Matter in Our Surroundings – Long Answer Questions (Class 9 Science)
Medium Level (Application & Explanation)
Q1. Explain the particle nature of matter and describe three key characteristics of particles that help us understand the different states of matter.
Answer:
The particle nature of matter states that all matter is made up of very tiny particles called atoms and molecules, which are in constant motion.
Continuous motion: Particles always move — in solids they vibrate, in liquids they slide past one another, and in gases they move freely and rapidly. This motion explains why temperature affects states.
Space between particles: There is empty space between particles. The space is least in solids, more in liquids, and greatest in gases. This difference explains compressibility and ability to flow.
Attractive forces: Particles attract each other. These forces are strongest in solids, weaker in liquids, and weakest in gases. The strength of attraction determines whether matter has a definite shape, definite volume, or neither.
Together, these characteristics explain why solids keep shape, liquids flow but keep volume, and gases expand to fill a container.
Q2. Using the particle model, explain why solids have definite shape and volume while liquids have definite volume but no definite shape. Give two everyday examples for each state to support your answer.
Answer:
In solids, particles are closely packed in a fixed arrangement and held by strong attractive forces. They only vibrate about fixed positions, so a solid maintains a definite shape and definite volume. Examples: ice and wood.
In liquids, particles are closer than in gases but not fixed. Attractive forces are weaker than in solids, allowing particles to move/slide past each other. Because of this motion, a liquid takes the shape of the container but keeps a definite volume. Examples: water and cooking oil.
The particle model explains why a solid piece of wood stays shaped but liquid water spreads to fit a glass — particle arrangement and freedom of movement control shape and volume.
Q3. Differentiate between evaporation and boiling. Describe three factors that affect the rate of evaporation and explain how each factor works.
Answer:
Evaporation is vaporization occurring only at the surface of a liquid at any temperature; some surface molecules with high kinetic energy escape to gas. Boiling happens throughout the liquid when its vapor pressure equals external pressure, forming bubbles.
Factors affecting rate of evaporation:
Temperature: Higher temperature means more molecules have enough kinetic energy to escape, so evaporation increases.
Surface area: Larger surface area exposes more molecules to air, increasing the number that can escape, so evaporation is faster.
Humidity (vapor concentration): When surrounding air is already humid (high vapour concentration), fewer molecules escape because the air cannot accept more vapour; low humidity speeds up evaporation.
Wind or air movement also removes vapour above the surface, increasing the rate by reducing local humidity.
Q4. Explain why gases are easily compressible but liquids and solids are not. Provide two practical applications that use compressibility of gases and two that rely on incompressibility of liquids or solids.
Answer:
Compressibility depends on the space between particles. In gases, particles are far apart with large empty space; pushing them closer reduces volume easily, so gases are easily compressible.
In liquids and solids, particles are close together with little empty space; applying pressure cannot significantly reduce their volume, so they are nearly incompressible.
Applications using gas compressibility: pneumatic tools (air compressors store and release compressed air) and inflatable tyres (air cushions and adjusts pressure).
Applications relying on incompressibility: hydraulic systems (brake fluids transmit force) and building materials (solids support loads without compressing).
This particle explanation shows how space between particles controls compressibility and engineering use.
Q5. Describe the processes of melting and freezing in terms of particle motion and energy changes. Use H₂O (water) as an example and mention one everyday situation where latent heat is involved.
Answer:
Melting: When a solid (e.g., ice) is heated, its particles gain kinetic energy. As energy increases, particles vibrate more strongly and the attractive forces weaken. At the melting point, particles overcome fixed positions and begin to move past each other, converting the solid into a liquid (water). Energy absorbed without temperature rise is called latent heat of fusion.
Freezing: The reverse — cooling removes kinetic energy, particles slow down, attractive forces pull them into fixed positions, and liquid becomes solid. Energy released during freezing equals the latent heat of fusion.
Everyday example: Ice melting in a drink absorbs heat from the drink, keeping it cool — latent heat plays a direct role in temperature control.
High Complexity (Analytical & Scenario-Based)
Q6. A puddle of water dries faster on a windy, sunny, and dry day than on a still, humid, and cloudy day. Explain this observation using the particle model of matter and factors affecting evaporation.
Answer:
On a windy, sunny, dry day: temperature is higher, so many water molecules have increased kinetic energy and more reach escape energy at the surface; sunlight supplies heat. Wind removes saturated air above the puddle, lowering local humidity and allowing more vapour to diffuse away. Low ambient humidity means air can accept more water vapour, increasing the net escape rate. Also, wind may spread the puddle into a slightly larger surface area, aiding evaporation.
On a still, humid, cloudy day: lower temperature reduces molecular energy, clouds block heat, and high humidity means the air is already rich in vapour — few surface molecules can escape. Minimal air movement prevents vapour removal, so evaporation slows.
Thus, the particle model (kinetic energy, surface escape, and vapour removal) explains why the puddle dries faster under warm, windy, dry conditions.
Q7. Explain with particle-level reasoning why a pressure cooker cooks food faster than an open pot. Mention the role of boiling point, vapour pressure, and particle collisions.
Answer:
In a pressure cooker, the lid traps steam, increasing the pressure inside. For liquid water, boiling occurs when its vapour pressure equals external pressure. With higher pressure, water must reach a higher temperature before its vapour pressure matches the external pressure — so water can boil above 100°C.
At higher temperature, water molecules have higher kinetic energy, causing faster movement and more energetic collisions with food particles. These collisions transfer heat more quickly into food, speeding up cooking.
Also, trapped steam reduces heat loss and creates a uniform high-temperature environment that penetrates food faster. Therefore, by raising pressure and boiling point, the cooker allows higher cooking temperatures and faster heat transfer at the molecular level.
Q8. Describe the heating curve of a substance when it is heated from solid at low temperature to gas at high temperature. Explain what happens to temperature and particle motion during each segment, including latent heat regions.
Answer:
Heating curve segments:
Solid heating (temperature rises): Energy supplied increases particles’ kinetic energy, so temperature rises; particles vibrate faster but remain fixed.
Melting plateau (temperature constant): Energy goes into breaking bonds (overcoming attractions) rather than raising temperature — this is the latent heat of fusion; particles begin to move past each other and form a liquid.
Liquid heating (temperature rises): Continued heating increases kinetic energy; particles move more freely and temperature rises.
Boiling plateau (temperature constant): Energy becomes latent heat of vaporization to convert liquid into gas; temperature stays constant at boiling point while bubbles form and particles escape into gas.
Gas heating (temperature rises): After vaporization, further energy increases kinetic energy of gaseous particles; temperature rises and particles move rapidly, far apart.
The plateaus represent energy used to change the state (bond strength change), not temperature.
Q9. A balloon filled with helium is released indoors and slowly drifts and mixes with room air. Explain this process using the ideas of diffusion and particle motion. Why might hydrogen spread faster than helium under the same conditions?
Answer:
Diffusion is the spontaneous mixing of gases due to random motion of particles. Helium atoms inside the balloon escape through any small opening or after the balloon bursts, and their high-speed random motion causes them to collide and spread into the surrounding air. Over time, helium atoms move from regions of higher concentration (near the release) to lower concentration (room), leading to even mixing without any bulk flow.
Lighter gases generally have higher average speeds at the same temperature (from kinetic theory), so they tend to spread faster. Hydrogen (H₂) is lighter than helium (He); at the same temperature, H₂ molecules move faster on average and therefore diffuse more rapidly than He atoms. Additionally, molecular size and interactions with air affect diffusion rate, but mass is the main reason hydroge...
