Matter

10^-10 m

Reason — A molecule is very small in size which is about 10^-10 m to 10^-9 m.

solids

Reason — The inter-molecular force of attraction between the particles is more in solids, less in liquids and least in gases.

in a liquid, move within its boundary.

Reason — The molecules in a solid are fixed at their positions and can only vibrate about their mean positions while in a liquid they can move within the boundary and moves freely in space in gas.

more dense

Reason — Solids have strong force of attraction (inter-molecular force) and least inter-molecular spaces between it's molecules which makes them more closely packed state and thus are more dense than liquids and gases.

weaker than in solids

Reason —

Solids : Molecules are tightly packed, and intermolecular forces are strongest, giving them a fixed shape and volume.

Liquids : Molecules are less tightly packed compared to solids, so the intermolecular forces are weaker — allowing them to flow but still maintain a definite volume.

Gases : Molecules are far apart, and intermolecular forces are negligible — they can expand to fill any container.

II

Reason — As drinking water belongs to liquid state which is less closely packed than solid and more packed than gas.

Both A and R are true and R is the correct explanation of A

Explanation

Assertion (A) is true because a metal water tank retains its shape when filled because solids are rigid and do not deform easily under normal conditions.

Reason (R) is true because solids have very little intermolecular space, so they cannot be compressed under ordinary pressure.

The Reason correctly explains the Assertion.

Therefore, both A and R are true and R is the correct explanation of A.

A is false but R is true

Explanation

Assertion (A) is false because packing sand in a bag does not double its volume; in fact, the volume may decrease slightly as the grains settle and occupy the gaps between them more efficiently.

Reason (R) is true because individual solid particles have a definite shape, size and volume.

Therefore, A is false but R is true.

(a) All the molecules of a substance are identical.

(b) The inter-molecular spacing is least in solids more in liquids and still more in gases.

(c) The molecular motion in liquid and gas is in zig-zag path.

(d) In a solid, the molecules vibrate on either side but they remain at their fixed positions.

(e) The inter-molecular forces are the weakest in gases.

(f) A solid exerts pressure downwards on its base.

(g) Gases are least dense.

(h) Solids are most rigid.

(a) False because different substances (e.g. air, water etc.) contain different types of molecules, so they are not identical

(b) False as intermolecular forces act only at very short distances (a few nanometers). At large distances, they become negligible.

(c) True since the particles of matter are not at rest, but they move randomly in all possible directions in a zig zag path.

(d) True as the inter-molecular forces are very weak in a gas which makes the molecules wide apart and hence, they can move freely in space.

(e) False because liquids are generally more viscous than gases because their molecules are closer together, leading to more friction between different layers.

Matter is defined as any substance that occupies space and has mass. It is made up of molecules which are very small in size (~ 10^-9 m).

Three states of matter are :

1. Solid
2. Liquid
3. Gas

A molecule is the smallest particle of a substance that is made up of one or more than one atoms of the same kind or of different kinds.

Monoatomic Molecule : Neon (Ne)

Diatomic Molecule : Hydrogen Molecule (H₂₎

The spacing between molecules of matter is called inter-molecular space.

The force of attraction between the constituent particles of matter is called intermolecular force of attraction.

Force of cohesion (Cohesive Force) : The force of attraction between the particles of same substances is called the force of cohesion or cohesive force.

Force of adhesion (Adhesive Force) : The force of attraction between the particles of two different substances is called the force of adhesion or adhesive force.

The three characteristics which decide the state of a substance as a solid, liquid or gas are :

1. Inter-molecular space
2. Force of attraction between the molecules
3. Movement of molecules

The molecules in a substance follow zig zag path when they are in motion.

(a) Solid $\xrightarrow {\textbf {Melting}}$ Liquid

(b) Liquid $\xrightarrow{\text {Boiling}}$ Gas

Experiment :

Take 100 ml of water in a measuring cylinder. Add 20 gram of salt in water gently and stir it well so as to dissolve the salt well in water. It is noticed that the level of water does not change as it can be seen from below figure. It shows that the particles of salt have occupied the spaces between the particles of water.

The following experiment can be performed to illustrate that molecules are not at rest, but they constantly move :

Take a beaker. Fill it partly with water. Add some lycopodium powder in the beaker containing water. Stir the contents of the beaker with a glass rod. Take out few drops of this suspension on a glass plate. Place the plate on the table and illuminate it with a table lamp. Observe the glass plate through a microscope. It is found that the fine particles of lycopodium powder move rapidly in a random manner and their path is zig zag as shown in figure below.

The reason is that the particles of water are in random motion which collide with the suspended fine particles of lycopodium powder and make them move in a zig zag path.

Five general properties of solids, liquids and gases are given below:

Features of molecular model for a solid are :

1. There is a strong force of attraction (strong inter-molecular force) between the molecules of a solid.
2. The molecules in a solid are closely packed with negligible intermolecular space, so solids cannot be compressed much. Their orderly arrangement gives them a definite shape.
3. The molecules vibrate on either side of their mean positions but they do not leave their positions. Therefore solids have a definite size.
4. The molecules of a solid are packed tightly and so they generally have a high density.

Thus, this model concludes that solids are rigid, they have a definite size, definite shape and a definite volume.

Features of molecular model for a liquid are :

1. The molecules in a liquid are loosely packed due to weaker attractive forces than in solids. They can move over each other, within the boundary of the liquid, so a liquid has a definite volume but no definite shape.
2. The inter-molecular space in a liquid is greater than that in a solid, so they generally have low density as compared to a solid, i.e., they are more compressible.
3. The motion of molecules in a liquid is irregular and random within the boundary of the liquid.

Thus, this model concludes that liquids do not have a definite shape, but have a definite volume and can flow from a higher to a lower level. They show the property of viscosity and surface tension because of the cohesive forces.

Features of molecular model for a gas are :

1. The molecules of a gas lie much farther apart than they lie in a liquid or a solid. Thus, the density of gases is very low.
2. There is negligible force of attraction between the molecules of a gas, so they are free to move in the entire space available to them.
3. The molecules of a gas move much faster than they move in liquids, and therefore they are infact in a state of incessant random motion, moving in all possible directions at all possible speeds.
4. The molecules of a gas are far apart and there is enough space available for compression. Thus, gases can easily be compressed.
5. During motion, the molecules of a gas collide with one another and also with the wall of the vessel. In each collision, the direction of motion of the molecule changes, so the momentum changes.
6. A gas exerts pressure on the wall of its container due to the continuous collisions of its molecules with the wall.

Thus, this model concludes that a gas has neither a definite shape nor a definite volume, but it can flow and is easily compressible.

A change of state of matter refers to the physical transformation of a substance from one state (solid, liquid, or gas) to another due to a change in temperature.

(a) The process of changing of a substance from the solid state into its liquid state on absorption of heat at a particular temperature, called the melting point, is called melting in which the heat energy absorbed by the substance increases the amplitude of vibrations of the molecules of the solid and a stage is reached at the melting point when the molecules acquire sufficient energy to overcome the force of attraction between them and they become free to move. The solid thus changes into a liquid.

Solid $\xrightarrow[\text{Heat absorbed}]{\text{Melting}}$ Liquid

(b) The process of change of a substance from the liquid state to its gaseous state at a particular temperature, called the boiling point, is called boiling in which the heat energy absorbed by a substance in liquid state increases the energy of its molecules due to which they begin to move rapidly. Thus a liquid changes into a gas.

Liquid $\xrightarrow[\text{Heat absorbed}]{\text{Boiling}}$ Gas

1. Matter is made up of atoms .
2. A solid cannot be compressed .
3. The intermolecular forces are less strong in liquids.
4. Liquids are less rigid.
5. A solid has definite shape.
6. Gases are highly compressible.

(i) The wooden block did not change its shape when pressed or squeezed which shows that solids have a fixed shape and resist deformation.

(ii) Neha observed that water takes the shape of its container while its quantity remains the same. This shows that liquids have no fixed shape but a fixed volume.

(iii) The smoke spread because it has particles that move freely in all directions and mix with air due to large spaces between them.

(iv) Following inferences can be drawn about the shape of matter from these observations :

• Solids have a fixed shape and size.
• Liquids have a definite volume but not a definite shape. They take the shape of their container.
• Gases have neither a definite volume nor a definite shape. They spread to fill the available space.