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

Q6: Read the source below and answer the questions that follow:

Anya filled a syringe (without a needle) with air and pushed the plunger down, compressing the air easily. She then tried the same with water but found it almost impossible to compress. She wondered why air behaved so differently from water.

(a) What happens to the air particles when Anya pushes the plunger? (1 mark)
(b) Why is water difficult to compress compared to air? (1 mark)
(c) What conclusion can Anya draw about the spaces between particles in gases and liquids? (2 marks)

Ans:
(a) Air particles move closer together, reducing the volume of air.
(b) Water particles are tightly packed with little space, making water nearly incompressible.
(c) Gases have large spaces between particles, allowing compression, whereas liquids have minimal spaces, making them hard to compress.

Q7: Read the source below and answer the questions that follow:

Karan dropped potassium permanganate crystals into two jars, one with hot water and one with cold water. He observed that the colour spread faster in the hot water, showing how temperature affects particle motion.

(a) Why does the colour spread faster in hot water? (1 mark)
(b) How does temperature affect the movement of particles in liquids? (1 mark)
(c) Explain why the speed of particle movement is important in practical applications like cooking or medicine. (2 mark)

Ans:
(a) Particles in hot water move faster, spreading the colour quickly.
(b) Higher temperature increases particle energy, causing faster movement and quicker mixing.
(c) Faster particle movement helps dissolve substances quickly, useful in cooking and medicines for better absorption.

Q8: Read the source below and answer the questions that follow:

Mia lit an incense stick in one corner of her room. After a few minutes, she could smell the fragrance throughout the room. She realised that gas particles carrying the scent were moving and spreading in all directions.

(a) How does the fragrance spread throughout the room? (1 mark)
(b) What does this tell us about the movement of gas particles? (1 mark)
(c) Why is the movement of gas particles important for ventilation and fresh air in rooms? (2 marks)

Ans:
(a) The fragrance spreads because gas particles carrying scent move freely through the room.
(b) Gas particles move randomly and spread in all directions.
(c) This movement helps in circulating fresh air, removing bad odours, and maintaining air quality in rooms.

Q9: Read the source below and answer the questions that follow:

Saanvi added sugar to one glass of water and sand to another. The sugar dissolved completely, while the sand settled at the bottom. She realised that the size of particles affects whether a substance dissolves in water.

(a) Why does sugar dissolve in water, but sand does not? (1 mark)
(b) Does the volume of water change when sugar dissolves? (1 mark)
(c) What does this show about the spaces between particles in liquids? (2 marks)

Ans:
(a) Sugar particles dissolve and fit between water particles; sand particles are too large and settle.
(b) The water volume increases slightly as sugar occupies spaces between water particles.
(c) Liquids have spaces between particles, allowing other small particles to enter and dissolve.

Q10: Read the source below and answer the questions that follow:

During a science demonstration, Tejas watched ice melt into water at 0°C and then water boil into steam at 100°C. He observed the changes in particle movement and learned how temperature affects states of matter.

(a) What is the melting point of ice in degrees Celsius? (1 mark)
(b) What changes happen to particles at the melting and boiling points? (2 marks)
(c) Why do different solids melt at different temperatures? (1 mark)

Ans:
(a) The melting point of ice is 0°C.
(b) At the melting point, particles gain energy and vibrate more to break free from fixed positions; at the boiling point, particles gain more energy and move freely as a gas.
(c) Different solids have different strengths of interparticle forces, so they require different temperatures to melt.