Case Based Questions: Particulate Nature of Matter | Science Curiosity Class 8 - New NCERT PDF Download

Case Study 1

Riya was playing at the beach, piling up sand to build a castle. She noticed that while sand could be shaped into a pile, water she poured from a bucket flowed and spread out. At home, she dissolved sugar in water and found it disappeared but made the water sweet. Riya wondered why sand could be piled but water couldn’t, and why sugar vanished in water but was still present.

Questions

1. Why can Riya pile sand but not water? (2 marks)
   Solution:
   Sand can be piled because its particles are tightly packed with strong interparticle forces, giving it a fixed shape. Water flows as its particles are loosely packed with weaker forces, taking the shape of the container.

2. Why did sugar disappear in water but still make it sweet? (2 marks)
   Solution:
   Sugar dissolves in water, breaking into tiny particles that spread among water particles, becoming invisible. The sweet taste shows sugar particles are still present in the solution.

3. How do the interparticle forces and spacing in sand (solid) and water (liquid) explain their different behaviors? (3 marks)
   Solution:
   Sand, a solid, has tightly packed particles with strong interparticle forces, holding them in a fixed shape, allowing piling. Water, a liquid, has loosely packed particles with weaker forces, letting them move freely and take the container’s shape. The larger interparticle spacing in water compared to sand allows it to flow, unlike sand’s rigid structure.

4. What happens to the volume of water when sugar is dissolved in it? (2 marks)
   Solution:
   The volume of water stays almost the same when sugar dissolves, as sugar particles fit into spaces between water particles, showing liquids have some interparticle spacing.

5. Why does sand settle at the bottom when mixed with water? (2 marks)
   Solution:
   Sand doesn’t dissolve in water because its particles have strong interparticle forces and don’t mix with water particles. Sand’s heavier particles settle due to gravity.

Case Study 2

During a science class, Arjun broke a piece of chalk into smaller pieces and ground them into a fine powder. He observed the powder under a magnifying glass and saw tiny chalk particles. Later, he poured 100 mL of water into different-shaped containers and noticed it took their shapes but kept the same volume. At home, he smelled incense smoke spreading across the room. Arjun wondered why chalk breaks into smaller particles, why water changes shape, and how smoke spreads.

Questions

1. Why does chalk break into smaller particles when ground? (2 marks)
   Solution:
   Chalk breaks into smaller particles because it is made of many tiny constituent particles. Grinding physically separates these particles, but they remain chalk, showing it’s a physical change.

2. Why does water take the shape of different containers but keep the same volume? (2 marks)
   Solution:
   Water’s particles are loosely packed and move freely, taking the container’s shape. The interparticle forces keep particles close enough to maintain a fixed volume of 100 mL.

3. How does the spreading of incense smoke demonstrate the behavior of gas particles, and what is the role of interparticle forces? (3 marks)
   Solution:
   Incense smoke spreads because gas particles move freely in all directions, filling the room. They have negligible interparticle forces, allowing maximum spacing and movement. Unlike solids or liquids, where stronger forces limit movement, gas particles’ weak forces let them spread quickly, carrying the fragrance everywhere.

4. Why can’t chalk particles dissolve in water like sugar? (2 marks)
   Solution:
   Chalk particles have strong interparticle forces and don’t mix with water particles, so they don’t dissolve. Sugar’s particles have weaker forces, allowing them to spread among water particles.

5. What happens to chalk particles if ground to the smallest possible size? (2 marks)
   Solution:
   If chalk is ground to the smallest size, it becomes tiny constituent particles that can’t be broken further without changing the substance. These are the basic building blocks of chalk.

Case Study 3

Meera conducted an experiment by adding a pinch of potassium permanganate to water and saw pink streaks spreading until the water turned uniformly pink. She tried compressing water in a syringe and found it hard to compress, but air in the syringe compressed easily. At home, she noticed a candle’s wax melting when heated and solidifying when cooled. Meera wondered why the pink color spread, why air compresses but water doesn’t, and how wax changes state.

Questions

1. Why did the pink color of potassium permanganate spread in water? (2 marks)
   Solution:
   The pink color spread because water particles are in constant motion, pulling and spreading potassium permanganate particles throughout the water, creating a uniform pink color.

2. Why was it easier to compress air than water in the syringe? (2 marks)
   Solution:
   Air compresses easily because gas particles have large interparticle spaces and weak forces, allowing them to move closer. Water’s particles are closer with stronger forces, making it nearly incompressible.

3. How does heating change the wax from solid to liquid, and what role do interparticle forces play? (3 marks)
   Solution:
   Heating gives wax particles more energy, causing them to vibrate more and weaken interparticle forces. At the melting point, these forces are overcome, allowing particles to move freely, changing the solid wax to a liquid. Strong interparticle forces in solid wax keep particles fixed, but heating reduces these forces, enabling the state change.

4. Why does water not compress like air in the syringe? (2 marks)
   Solution:
   Water’s particles are closely packed with strong interparticle forces, leaving little space to compress. Air’s particles have large spaces and weak forces, allowing compression.

5. What would happen if Meera heated the melted wax further? (2 marks)
   Solution:
   Further heating the melted wax would increase particle energy, weakening interparticle forces more. At the boiling point, wax could turn into vapor, with particles moving freely as a gas.

Case Study 4

Vikram was cleaning his room and noticed that spilled juice spread out on the floor, but a fallen book stayed in place. He burned an incense stick and saw its smoke fill the room. In class, he learned that heating camphor makes its smell spread faster. Vikram wondered why juice flows but the book doesn’t, how smoke fills the room, and why heating camphor affects its smell.

Questions

1. Why does spilled juice spread out but the book stays in place? (2 marks)
   Solution:
   Juice, a liquid, has loosely packed particles that move freely, spreading on the floor. The book, a solid, has tightly packed particles with strong forces, keeping its shape and position.

2. Why does the smoke from the incense stick fill the entire room? (2 marks)
   Solution:
   Smoke particles, like gas, have weak interparticle forces and large spaces, allowing them to move freely and spread throughout the room, filling all available space.

3. Why does heating camphor make its smell spread faster, and how does this relate to particle movement? (3 marks)
   Solution:
   Heating camphor gives its particles more energy, increasing their movement and weakening interparticle forces. This allows camphor particles to turn into vapor faster and spread quickly through the air, carrying the smell. Gas particles move faster with heat, spreading the fragrance more rapidly compared to when camphor is cold.

4. Why can’t Vikram pile juice like he can pile sand? (2 marks)
   Solution:
   Juice’s particles are loosely packed with weak forces, causing it to flow and take the container’s shape. Sand’s particles are tightly packed with strong forces, allowing it to hold a pile’s shape.

5. What happens to the interparticle spacing when juice evaporates? (2 marks)
   Solution:
   When juice evaporates, its particles gain energy, move farther apart, and turn into a gas. The interparticle spacing increases greatly, becoming much larger than in the liquid state.