Particulate Nature of Matter

Correct Option: (iv) closely packed in solids and move past each other in liquids.

Justification:
In solids, the constituent particles are tightly packed and held in fixed positions by very strong interparticle forces of attraction; they can only vibrate about their mean positions but cannot move past each other.

In liquids, the particles are still relatively close together but the interparticle forces are slightly weaker, allowing the particles to move past each other freely within a limited space. This is why liquids flow and take the shape of their container.

Options (i), (ii), and (iii) are incorrect because:
- Particles in liquids are NOT stationary — they move freely.
- Particles in solids are NOT far apart — they are closely packed.
- Particles in solids do NOT always move freely — they only vibrate about fixed positions.

(i) TRUE.
Melting ice into water is indeed an example of transformation of a solid (ice) into a liquid (water). When ice is heated to its melting point (0°C), it absorbs heat energy and changes into liquid water.

(ii) TRUE.
During melting, heat energy is supplied to the solid. This energy weakens the strong interparticle forces of attraction, allowing particles to move more freely. Hence, interparticle attractions decrease during the melting process.

(iii) TRUE.
Solids have a fixed (definite) shape and a fixed (definite) volume. This is because the particles in solids are tightly packed and held in fixed positions by strong interparticle forces, preventing them from moving freely.

(iv) TRUE.
In solids, the interparticle forces of attraction are very strong, and the particles are closely packed, leaving very little (minimum) interparticle space between them.

(v) TRUE.
When camphor is heated, it sublimes (converts directly from solid to vapour/gas). The gaseous particles of camphor move freely in all directions and spread throughout the room, so the fragrance reaches all corners.

(vi) FALSE.
Corrected statement: On heating camphor, we are adding energy to it, which causes it to sublime (convert into vapour). The camphor vapour (gas particles) then diffuses and spreads throughout the room. The fragrance we smell is due to the camphor vapour particles reaching our nose — the energy is NOT released as a smell. The smell is caused by the movement and diffusion of camphor vapour particles.

Correct Option: (iii) Nothing of the chair will remain.

Justification:
All matter is made up of constituent particles (atoms and molecules). The chair — whether made of wood, metal, or plastic — is entirely composed of these constituent particles. If all the constituent particles were removed, there would be nothing left of the chair. The chair exists only because of the collective arrangement of its constituent particles. Hence, removing all particles means the chair ceases to exist entirely.

Option (i) is incorrect because the chair is made entirely of particles, so removing them changes everything.
Option (ii) is incorrect because the chair would not merely weigh less — it would not exist at all.

Given: We need to explain why gases mix easily but solids do not.

Concept Used: Interparticle spacing and freedom of movement of particles in different states of matter.

Explanation:

Gases mix easily because:
- The interparticle forces of attraction in gases are negligible (almost zero).
- The particles of gases are very far apart from each other, resulting in maximum interparticle spacing.
- Gas particles are completely free to move in all directions with high speed.
- When two gases are placed together, their particles intermingle freely and spread throughout the available space. This process is called diffusion.
- Example: When an incense stick is lit in one corner of a room, its fragrance (vapour particles) spreads to all corners due to free movement of gas particles.

Solids do not mix easily because:
- The interparticle forces of attraction in solids are very strong.
- The particles are tightly packed with minimum interparticle spacing.
- The particles of solids are held in fixed positions and can only vibrate about their mean positions — they cannot move freely from one place to another.
- Therefore, the particles of one solid cannot intermingle with the particles of another solid easily.

Conclusion: The negligible interparticle attraction and maximum interparticle spacing in gases allow their particles to move freely and mix easily, whereas the strong interparticle attraction and minimum interparticle spacing in solids prevent their particles from moving and mixing.

Given: Milk (liquid) flows and spreads when spilled; glass tumbler (solid) retains its shape.

Concept Used: Properties of solids and liquids based on interparticle forces and particle movement.

Justification:

Why milk flows and spreads out (Liquid behaviour):
- Milk is a liquid. In liquids, the interparticle forces of attraction are slightly weaker than in solids.
- The particles of milk are free to move past each other within a limited space.
- Because the particles can move freely, milk does not have a fixed shape.
- When spilled, the milk particles flow under gravity and spread out, taking the shape of the surface (the table) on which they are placed.
- Liquids and gases are called fluids because they can flow.

Why the glass tumbler retains its shape (Solid behaviour):
- The glass tumbler is a solid. In solids, the interparticle forces of attraction are very strong.
- The particles are tightly packed and held in fixed positions; they can only vibrate about their mean positions but cannot move past each other.
- Because the particles cannot move freely, the solid maintains a definite, fixed shape and volume.
- Therefore, even when the tumbler falls, it retains its original shape (unless it breaks due to impact).

Conclusion: The difference in behaviour is due to the difference in interparticle forces and freedom of movement of particles — weaker forces and free movement in liquids (milk) allow it to flow and spread, while strong forces and fixed positions in solids (glass tumbler) allow it to retain its shape.

Given: Ice (solid) → Water (liquid) → Water vapour (gas)

Concept Used: Change in interparticle spacing and arrangement during change of state.

Diagram Description (to be drawn in the answer sheet):

Stage 1 — Ice (Solid):
Draw closely packed particles arranged in a regular, orderly pattern. The particles are touching or very close to each other with minimum space between them. Label: *Ice — particles tightly packed, fixed positions, strong interparticle forces.*

$$\text{[Closely packed, regular arrangement — SOLID]}$$

Stage 2 — Water (Liquid):
Draw particles that are still close together but arranged randomly (no fixed pattern). There is slightly more space between particles compared to ice. The particles are not in fixed positions. Label: *Water — particles close but free to move, slightly more interparticle space, weaker interparticle forces.*

$$\text{[Random arrangement, slightly more space — LIQUID]}$$

Stage 3 — Water Vapour (Gas):
Draw particles that are very far apart from each other, scattered randomly throughout the available space. Very large interparticle spacing. Label: *Water vapour — particles far apart, move freely in all directions, negligible interparticle forces.*

$$\text{[Very far apart, random, maximum space — GAS]}$$

Process Labels:
- Ice → Water: Melting (at 0°C; heat is absorbed; interparticle forces weaken)
- Water → Water Vapour: Boiling/Evaporation (at 100°C for boiling; heat is absorbed; interparticle forces become negligible)

Summary of changes:

| State | Interparticle Space | Interparticle Force | Particle Movement |
|-------|-------------------|--------------------|-----------------|
| Ice (Solid) | Minimum | Very Strong | Only vibration |
| Water (Liquid) | Moderate | Moderate | Move within limited space |
| Water Vapour (Gas) | Maximum | Negligible | Move freely in all directions |

Concept Used: Arrangement of particles in the three states of matter — solid, liquid, and gas.

(i) Aluminium foil — SOLID

Description of diagram to draw:
Draw particles (small circles) arranged in a very close, regular, and orderly pattern. The particles should be touching or nearly touching each other, with minimum space between them. The arrangement should look like a neat grid or lattice.

- Particles: Closely and tightly packed
- Arrangement: Regular/ordered
- Interparticle space: Minimum
- Interparticle force: Very strong
- Movement: Particles only vibrate about fixed positions

(ii) Glycerin — LIQUID

Description of diagram to draw:
Draw particles (small circles) that are close to each other but arranged randomly (no fixed pattern). There should be slightly more space between particles compared to the solid. The particles should appear to be in a disordered arrangement.

- Particles: Close but not as tightly packed as solid
- Arrangement: Random/disordered
- Interparticle space: Moderate (more than solid, less than gas)
- Interparticle force: Moderate (weaker than solid)
- Movement: Particles can move past each other freely within limited space

(iii) Methane gas — GAS

Description of diagram to draw:
Draw particles (small circles) that are very far apart from each other, scattered randomly throughout the entire available space (the container). Very large gaps between particles.

- Particles: Very far apart
- Arrangement: Completely random
- Interparticle space: Maximum
- Interparticle force: Negligible
- Movement: Particles move freely and rapidly in all directions

Note: In all three diagrams, each circle represents one constituent particle of the respective substance. The key difference is the spacing and arrangement of these circles.

Given: A candle just extinguished after burning — different states of wax are visible.

Concept Used: The three states of matter and their particle arrangements.

Identification of different states of wax in the candle (Fig. 7.16a):

In a candle that has just been extinguished after burning, wax can be observed in three states:

1. Solid wax — The main body of the candle (the hard, unmelted portion of the candle stick). This is the solid state of wax.

2. Liquid wax — The melted wax pooled around the wick at the top of the candle (the soft, molten portion near the flame area). This is the liquid state of wax.

3. Wax vapour (Gaseous state) — The smoke/vapour rising from the wick just after the candle is extinguished. This is the gaseous state of wax.

Matching with particle arrangement (Fig. 7.16b):

| State of Wax | Location in Candle | Particle Arrangement |
|---|---|---|
| Solid wax | Hard body of the candle | Closely packed, regular arrangement, particles in fixed positions, minimum interparticle space |
| Liquid wax | Melted wax pool near the wick | Particles close but randomly arranged, free to move past each other, moderate interparticle space |
| Wax vapour (Gas) | Smoke/vapour rising from wick | Particles very far apart, scattered randomly, maximum interparticle space, move freely |

Conclusion: The same substance (wax) exists in all three states simultaneously in a burning/just-extinguished candle, demonstrating how interparticle forces and spacing change with the state of matter.

Given: Ocean water tastes salty but salt is not visible in it.

Concept Used: Dissolution of substances and the particulate nature of matter.

Explanation:

All matter is made up of extremely small constituent particles. When salt (sodium chloride) dissolves in water:

1. The water particles are in constant motion. They pull out the constituent particles of salt from the salt crystals.

2. The salt breaks down into its constituent particles, which are extremely tiny — too small to be seen with the naked eye.

3. These tiny salt particles spread uniformly throughout the water and occupy the spaces between the water particles (interparticle spaces in the liquid).

4. Since the salt particles are extremely small, they cannot be seen individually, but they are present throughout the water.

5. When we taste the ocean water, these dissolved salt particles come in contact with our taste buds, and we sense the salty taste.

This is similar to Activity 7.2 in the chapter, where sugar dissolved in water could not be seen but could be tasted — proving that the constituent particles of sugar were present throughout the solution.

Conclusion: The salt in ocean water has dissolved into its constituent particles, which are too small to be seen but are uniformly distributed throughout the water. Their presence is detected by taste, which is why ocean water tastes salty even though the salt is not visible.

Given: Grains of rice and rice flour take the shape of the container when placed in different jars.

Answer: Both grains of rice and rice flour are solids, not liquids.

Explanation:

At first glance, it may seem that rice grains and rice flour behave like liquids because they take the shape of the container. However, this is a misleading observation. Let us analyse carefully:

Why they appear to behave like liquids:
- Rice grains and rice flour are made up of a very large number of individual solid particles (each grain or flour particle is itself a solid).
- When poured into a container, the collection of these many small solid particles rearranges itself to fill the bottom of the container, giving the appearance of taking the container's shape.
- This is a property of the bulk collection of small solid particles, not of individual particles.

Why they are actually solids:
- Each individual grain of rice or each individual particle of rice flour has a fixed shape and fixed volume of its own — these are properties of solids.
- The individual particles do not flow into each other or merge — they remain separate solid units.
- Unlike liquids, the individual particles cannot move past each other freely; they simply slide over one another as a collection.
- If you look at a single grain of rice, it has a definite shape and does not change its shape on its own.
- The interparticle forces within each grain are strong (solid characteristics).

Key Distinction:
- In a liquid, the constituent particles themselves flow and take the shape of the container.
- In rice grains/rice flour, it is the collection of many individual solid particles that rearranges — each particle itself remains a solid with fixed shape and volume.

Conclusion: Rice grains and rice flour are solids. The apparent shape-taking behaviour is due to the rearrangement of a large number of small solid particles as a bulk collection, not due to the flow of individual particles as in a true liquid.