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NCERT Class 12 Chemistry Textbook Chapter 1 With Answer PDF Free Download
Chapter 1: The Solid State
We are mostly surrounded by solids and we use them more often than liquids and gases. For different applications, we need solids with widely different properties.
These properties depend upon the nature of constituent particles and the binding forces operating between them.
Therefore, the study of the structure of solids is important.
The correlation between structure and properties helps in discovering new solid materials with desired properties like high-temperature superconductors, magnetic materials, biodegradable polymers for packaging, compliant solids for surgical implants, etc.
From our earlier studies, we know that liquids and gases are called fluids because of their ability to flow. The fluidity in both of these states is due to the fact that the molecules are free to move about.
On the contrary, the constituent particles in solids have fixed positions and can only oscillate about their mean positions.
This explains the rigidity in solids. In crystalline solids, the constituent particles are arranged in regular patterns.
In this Unit, we shall discuss different possible arrangements of particles resulting in several types of structures.
The correlation between the nature of interactions within the constituent particles and several properties of solids will also be explored.
How these properties get modified due to the structural imperfections or by the presence of impurities in minute amounts would also be discussed.
In Class XI you have learned that matter can exist in three states namely, solid, liquid, and gas.
Under a given set of conditions of temperature and pressure, which of these would be the most stable state of a given substance depends upon the net effect of two opposing factors.
Intermolecular forces tend to keep the molecules (or atoms or ions) closer, whereas thermal energy tends to keep them apart by making them move faster.
At sufficiently low temperature, the thermal energy is low and intermolecular forces bring them so close that they cling to one another and occupy fixed positions.
These can still oscillate about their mean positions and the substance exists in a solid-state. The following are the characteristic properties of the solid state:
(i) They have definite mass, volume, and shape.
(ii) Intermolecular distances are short.
(iii) Intermolecular forces are strong.
(iv) Their constituent particles (atoms, molecules, or ions) have fixed positions and can only oscillate about their mean positions.
(v) They are incompressible and rigid.
Solids can be classified as crystalline or amorphous on the basis of the nature of order present in the arrangement of their constituent particles.
A crystalline solid usually consists of a large number of small crystals, each of them having a definite characteristic geometrical shape.
Molecules are the constituent particles of molecular solids. These are further subdivided into the following categories:
(i) Nonpolar Molecular Solids: They comprise either atoms, for example, argon and helium, or the molecules formed by nonpolar covalent bonds for example H2, Cl2 and I2.
In these solids, the atoms or molecules are held by weak dispersion forces or London forces which you have learned in Class XI.
These solids are soft and non-conductors of electricity. They have low melting points and are usually in a liquid or gaseous state at room temperature and pressure.
(ii) Polar Molecular Solids: The molecules of substances like HCl, SO2, etc. are formed by polar covalent bonds.
The molecules in such solids are held together by relatively stronger dipole-dipole interactions. These solids are soft and non-conductors of electricity.
Their melting points are higher than those of nonpolar molecular solids yet most of these are gases or liquids under room temperature and pressure. Solid SO2 and solid NH3 are some examples of such solids.
(iii) Hydrogen-Bonded Molecular Solids: The molecules of such solids contain polar covalent bonds between H and F, O, or N atoms.
Strong hydrogen bonding binds molecules of such solids as H2O (ice). They are non-conductors of electricity.
Generally, they are volatile liquids or soft solids under room temperature and pressure.
Metalic Solid
Metals are orderly collections of positive ions surrounded by and held together by a sea of free electrons. These electrons are mobile and are evenly spread out throughout the crystal.
Each metal atom contributes one or more electrons towards this sea of mobile electrons. These free and mobile electrons are responsible for the high electrical and thermal conductivity of metals.
When an electric field is applied, these electrons flow through the network of positive ions. Similarly, when heat is supplied to one portion of the metal, the thermal energy is uniformly spread throughout by free electrons.
Another important characteristic of metals is their luster and color in certain cases. This is also due to the presence of free electrons in them. Metals are highly malleable and ductile.
| Author | NCERT |
| Language | English |
| No. of Pages | 34 |
| PDF Size | 17.9 MB |
| Category | Chemistry |
| Source/Credits | ncert.nic.in |
NCERT Solutions Class 12 Chemistry Chapter 1 The Solid State
Q1. Define the term ‘amorphous’. Give a few examples of amorphous solids.
Ans:
Amorphous solids are solids without a regular/definitive arrangement of its constituent particles (ions, atoms or molecules) and they possess something called the short-range order i.e., a regular and periodically repeating arrangement is seen only over short distances, e.g., rubber, glass.
Q2. What makes a glass different from a solid such as quartz? Under what conditions could quartz be converted into glass?
Ans:
The arrangement of the constituent particles differentiates glass from quartz. The constituent particles in glass have a short-range order, but the constituent particles of quartz possess long-range orders.
Quartz is converted into glass by heating it and then rapidly cooling it.
Q3. Classify each of the following solids as ionic, metallic, molecular, network (covalent), or amorphous.
(a) Tetra phosphorus decaoxide (P4O10)
(b) Ammonium phosphate (NH4)3PO4
(c) SiC
(d) I2
(e) P4
(f) Plastic
(g) Graphite
(h) Brass
(i) Rb
(j) LiBr
(k) Si
Ans:
Metallic : ( h ) Brass, ( i ) Rb
Molecular : (a) Tetra phosphorus decaoxide (P4O10), (d) I2, (e) P4.
Ionic : ( b ) Ammonium phosphate (NH4)3PO4, ( j ) LiBr
Amorphous : ( f ) Plastic
Covalent : ( c ) SiC, ( g ) Graphite, ( k ) Si
Q4. (i) What is meant by the term ‘coordination number’?
(ii) What is the coordination number of atoms:
(a) in a cubic close-packed structure?
(b) in a body-centered cubic structure?
Ans:
(i) Coordination number is the number of nearest neighbours of a particle.
(ii)
(a) coordination number =12
(b) coordination number = 8
Q5. How can you determine the atomic mass of an unknown metal if you know its density and the dimension of its unit cell? Explain.
Ans:
Given,
We know the dimension and density of its unit cell.
Let,
The edge length of a unit cell = a
The volume of the cell = a3
Density = d
Atomic mass = M
Mass of unit cell = No. of atoms in unit cell x Mass of each atom = Z × m
Mass of an atom present in the unit cell, m = M/ Na
where Na is the Avogadro’s number.
We know,
d = Mass of unit cell / Volume of unit cell = Zm/a3 = Z.M / a3Na
Therefore, Atomic mass, M =( da3 Na) / Z
NCERT Class 12 Chemistry Textbook Chapter 1 With Answer PDF Free Download