Introduction to Solid State

Solid state

Solids are characterized by their definite volume and shape, rigidity and incompressibility. In general, the substance in the solid state is denser than they are in the liquid and gaseous liquid. However, water is one of the expectations that its solid state is less dense than the liquid state. The ability of solids to retain their shape due to sufficiently strong forces of attraction within them. In the case of liquids, the force of gravity is sufficient to cause a liquid to acquire the shape of its container. Gases have such weak intermolecular forces that they disperse uniformly into a container of any volume. Hence, the constituent particles of a solid are closely packed. Unlike to liquids and gases, particles of solid cannot move freely. However, they can vibrate about their positions. On heating these vibrations increase and a stage will come then the vibrations are so much that the particles can overcome the attractive forces and start becoming free and the solids melt to form the liquid.

Classification of solids

Solids are classified on the basis of their properties. On the basis of their physical structures, they are classified as crystalline solids and amorphous solids.

1. Crystalline solids: The solids whose constituents (atoms, ions or molecules) are arranged in orderly arrangements in a three-dimensional pattern are called crystalline solids. For example, NaCl, CuSO₄.5H₂O, oxalic acid crystals, copper wire etc. Glass is not a crystalline sloid. In fact, glass is a super-cooled liquid. It has no sharp melting point.

2. Amorphous Solids: The substances whose constituents (atoms, molecules, or ions) are not orderly arranged in the three-dimensional pattern are called amorphous solids. For example, CaCO₃ powder, soda ash etc.

Differences between Crystalline and Amorphous solids

Classification of Crystalline Solid on the Basis of Dominant Types of Bond

Crystalline solids may be classified into four types depending upon the nature of bonds present in them.

1. Molecular crystals: In molecular crystals, the molecules are held by weak Vander Waals' forces in the form of solid. For example dry ice, ice, iodine etc.

They possess the following characteristics:

a. They are generally soft in nature.

b.They have low melting points.

c. They are bad conductors of electricity.

d. They have the low heat of vaporization.

e. They are volatile in nature.

2. Covalent crystals: In covalent crystals, atoms or molecules of crystals are linked to one another by covalent bonds. For example diamond, silica, carborundum.

Their important characteristics are:

a. They are hard.

b. They possess high melting points.

c. They are generally poor conductors (exception: graphite)

d. They possess higher density than the molecular crystals.

e. They have the high heat of fusion.

3. Ionic crystals: The crystals in which constituents (ions) are orderly arranged and are linked with each other by the ionic forces are called ionic crystals. For example NaCl, KCl, NaNO₃, LiF etc.

They possess the following characteristics:

a. They are very hard and brittle.

b. They have high boiling point and melting point.

c. They have high density.

d. They behave as poor conductors at solid state but conduct electricity at molten state.

e. They have the high heat of vaporization.

4. Metallic solids: In metallic solids, the constituents (positively charged metal ions) are surrounded by a sea of mobile electrons. For example nickel, zinc, copper etc. There is a force of attraction between the positively charged ions and the electrons. They show the following properties:

a. They may be hard as well as soft.

b. They are good conductors of heat and electricity.

c. They have the metallic lustre.

d. They are malleable and ductile.

e. They have the moderate heat of fusion.

Crystal lattice or space lattice

A space lattice is an array of structural units of a crystal in three-dimensional spaces. When a crystal is viewed in molecular level, the structural units simply appear as points and space lattice shows how these points are arranged in a crystal. So, space lattice is simply the regular periodic arrangement of points. If the three-dimensional distribution of particles in a crystal is represented diagrammatically, in which each particle is represented as a point, the arrangement is called space lattice or crystal lattice.

Unit cell

A crystalline material is composed of an array of identical units. The smallest unit which possesses all of the properties of the crystal is the unit cell. From a unit cell, the entire crystal may be built by allowing other similar unit cells to rest on the different faces of the first unit cell.A simple arrangement of points in a three-dimensional space lattice of a crystal along with the unit cell is shown below:

It must be remembered here that only points and not the lines which constitute the space lattice. Lines are shown in order to understand the shape or geometry of the lattice.

In each crystal lattice, t is possible to identify the simplest, three-dimensional set of points which when repeated along the directions of its edges, generates the entire crystal lattice. These three-dimensional simplest sets of points are called the unit cell of the crystal. In the above figure, the unit cell is shown with the help of thick lines. Generation of the crystal lattice from its unit cell is similar to the construction of a building from a large number of identical bricks. A unit cell is characterized by its dimensions along the three edges-a, b and c and the angles between the edges-α,β andγ i.e.;α, β, and γ are the angles between edges b and c; c and a; and a and b respectively.

Representation of a unit cell of simple cube

Seven Types of Crystal System

A unit cell in a crystal lattice having the lattice points only at its corner is known as a primitive or simple unit cell. There are seven types of such primitive unit cells which are also known as crystal systems. There are seven types of crystal system. This crystal system, their dimensional characteristics, and example for each is given below in the table.

Here, a, b and c represent the lengths of the intersecting edges at three different co-ordinates andα,β, andγ represent the angle between the edges, called interfacial angles. (The angles are measured by means of goniometer).

Table for seven crystal system

In the case of the cubic system, which is the simplest crystal system, there are three types of lattices depending upon the unit cells. These are simple cubic unit cells (lattice points are at the corners of the cube), body-centered cubic unit cells (bcc, lattice points are at the center of the body and at the corners of the cube) and face-centered cubic unit cells (fcc, lattice points are at the center of each face of the cube and at the corners of the cube).

Water of Crystallization

In hydrated crystalline solids, certain numbers of water molecules are associated as the part of the crystal. Such numbers of water molecules are called water of crystallization. Hence, the water of crystallization may be defined as the certain number of water molecules which are associated as the part of the constituent per molecule of solids. The substance having water of crystallization is called hydrated substance. For example blue vitriol (CuSO₄.5H₂O), green vitriol (FeSO₄.7H₂O). The substances which have lost all of the water crystallization and do not have the water of crystallization are called anhydrous substances. For example, CuSO₄ is the anhydrous copper sulphate.

Determination of Water of Crystallization

The water of crystallization of crystalline substances can be determined by the gravimetric method. A small quality of a crystalline substance (e.g., NaCl) is taken in a weighed crucible. The substance in the crucible is weighed. The crucible is heated gradually for about an hour. Then the crucible is cooled and weighed. The process of heating , cooling and weighing are repeated till a constant weight is obtained. Then the following calculations are made to find the water of crystallization molecules in the substance.

let,

Weight of empty crucible = W₁gm

Weight of crucible and hydrated crystal = W₂ gm

Weight of hydrated crystal = (W₂ - W₁) gm

Weight of crucible after heating = W₃ gm

Weight of anhydrous substance = (w₃ - w₁) gm

Weight of water = weight of hydrated crystal - weight of anhydrous substance

= (W₂ - W₁)gm - (W₃ - W₁)gm = (W₂ -W₃) gm

(W₃- W₁) gm of anhydrous substance is associated with (W₂ - W₃) gm of H₂O.

∴ M gm of anhydrous substance is associated with W₂ - W₃⁄ W₃ - W₁* M gm of H₂O.

Hence, number of moles of water of crystallization = W₂ - W₃⁄ 18 (W₃ - W₁) * M

(∴ no. of moles = Weight in gm ⁄ Molar mass)

Where, M is the molar mass of the anhydrous substance and 18 is the molar mass of water.

Polymorphism and Isomorphism

The solid substances which exist in more than one form are called polymorphous substances and this phenomenon of occurring a solid substance in different physical forms is called polymorphism. Carbon, silica, calcium carbonate etc. show the phenomenon.

When two or more different solids crystallize in the same crystalline form, they are said to be isomorphous with each other and the phenomenon is called isomorphism. For e.g.: Epsom salt (MgSO₄.7H₂O) and white vitriol (ZnSO₄.7H₂O) are isomorphous with each other.

Efflorescence, Hygroscopy, and Deliquescence

Due to the pressure of water vapour the crystal, which is greater than the pressure of water vapour in the atmosphere, the crystal constantly loses its water of crystallization and itself crumbles to powdery form. Such substances are called efflorescent substances and the phenomenon is called efflorescence. Washing soda (Na₂CO₃.10H₂O) is the example of efflorescent substances.

Any substance which absorbs moisture from the air when exposed to it is called a hygroscopic substance and this phenomenon is called hygroscopy. Anhydrous copper sulphate (CuSO₄) is the example of hygroscopic substance.

Those hygroscopic solids which absorb moisture from the air up to such an extent that they become wet and turn into liquids are called deliquescent substances and this phenomenon is called deliquescence. In a deliquescent substance, the vapour pressure of water in the crystals is less than the outward pressure of the water vapour present in the atmosphere. Calcium chloride, caustic soda, zinc chloride are the example of deliquescent substances.

Reference:

Gewali, Mohan Bikram and Rishi Tiwari. principles of chemistry. second edition. buddha academic publishers and distributers pvt. ltd,