SCIENCE CLASS- 8
CHAPTER-7 (Particulate Nature of Matter)
Question 1. Choose the correct option.
The primary difference between solids and liquids is that the constituent particles are:
Answer: (iv) Closely packed in solids and move past each other in liquids.
Explanation:
In solids, particles are tightly packed and can only vibrate about their fixed positions. In liquids, particles are less tightly packed and can move past one another, allowing liquids to flow.
Question 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.
Answer: True
Explanation: Melting is the process by which a solid changes into a liquid on heating.
(ii) Melting process involves a decrease in interparticle attractions during the transformation.
Answer: True
Explanation: During melting, particles gain energy and move farther apart, reducing the force of attraction between them.
(iii) Solids have a fixed shape and a fixed volume.
Answer: True
Explanation: Strong intermolecular forces keep the particles fixed in position, giving solids a definite shape and volume.
(iv) The interparticle interactions in solids are very strong, and the interparticle spaces are very small.
Answer: True
Explanation: Particles in solids are closely packed and held together by strong forces of attraction.
(v) When we heat camphor in one corner of a room, the fragrance reaches all corners of the room.
Answer: True
Explanation: Camphor changes into vapour, and its particles spread throughout the room by diffusion.
(vi) On heating, we are adding energy to the camphor, and the energy is released as a smell.
Answer: False
Correct Statement:
On heating, camphor gains energy and changes directly into vapour. The smell is due to the diffusion of camphor vapour particles in air, not because energy is released as smell.
Question 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.
Answer: (iii) Nothing of the chair will remain.
Justification:
All matter is made up of particles. If all the particles of the chair are removed, the chair itself will cease to exist because there will be no matter left to form it.
Question 4. Why do gases mix easily, while solids do not?
Answer:
Gases mix easily because their particles are far apart and move freely in all directions. This rapid motion allows gases to diffuse into one another quickly.
In solids, particles are closely packed and held firmly in fixed positions by strong forces of attraction. Therefore, solids do not mix easily.
Question 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.
Answer:
Milk is a liquid. The particles in a liquid can move freely past one another. Therefore, milk flows and spreads when spilled.
The glass tumbler is a solid. Its particles are tightly packed and remain fixed in position. Therefore, it retains its shape and does not flow.
Question 6. Represent diagrammatically the changes in the arrangement of particles as ice melts and transforms into water vapour.
Answer:
Ice (Solid)
● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
Particles are closely packed in a regular arrangement.
Water (Liquid)
● ● ● ● ● ● ● ● ●
Particles are close together but can move past each other.
Water Vapour (Gas)
● ●
●
●
●
Particles are far apart and move freely in all directions.
Question 7. Draw a picture representing particles present in the following.
(i) Aluminium foil
Answer:
● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
Particles are tightly packed because aluminium foil is a solid.
(ii) Glycerine
Answer:
● ● ● ● ● ● ● ● ● ●
Particles are close together but can move because glycerine is a liquid.
(iii) Methane gas
Answer:
● ●
●
●
●
Particles are widely separated because methane is a gas.
Question 8. Observe Fig. 7.16(a) which shows the image of a candle that was extinguished after burning for some time. Identify the different states of wax in the figure and match them with Fig. 7.16(b) showing the arrangement of particles.
Answer:
The candle contains wax in all three states of matter.
- Solid Wax: Unmelted candle wax.
- Liquid Wax: Melted wax near the wick.
- Wax Vapour: Smoke-like vapour above the wick.
Matching with particle arrangements:
- Top diagram (closely packed particles) → Solid wax
- Middle diagram (particles close but irregular) → Liquid wax
- Bottom diagram (widely spaced particles) → Wax vapour
Question 9. Why does the water in the ocean taste salty, even though the salt is not visible?
Answer:
Salt dissolves completely in water and forms a homogeneous solution.
The salt particles become uniformly distributed throughout the water and are too small to be seen with the naked eye.
Although invisible, the dissolved salt gives the ocean water its salty taste.
Question 10. Grains of rice and rice flour take the shape of the container when placed in different jars. Are they liquids or solids? Explain.
Answer:
Grains of rice and rice flour are solids.
Reason:
- Each grain of rice has a definite shape and volume.
- Each particle of rice flour is also a solid particle.
- When many solid particles are placed together, they appear to take the shape of the container because the particles can slide over one another.
- However, each individual particle remains a solid.
Therefore, rice grains and rice flour are solids, not liquids.
Key Concepts Learned
- Matter is made up of tiny particles.
- Particles of solids are closely packed and have strong intermolecular forces.
- Particles of liquids can move past one another.
- Particles of gases are far apart and move freely.
- Diffusion is fastest in gases and slowest in solids.
- Melting converts solids into liquids.
- Heating increases the kinetic energy of particles.
- Solids have definite shape and volume, whereas liquids have definite volume but no definite shape.