Solids -The State of Matter

Definition of Solids

“Solids are those compounds which are rigid, tough, have a definite shape and definite volume”.

The atoms, ions and particles that make up a solid are closely packed. They are held together by strong cohesive forces. The constituent atoms, ions or molecules of solids can not move at random. There exists a well-ordered arrangement in solids.

Types of Solids

Solids can be classified on the basis of the regular plans of constituent atoms, ions or molecules. There are two kinds of solids in this regard.

(i) Crystalline Solids

Those solids in which atoms, ions or molecules are arranged in a definite3-dimensional pattern are called crystalline solids. This repeating regular geometrical pattern of the structure extends 3- dimensionally.


(ii) Amorphous Solids

All solids are not crystalline. The word amorphous means shapeless.

” Amorphous substances are those whose constituent atoms, ions, or molecules do not possess a regular orderly arrangement”.

The best examples are glass, plastics, rubber, glue, etc. These compounds have solid-state properties and virtually total maintenance of shape and volume. However, they do not have an ordered crystalline state.

Numerous crystalline solids can be changed into amorphous solids by melting them and then cooling the molten mass rapidly. In this way, the constituent particles do not find time to organize themselves.

A long-range regularity does not exist in amorphous solids but they can possess little areas of organized arrangements. These crystalline parts of otherwise amorphous solids are referred to as crystallites.

Amorphous solids don’t have sharp melting points that is why particles of glass soften over a temperature range and can be moulded and blown into numerous shapes. They do not possess definite heats of fusion.

Properties of Crystalline Solids
Geometrical shape

All the crystalline solids have a definite, unique geometrical shape due to definite and orderly plan of atoms, ions or particles in the three-dimensional area. For a given crystal, the interfacial angles, at which the surfaces converge, are constantly the same no matter in which shape they are grown. The faces and angles stay particular even when the material is ground to a fine powder.

Melting Points

Crystalline solids have sharp melting points and can be identified from their definite melting points.

Cleavage Planes

Whenever the crystalline solids are broken, they do so along definite planes. These planes are called the cleavage planes and they are inclined to one another at a particular angle for an offered crystalline solid. The value of this angle differs from one solid to another solid.


A few of the crystals reveal variation in physical properties relying on the direction. Such properties are called anisotropic properties and the phenomenon is referred to as anisotropy. The physical properties of crystalline solids like refractive index, coefficient of thermal expansion, electrical and thermal conductivities are in some cases anisotropic in nature for some crystals.

The variation in these properties with direction is because of truth that the orderly plan of the particles in crystalline solids is different in different instructions. For example, the electrical conductivity of graphite is greater in one direction than in another.

In fact, electrons in graphite are mobile for electrical conduction parallel to the layers. For that reason, its conductivity in this direction is far much better than perpendicular to the other directions. Likewise, cleavage itself is an anisotropic behaviour.



The repetition of faces, angles or edges when a crystal is rotated by 360 ° along its axis is called symmetry. This a crucial property of the crystal and there are different kinds of symmetry elements discovered in crystals like center of symmetry, the plane of symmetry and axis of symmetry, and so on.

Habit of a Crystal

The shape of a crystal in which it normally grows is called the habit of a crystal. Crystals are normally acquired by cooling the saturated option or by slow cooling of the liquid compound. These are formed by growing in different directions.

If the conditions for growing a crystal are kept, then the shape of the crystal always remains the same. If the conditions are altered the shape of the crystal may change. For instance, a cubic crystal of NaCl ends up being needle-like when 10% urea is present in its solution as an impurity.


Isomorphism is the phenomenon in which two different substances exist in the very same crystalline type. These different compounds are called isomorphs of each other.

A crystalline type is independent of the chemical nature of the atoms and depends just on the variety of atoms and their method of combinations.

Primarily the ratio of atoms in numerous compounds is such that isomorphism is possible. Their physical and chemical properties are rather different from each other. Anyway, isomorphic substances take shape together in all proportions in uniform mixtures. The following examples tell us the nature of the compound, their crystalline kinds, and the ratio of their atoms.

The structures of the negatively charged ions like NO3-1 and CO32-, are the same. Similarly shapes of SO42- and CrO42- are also alike. CO32-, and NO3-1are triangular planar units, while SO42and CrO42-are both tetrahedral.


Polymorphism is a phenomenon in which a compound exists in more than one crystalline form. That compound which exists in more than one crystalline form is called a polymorphic, and these forms are called polymorphs, and these forms are called polymorphs of each other.

Polymorphs have exact same chemical properties, but they differ in the physical properties. The difference in physical properties is due to the different structural arrangements of their particles.

The following substances are very important polymorphs.




The existence of an element in more than one crystalline form is called allotropy and these kinds of the element are called allotropes or allotropic kinds. Sulphur, phosphorus, carbon and tin are some major examples of elements which reveal allotropy.


Transition Temperature

It is that temperature at which two crystalline forms of the same substance can co-exist in balance with each other. At this temperature level, one crystalline type of substance changes to another. Above and below this temperature, only one type exists.

A few examples for those substances which show allotropy and have a transition temperature are listed below.

(i) Grey Tin (cubic)⇌White tin (Tetragonal)


(ii) Sulphur S8 (rhombic)⇌Sulphur S8(monoclinic)


(iii) KNO3 (orthorhombic) ⇌ KNO3(rhombohedral)