Matter exists in three states solid, liquid and gas. The main difference between these three states is the distance between their constituent particles. The constituent particles can be atoms, molecules or ions. In the gaseous state the constituent particles are present at a greater distance from each other than in the liquid state at a smaller distance. In the solid state the constituent particles are at the shortest distance. The constituent particles have strong bonds and have a dense structure. Particles do not move due to strong bonds. Matter in the solid state has definite shape and volume.

In this chapter, we will study the characteristics of solids, the difference between different types of solids, the non-consolidated structure of crystals, the relationship between the density of solids and the properties of unit cells, the electrical and magnetic properties of solids, etc. The different structures of solids determine their properties. Due to the structure, new solid materials with some desirable properties such as high thermal superconductors, magnetic materials, biodegradable polymers for packing (in place of polyethylene), bio-flexible for surgical implantation (solids easily digestible in the body, etc.) should be discovered. Which can play an important role in the future development of science.

General characteristics of solid state

• Why is the solid state of a substance more stable than the liquid and gas state?
• Two opposing forces act in the stability of every substance.
• Intermolecular forces try to bring molecules (atoms or ions) closer to each other than to the nearest distance.
• Thermal energy tries to keep them fast moving. Thermal energy is low at low temperature. As a result intermolecular forces bring the constituent particles to the nearest distance and fix their position.
• In this way matter comes to solid state. In the solid state also the constituent particle can oscillate with respect to its mean position.
• The general characteristics of solids are as follows:
• Solids have definite mass, volume, shape and shape.
• Solids are hard and incompressible. Intermolecular forces are strong.
• The distances between the constituent particles (molecules, ions or atoms) are small.
• In solids the constituent particles have a fixed position, but they can oscillate with respect to their mean position.
• The density of a solid is greater than that of its liquid and gas state.

---

Solids can be mainly divided into two parts.

1. Crystalline Solids
2. Non-crystalline or Amorphous solids

Crystalline Solids

• There is a definite and regular arrangement of constituent particles (molecules, atoms or ions) in non-crystalline solids.
• There is a range, that is, there is a definite pattern of arrangement of particles and this pattern is repeated at equal intervals throughout the crystal. For example, NaCl, KCl, Na2SO4, K2CO3, Fe, Au, Cu quartz etc.
• Crystalline solids have fixed melting points and have discrete cooling curves.
• Crystalline solids are called true solids.
• Crystalline solids are generally a group of very large number of small crystals.
• Crystalline solids are amorphous solids in nature

Non-crystalline or Amorphous solids

• In amorphous solids, the constituent particles do not have any definite arrangement and have no definite geometry and shape.
• The arrangement of the constituent particles in these solids is short range.
• These solids have an irregular pattern of arrangement of the constituent particles whose repetition is found only over short distances. For example, the melting points of amorphous solids like glass, plastic rubber resin etc. are not fixed.
• Their cooling curves are continuous.
• The structure of amorphous solids is similar to that of liquids, they have the nature of flowing (flow) like liquids, but the flow is very slow. That’s why they are called super cooled liquids or pseudo solids.
• Amorphous solids are isotropic in nature.
• Some amorphous solids are melted and cooled slowly and they turn into crystalline solids.

Uses of amorphous solids

• Glasses are amorphous solids. They are used in making household and laboratory equipment, decorative items etc.
• Both natural and synthetic rubber are amorphous solids. They are used in making tyres, tubes, shoes, toys etc.
• They are used in photovoltaic cells to convert sunlight into electricity by amorphous silicon.
• Synthetic polymers are amorphous and are widely used in various fields.

Difference Between Crystalline and Non-Crystalline

The main difference found in both types of solids and the reason for the difference is detailed as follows:

Melting Point

Crystalline solids have definite and sharp melting points. That is, they change from solid to liquid state at a certain temperature. Similarly, if a liquid is cooled, it turns into a solid at a certain temperature. The melting points of amorphous solids are not fixed. When these solids are heated, they first become soft and then they start flowing. The above process is observed when glass or plastic is heated.

Cooling Curve

Discrete Cooling Curve

Crystalline solids have a fixed melting point. Their temperature remains constant until they are completely liquefied. On cooling the molten solid, it starts to solidify at a certain temperature and until it is completely solidified, its temperature remains constant, after which its temperature starts decreasing. Hence their cooling curve is discontinuous.

Example- Sugar is a crystalline solid. Its melting point is 458K, so it is in the solid state at a temperature less than 458K and starts melting at 458K and when it is not completely melted, its temperature remains 458K. Above 458 temperature it turns into a completely liquid state.

Continuous Cooling Curve

Amorphous solids first soften on increasing temperature and melt and turn into liquid with increase in temperature. On cooling molten amorphous solids, their temperature decreases continuously and there is no such point at which the temperature is constant. Hence their cooling curves are continuous. For example, if glass or plastic is heated. So they first become soft, and on heating more, they start flowing after converting into liquid.

Isotropy and Anisotropy

Anisotropy

In addition to the cubic crystal structure, all other crystalline solids have different values ​​of physical properties such as conductivity, refractive index, hardness, tensile strength, etc. in different directions. This property is called anomalous. Therefore, crystalline solids are unidirectional.

If in a crystalline solid, the beam of light is passed in the direction AB, then rows of identical constituent particles are present on either side of the light path, while in the CD direction both types of constituent particles are present in alternate order. Therefore, due to different effects of constituent particles in AB direction and CD direction, different values ​​of refractive index of light are obtained. Similarly, the values ​​of other physical properties are also found to be different, which is called anomalous.

Isotropy

Due to the absence of a definite arrangement of constituent particles in amorphous solids, the values ​​of their physical properties such as hardness, refractive index, conductivity, tensile strength, mechanical strength, etc. are the same in each direction. This property is called isomorphism. Therefore, amorphous solids are isotropic.

The reason for isomorphism: The reason for isomorphism is that the constituent particles in amorphous solids do not have any definite arrangement, but they remain disordered. Therefore, in finding the refractive index, the rays of light are affected by both types of particles from each direction. Therefore the values ​​of their physical properties remain the same in each direction.

Cleavage

There is a clear cleavage in crystalline solids when cut with a sharp-edged instrument such as a knife. But in amorphous solids the cleavage is irregular. The newly generated surfaces are flat and smooth in two fragments formed by the cleavage of a crystalline solid. In the fission of amorphous solids, the newly generated surfaces are not flat and smooth.

Amorphous solids can be converted into crystalline solids at any temperature on heating. For example, glass objects found in ancient civilizations or the glass in the windows of old buildings appear milky. This is due to some crystallization in the glass in them.