solid

Question 1

Solids

Answer 1

A state of matter which is rigid and hard having a definite shape and definite volume is called solid.

Question 2

Properties of Solids:

Answer 2

➢ Solids possess only vibrational motion and hence, vibrational kinetic energy.
➢ Solids have definite shape and definite volume because particles are firmly held at their positions due to strong attractive forces between them.
➢ Solids are not compressible and they cannot diffuse into each other because particles are very close to each other and are tightly packed.

Question 3

Types of Solids:

Answer 3

Solids are classified into two types on the basis of arrangements of constituent atoms, ions or molecules.
i) Crystalline solids
ii) Amorphous solids

Question 4

(i) Crystalline Solids:

Answer 4

The solids in which constituent particles (atoms, ions or molecules) are arranged in a definite three-dimensional pattern are called crystalline solids.
This recurring (repetitive) three dimensional pattern of structure extends throughout the crystal.
Example: NaNO3, KNO3.

Question 5

(ii) Amorphous Solids:

Answer 5

The solids whose constituent particles (atoms, ions or molecules) do not possess a regular orderly arrangement are called amorphous solids.
They are also called pseudosolids.
Example: Glass, Plastics, Rubber, Glue etc.

Question 6

Properties of amorphous solids:

Answer 6

Amorphous solids do not have sharp melting point.
Example: Glass softens over a temperature range and can be moulded and blown into various shapes.
They do not possess definite heats of fusion.
They may have hardness and elasticity.
They are isotropic in nature.
They do not have fixed geometrical shape.

Question 7

Conversion of crystalline compounds into amorphous:

Answer 7

Many crystalline solids can be converted into amorphous solids by melting them and then cooling the molten mass rapidly.
This is also called super-cooling. In this way, the constituent particles do not find time to arrange themselves.
Amorphous solids can possess small regions of orderly arrangements. These crystalline parts of otherwise amorphous solids are called crystallites.
Properties of Crystalline Solids:

Question 8

1. Geometrical Shape:

Answer 8

All the crystalline solids have a definite, distinctive geometrical shape due to definite and orderly arrangement of atoms, ions or molecules in three-dimensional space.
The interfacial angles (at which faces intersect each other) are always same, no matter in which shape they are grown.
The interfacial angles remain same even when material is ground to fine powder.

Question 9

2. Melting Points:

Answer 9

Crystalline solids have sharp melting points by which they can be identified.

Question 10

Cleavage Planes:

Answer 10

The direction through which a crystal can be broken by applying pressure is called cleavage plane.
The process of breaking down a bigger crystal structures into smaller similar ones by the application of external pressure is called cleavage.
Cleavage is an anisotropic property.

Question 11

4. Anisotropy:

Answer 11

A property whose intensity depends upon the direction of crystal is called Anisotropy.
Anisotropic properties: Different anisotropic properties are
* Refractive index
* Co-efficient of thermal expansion
* Electrical conductivities
* Thermal conductivities
Reason:
In crystalline solids, the orderly arrangement of particles is different in different directions. This is the base of variation of properties with direction.
Example:
* The electrical conductivity in graphite is far better parallel to the layers than perpendicular to layers due to the presence of mobile electrons in this direction.
* Reflection in diamond.
* Cleavage of crystalline solids.

Question 12

5. Symmetry:

Answer 12

The repetition of faces, angles or edges when a crystal is rotated by 360o along its axis is called symmetry”.
Symmetry elements: There are various types of symmetry elements like:
* Center of symmetry
* Plane of symmetry
* Axis of symmetry

Question 13

6. Habit Of A Crystal:

Answer 13

The shape of a crystal in which it grows under identical conditions of temperature, pressure and concentration is called habit of a crystal.
Example:
The habit of crystal of NaCl in pure state is to make cubic crystal.
Change in Habit: When some impurities are present or under different conditions of temperature and pressure, a crystalline solid does not generate the same crystals which is called change in habit of a crystal.
Example:
NaCl generates needle-like crystals when 10% urea is present in its solution as an impurity.

Question 14

7. Isomorphism:

Answer 14

The existence of two or more substances in the same crystalline from is called isomorphism.
The different substances are called isomorphs.
Example:
i) NaNO3 and KNO3 Rhombohedal
ii) K2SO4 and K2 Cr2O4 Orthorhombic
Isomorphs have different physical and chemical properties.
A crystalline form is independent of chemical nature of atoms.
A crystalline form depends upon
* No. of atoms (atomic ratio)
* Way of combination of atoms (geometry)
Reason:
➢ If number of atoms and their way of combination is same in different substances then they exhibit isomorphism.
Example:
➢ K2SO4 and K2CrO4 show orthorhombic crystals. Because both have
i) Same atomic ratio (2 : 1: 4)
ii) Same geometry of anion (tetrahedral)

Question 15

8. Polymorphism:

Answer 15

The existence of a substance in more than one crystalline form under different conditions of temperature and pressure is called polymorphism.
Polymorphs: These crystalline forms are called polymorphs.
* Polymorphs have same chemical properties due to having same chemical composition.
* Polymorphs have different physical properties due to different structural arrangement of particles.
(i) Examples:
(a) CaCO3 Aragonite (orthorhombic)
Calcite (Rhombohedral or trigonal)
(b) AgNO3 Rhombohedral, orthorhombic

Question 16

9. Allotropy:

Answer 16

The existence of an element in more than one crystalline forms is called Allotropy
Allotropes: These crystalline forms are called allotropes.
* Allotropes have same chemical properties due to same chemical nature of particles.
* Allotropes have different physical properties due to different structural arrangement of atoms.
Examples:
(i) Sulphur Rhombic, monoclinic
(ii) Carbon Graphite (hexagonal), Diamond (cubic)
(iii) Tin Grey tin (cubic), white tin (tetragonal)

Question 17

10. Transition Temperature:

Answer 17

➢ The temperature at which two or more crystalline forms of a substance co-exist in equilibrium with each other is called transition temperature.
➢ Above which or below which only one form can exist.
Note: Transition temperature of a substance is always less than its melting point.

Question 18

Crystal Lattice:

Answer 18

A particular three dimensional arrangement of particles in a crystal is
called lattice/space lattice or crystal lattice.
Lattice Point/Lattice Sites:
The space or position occupied by the particles (atoms, ions or molecules)
in the crystalline solid is called lattice point or lattice site”.

Question 19

Unit Cell

Answer 19

The smallest part of a crystal that describes the characteristics of the
entire crystal is called unit cell.
The smallest unit or volume of crystal, which when repeated in three
dimensions generates an entire crystal is known as unit cell.
Significance:
* It is the smallest block/geometrical figure of a crystal.
* The entire crystal can be built up by repeating it in three dimensions.
* It shows the structural properties of a crystal.
* The arrangement of atoms in a unit cell describes their arrangement in whole crystal.
* The quantitative aspects of a crystal lattice are deduced from the size and shape of its unit cell.
* There are three unit cell lengths a, b and c and three unit cell angels (α,β ,γ ) .

Question 20

Ionic Solids

Answer 20

The solids in which ions are held together by electrostatic forces / ionic bonds are called as ionic solids.
Examples: NaCl, KBr, etc.
Properties Of Ionic Solids:
(i) Physical State:
The arrangement of ions in ionic solids is three-dimensional.
The cations and anions are arranged in a well-defined geometrical pattern and due to strong
electrostatic forces.
So, they are crystalline solids at room temperature.
(ii) High melting and boiling points:
Ionic solids are very stable compounds.
Very high energy is required to separate the anions and cations from each other against the strong electrostatic forces of attraction.
So, they have high melting and boiling points and are non-volatile.
(iii) Non directional:
Ionic solids do not exist as individual neutral independent molecules.
Their cations and anions attract each other and these forces are non-directional.
(iv) Structure:
The structure of an ionic crystal depends upon the radius ratio of cations and anions.
Example:
NaCl and CsF have same geometry because the radius ratio in both the cases is same.
(v) Close packing:
The close packing of ions enables them to occupy minimum space. A crystal lattice is developed when ions arrange themselves systematically in an alternate manner.
(vi) Formula mass:
In case of ionic crystals, we always talk about formula mass of substances and not the molecular mass because they do not exist as molecules.
(vii) Conductivity:
* In solid state: They are non-conductors in solid state because strong electrostatic force hold the ions at fixed positions and ions are not free to move to conduct electricity.
* In molten state or solution form: They are conductors in molten state or solution form because in these forms, ions are free to move to conduct electricity. .
(viii) Brittleness and Hardness:
* Arrangement of ions: They are highly brittle because cations and anions are present in parallel layers such that oppositely charged ions lie over each other.
* Application of pressure: On application of external force, one row slides or passes over the other row such that like ions come in front of each other causing repulsion. Hence, ionic solids become brittle.
(ix) Isomorphism and polymorphism:
The properties of isomorphism and polymorphism are also associated with ionic crystals.
(x) High Density:
Ionic solids have mostly high density due to close packing of ions. Compounds having ionic crystals give ionic reactions in polar solvents and these are very fast reactions.
(xi) Solubility:
They are soluble in polar solvents and insoluble in non-polar solvents.

Question 21

Ionic Solids

Answer 21

The solids in which ions are held together by electrostatic forces / ionic bonds are called as ionic solids.
Examples: NaCl, KBr, etc.
Properties Of Ionic Solids:
(i) Physical State:
The arrangement of ions in ionic solids is three-dimensional.
The cations and anions are arranged in a well-defined geometrical pattern and due to strong
electrostatic forces.
So, they are crystalline solids at room temperature.
(ii) High melting and boiling points:
Ionic solids are very stable compounds.
Very high energy is required to separate the anions and cations from each other against the strong electrostatic forces of attraction.
So, they have high melting and boiling points and are non-volatile.
(iii) Non directional:
Ionic solids do not exist as individual neutral independent molecules.
Their cations and anions attract each other and these forces are non-directional.
(iv) Structure:
The structure of an ionic crystal depends upon the radius ratio of cations and anions.
Example:
NaCl and CsF have same geometry because the radius ratio in both the cases is same.
(v) Close packing:
The close packing of ions enables them to occupy minimum space. A crystal lattice is developed when ions arrange themselves systematically in an alternate manner.
(vi) Formula mass:
In case of ionic crystals, we always talk about formula mass of substances and not the molecular mass because they do not exist as molecules.
(vii) Conductivity:
* In solid state: They are non-conductors in solid state because strong electrostatic force hold the ions at fixed positions and ions are not free to move to conduct electricity.
* In molten state or solution form: They are conductors in molten state or solution form because in these forms, ions are free to move to conduct electricity. .
(viii) Brittleness and Hardness:
* Arrangement of ions: They are highly brittle because cations and anions are present in parallel layers such that oppositely charged ions lie over each other.
* Application of pressure: On application of external force, one row slides or passes over the other row such that like ions come in front of each other causing repulsion. Hence, ionic solids become brittle.
(ix) Isomorphism and polymorphism:
The properties of isomorphism and polymorphism are also associated with ionic crystals.
(x) High Density:
Ionic solids have mostly high density due to close packing of ions. Compounds having ionic crystals give ionic reactions in polar solvents and these are very fast reactions.
(xi) Solubility:
They are soluble in polar solvents and insoluble in non-polar solvents.