Particulate Nature of Matter NCERT Solutions | Science Curiosity Class 8 - New NCERT PDF Download

Probe and Ponder

1. Why is it possible to pile up stones or sand, but not a liquid like water?
Ans: 

• ​Stones and sand are solids, where particles are tightly packed with strong interparticle attractions, giving them a fixed shape and allowing them to be piled up without flowing. 
• Water is a liquid, with weaker interparticle attractions, so its particles can move freely and flow, taking the shape of the container or spreading out, making piling impossible.

2. Why does water take the shape of folded hands but lose that shape when released?
Ans: 

• Water is a liquid, so its particles have enough freedom to move and adapt to the shape of the container (like folded hands). 
• When released, the particles flow due to gravity and weaker interparticle forces, losing the shape because liquids do not have a fixed form—they only have a fixed volume.

3. We cannot see air, so how does it add weight to an inflated balloon?
Ans: 

• ​Air is a gas made of tiny, invisible particles (like nitrogen and oxygen molecules) that are far apart and in constant motion. 
• When a balloon is inflated, these particles occupy space inside, adding mass (and thus weight) to the balloon, even though the particles themselves are too small to see.

4. Is the air we breathe today the same that existed thousands of years ago?
Ans: 

• ​Yes, in terms of composition, the air we breathe is largely the same mixture of gases (nitrogen, oxygen, etc.) that has existed for thousands of years, as matter is conserved and particles cycle through natural processes like respiration and photosynthesis. 
• However, human activities have slightly altered its composition, adding pollutants.

5. Share your questions?
Ans: 

• How small are the tiniest particles of matter, and can we ever see them?
• Why do some solids melt easily while others need very high temperatures?
• If gases have no fixed volume, how do they stay contained in the atmosphere?
• What happens to the particles when a substance changes from solid to liquid?
• Why don't all solids dissolve in water like sugar does?

Keep the Curiosity Alive

1. Choose the correct option.
The primary difference between solids and liquids is that the constituent particles are: 
(i) closely packed in solids, while they are stationary in liquids. 
(ii) far apart in solids and have fixed position in liquids. 
(iii) always moving in solids and have fixed position in liquids. 
(iv) closely packed in solids and move past each other in liquids.
Ans: (iv) closely packed in solids and move past each other in liquids.
In solids, particles are fixed in position due to strong attractions, while in liquids, particles can slide past one another, allowing flow.

2. Which of the following statements are true? Correct the false statements.

(i) Melting ice into water is an example of the transformation of a solid into a liquid. 
Ans: True
(ii) Melting process involves a decrease in interparticle attractions during the transformation. 
Ans: False
Melting involves overcoming interparticle attractions with heat energy, but the attractions weaken as particles move farther apart.
(iii) Solids have a fixed shape and a fixed volume. 
Ans: True 
(iv) The interparticle interactions in solids are very strong, and the interparticle spaces are very small.
Ans: True

(v) When we heat camphor in one corner of a room, the fragrance reaches all corners of the room. 
Ans: True

(vi) On heating, we are adding energy to the camphor, and the energy is released as a smell.
Ans: False
Heating adds energy to camphor particles, causing sublimation (solid to gas), and the gas particles spread due to constant motion, carrying the fragrance.

3. Choose the correct answer with justification. If we could remove all the constituent particles from a chair, what would happen?
(i) Nothing will change. (ii) The chair will weigh less due to lost particles. (iii) Nothing of the chair will remain.
Ans: A chair is made entirely of constituent particles (atoms and molecules of wood or other materials). Removing all particles would leave no matter behind, as matter is composed of these tiny units—there would be no structure, mass, or volume left.

4. Why do gases mix easily, while solids do not?

• ​Gases mix easily because their particles have negligible interparticle attractions, maximum spacing, and constant random motion, allowing them to spread and intermingle freely in all directions. 
• Solids do not mix easily as their particles are tightly packed with strong attractions, fixed positions, and only vibrational motion, preventing them from blending without external force like grinding.

5. When spilled on the table, milk in a glass tumbler flows and spreads out, but the glass tumbler stays in the same shape. Justify this statement.
Ans: 

• ​Milk is a liquid, with particles that have weaker interparticle attractions and can move past each other, allowing it to flow and spread under gravity. 
• The glass tumbler is a solid, with particles tightly packed and held by strong attractions in fixed positions, maintaining its rigid shape and volume regardless of external forces like spilling.

6. Represent diagrammatically the changes in the arrangement of particles as ice melts and transforms into water vapour.
Ans: 

• Ice (Solid): Particles closely packed in a fixed lattice, vibrating in place. (Imagine a grid of dots tightly together.)

• Water (Liquid): Particles slightly farther apart, sliding past each other but still close. (Dots loosely arranged, with some movement arrows.)

• Water Vapour (Gas): Particles far apart, moving freely in all directions. (Scattered dots with long arrows showing random motion.)
  The transformation: Heat weakens attractions, increasing particle spacing and motion from solid to liquid (melting) and liquid to gas (boiling/evaporation).

7. Draw a picture representing particles present in the following:
(i) Aluminium foil (Solid): Closely packed particles in a regular pattern, with minimal spacing and only vibrations.
(ii) Glycerin (Liquid): Particles close but irregular, with some sliding motion and small spaces.
(iii) Methane gas (Gas): Widely spaced particles moving randomly in all directions, with large empty spaces.

8. Observe Fig. 7.16a which shows the image of a candle that was just extinguished after burning for some time. Identify the different states of wax in the figure and match them with Fig. 7.16b showing the arrangement of particles.

Ans:

• Solid wax (at base): Rigid, unmelted portion—matches tightly packed particles.
• Liquid wax (melted pool): Flowing around wick—matches loosely arranged particles with movement.
• Gaseous wax (vapour/smoke): Rising as fumes—matches widely spaced, freely moving particles.
  The figure shows transitions: solid to liquid (melting) and liquid to gas (evaporation), with particle arrangements changing from fixed to mobile.

9. Why does the water in the ocean taste salty, even though the salt is not visible? Explain.
Ans: 

• Ocean water tastes salty because salt (sodium chloride) dissolves into tiny particles that mix uniformly with water particles, occupying interparticle spaces. 
• The salt particles are too small to see but are detected by taste buds. 
• This is a solution where solute (salt) particles are distributed evenly in the solvent (water) without changing visibility.

10. Grains of rice and rice flour take the shape of the container when placed in different jars. Are they solids or liquids? Explain.
Ans: 

• ​They are solids. Both rice grains and flour consist of solid particles with strong interparticle attractions and fixed shapes individually.
• However, as a collection of many small particles, they can flow and adapt to the container's shape due to gravity, similar to liquids, but they remain solids because their particles do not truly flow past each other like liquid particles.

Discover, Design, and Debate

1. Fix a balloon over the neck of a bottle and put the bottle in hot water. Explore what will happen?
Ans: The balloon inflates. Hot water heats the air inside the bottle, increasing the kinetic energy of air particles, causing them to move faster and spread out (expand). This raises the pressure inside, pushing air into the balloon. It demonstrates gas particles' response to heat, with increased motion and spacing.

2. Design and create simple models to represent particles of solids, liquids, and gases showing interparticle spacing using clay balls, beads, etc.

• Solid: Arrange clay balls in a tight grid (e.g., in a box), touching each other to show minimal spacing and fixed positions.

• Liquid: Place beads in a shallow tray, close but movable when shaken, illustrating slight spacing and flow.

• Gas: Scatter beads loosely in a large container, shaking to show random motion and maximum spacing.
  These models visualize how interparticle spacing increases from solids to gases, affecting properties like shape and volume.

3. Pretend to be particles of solids, liquids, and gases, at different temperatures—create and perform a role-play/dance showing particles in motion.
Ans: 

• Solid (low temperature): Students stand close, holding hands tightly, only vibrating in place (small shakes).

• Liquid (medium temperature): Loosen grips, move arms while sliding around in a group, showing flow but staying somewhat together.

• Gas (high temperature): Break apart, run freely in all directions with energetic jumps, simulating random motion.
  Increase "temperature" by speeding up movements to show phase changes, like "melting" from solid to liquid.

4. Debate in the class — ‘Gases can spread and fill all the available space’. Is this property of gases beneficial or harmful?
Ans: 
Beneficial side: Gases spreading enables essential processes like oxygen diffusion in breathing, fragrance dispersal in perfumes, and even weather patterns (wind). It aids in cooking (gas stoves) and inflation (tires/balloons), making life convenient and supporting ecosystems.
Harmful side: It can spread pollutants or toxic gases quickly (e.g., air pollution, gas leaks causing accidents), leading to health risks or environmental damage like greenhouse gases contributing to climate change.
Overall, the property is mostly beneficial when controlled but harmful if unmanaged—debate could conclude with the need for safety measures like ventilation.