopic attractions produce measurable macroscopic properties.
21. 21. Explain why the diffusion of perfume is faster in a warm room compared to a cold room, using particle ideas.
    Model Answer: In a warm room molecules (both perfume and air) possess higher average kinetic energy, moving faster and colliding more frequently. Faster motion increases the net rate at which perfume molecules spread into the surrounding air, hence scent is detected quicker. In a cold room molecular speeds are lower, diffusion is slower, and scent spreads more gradually. This behavior follows directly from the particle model linking temperature to kinetic energy and motion.
22. 22. Give a full explanation of why liquids have definite volume but no definite shape, and relate this to molecular arrangement and motion.
    Model Answer: Liquids have definite volume because particles are closely packed enough that overall space occupied remains nearly constant; though they are not in fixed positions molecular attractions still keep them close preventing significant compression. They have no definite shape because particles can move/slide past each other, allowing the liquid to conform to the shape of its container. On a molecular level, liquid particles have sufficient kinetic energy to overcome some intermolecular bonds, enabling flow while remaining close enough to maintain volume.
23. 23. What is meant by 'surface phenomenon' in the context of evaporation? Provide an activity that students can perform to verify this property.
    Model Answer: A surface phenomenon involves changes occurring only at the surface; evaporation is surface phenomenon because only molecules at the liquid surface can escape into vapour. An activity: place equal volumes of water in two dishes, one with larger surface area than the other, leave both for several hours; the dish with larger surface area will lose more water due to greater evaporation. This demonstrates evaporation depends on surface area and thus occurs at the surface.
24. 24. Explain how humidity affects drying of clothes and describe measures to improve drying rate in humid conditions.
    Model Answer: Humidity is the amount of water vapour present in air; high humidity reduces the vapour pressure gradient between wet clothes and surrounding air, slowing evaporation and thus drying. To improve drying in humid conditions use methods that increase evaporation: provide higher temperature (warm dryer), increase airflow (fan or wind), increase surface area (spread clothes out), or use dehumidifiers to lower ambient humidity. These methods restore a larger driving gradient for water molecules to escape from fabric into air.
25. 25. Provide an exam-style answer discussing how a viscosity difference between liquids can be explained using molecular interactions.
    Model Answer: Viscosity measures a liquid's resistance to flow and is influenced by intermolecular forces and molecular size/shape. Stronger intermolecular attractions (e.g., hydrogen bonds in glycerol) increase resistance to movement, raising viscosity. Larger or more entangled molecules (polymers, oils) increase friction and hinder flow, also raising viscosity. Temperature reduces viscosity because higher thermal energy helps molecules overcome attractions and move past each other more easily. Thus molecular interactions and structure determine macroscopic viscosity.
26. 26. How does the particle model account for thermal expansion? Give two engineering examples where thermal expansion must be considered.
    Model Answer: On heating, particles vibrate more vigorously and average separations increase, causing materials to expand in volume or length — this is thermal expansion. Engineering examples: (1) Bridges include expansion gaps to prevent structural damage when metal expands in summer; (2) Railway tracks use small gaps or expansion joints to avoid buckling. In precision devices, temperature control is critical to maintain dimensional tolerances because expansion can affect function.
27. 27. Describe the role of evaporation in natural cooling processes and in one man-made cooling device.
    Model Answer: Evaporation removes higher-energy molecules from a liquid’s surface, reducing average kinetic energy and producing cooling. Natural cooling: human perspiration evaporates from skin surfaces, dissipating body heat. Man-made device: an evaporative cooler (swamp cooler) draws warm air through wet pads; water evaporation absorbs heat from the air stream, lowering temperature and providing cooling in dry climates. Evaporation is central to both phenomena and relies on ambient humidity and airflow to be effective.
28. 28. Explain, with examples, why diffusion is slower in liquids than gases and how this affects biological processes.
    Model Answer: Diffusion is slower in liquids because particles are more closely packed and experience greater friction and intermolecular attractions compared to gases where particles are farther apart and move faster. For example, smell disperses much faster through air (gas) than a solute spreads through water (liquid). In biology, slower diffusion in liquids affects processes such as nutrient transport in cells or aquatic diffusion; therefore, organisms have evolved adaptations (large surface areas, active transport, circulation systems) to overcome diffusion limits in liquid environments.
29. 29. What practical precautions should be taken when carrying out experiments involving evaporation and boiling in the laboratory?
    Model Answer: Safety measures include: (1) wear protective equipment — lab coat, goggles and heat-resistant gloves; (2) avoid direct exposure to steam and boiling liquids; use tongs or clamps when handling hot apparatus; (3) ensure good ventilation to prevent vapour accumulation; (4) do not leave boiling setups unattended; (5) use lids or splash guards to prevent splattering; (6) secure long hair and loose clothing. These precautions minimize burn risks and prevent inhalation or accidental contact with hot liquids and vapours.
30. 30. Summarize the chapter 'Matter in Our Surroundings' in a structured answer suitable for a 5-mark board question.
    Model Answer (structured):

    1. Definition: Matter occupies space and has mass. (1–2 lines)
    2. States: Three common states — solid (definite shape & volume), liquid (definite volume, shape of container), gas (no definite shape or volume). Explain particle arrangement briefly. (2–3 lines)
    3. Changes of state: Melting, freezing, evaporation, boiling, condensation, sublimation — give short definitions and examples. (2–3 lines)
    4. Particle view: Matter made of tiny particles in constant motion; kinetic energy increases with temperature; diffusion and Brownian motion support this idea. (2–3 lines)
    5. Applications & importance: Evaporation used in cooling; diffusion in smell and mixing; thermal expansion in everyday devices. (1–2 lines)

    Conclude with one-line remark: Understanding states and particle behavior forms the foundation for later chemistry and physics topics. (1 line)