Matter is defined as anything that occupies space and has mass. It exists in three forms around us – solid, liquid, and gas. There are general characteristics of the solid state of matter that distinguish it from other states. Matter exists in solid form under high pressure and low temperature conditions. Matter in a solid state has a higher density compared to that in other states, barring a few exceptions. Solids can also be of two types – crystalline and amorphous. Both have different properties, arising out of differences in structures. Crystalline solids can further be classified into four types based on the nature of intermolecular forces.

Solid State of Matter

Matter can be changed from one state to another by modifying external conditions like pressure and temperature. Let’s take the example of water. At low temperatures (below 0 degrees celsius), it exists in solid form, which we call ice. At room temperature, it exists in liquid form. At high temperatures (above 100 degrees celsius), it exists in vapour form (the gaseous form of water). Similarly, every compound can change its state when the surrounding temperature or pressure is changed. 

The matter’s stability in a given state depends on the following two factors:

• Intermolecular Forces: The forces of interaction between two molecules in a given state of matter are known as the intermolecular forces of attraction. The higher the intermolecular forces of attraction, the closer in distance the molecules would be, i.e., they would be more densely packed. One of the general characteristics of the solid state of matter is high intermolecular forces of attraction, due to which the particles are close together.

• Thermal Energy: It is a form of energy that the particles possess by virtue of their kinetic movement. It depends on the surrounding temperature. When temperature increases, more heat is transferred from the environment to molecules. Thus, they vibrate more vigorously at their positions. The higher is the thermal energy, the farther away the particles would be. Hence, its effect is somewhat opposite to that of intermolecular forces.

General Characteristics of Solid State

The following general characteristics of solid state apply to matter:

1. Definite volume, mass, and shape.
2. High rigidity.
3. Negligible compressibility.
4. High density (This is not true for water, wherein the density of ice is lower than liquid water).
5. On application of force, they can break into several pieces, but their shape does not change.
6. Short intermolecular distance.
7. High intermolecular forces.

Types of solid state

Solid-state compounds can be categorised into two types based on the arrangement of constituent particles:

1. Crystalline solids
2. Amorphous solids

Let’s discuss the two in detail.

• Crystalline Solids or True Solids

When the particles of a solid are arranged in a definite pattern such that they have a repeating pattern, they are said to form crystalline solids. A repeating unit (known as a unit cell) repeats in all directions to give a solid its 3-D structure. The particles are assembled in a highly-ordered manner, i.e., they exhibit long-range order. They have highly defined faces and edges. Crystalline solids are anisotropic and have a sharp melting point. 

• Amorphous Solids or Supercooled Liquids

The term ‘amorphous’ means irregular or shapeless. Amorphous solids, by their very name, are the ones that do not have a regular arrangement of constituent particles. They lack geometry, and upon breaking, they form fragments with no defined surfaces. The surfaces are curved and irregular.

They have a short-range order. The intermolecular forces between constituent particles are not identical, i.e., the distance between two particles may vary. To get an amorphous solid, we solidify the liquid very rapidly. Therefore, these solids are also called supercooled liquids.

Amorphous solids are isotropic in nature, i.e., the value of any physical property does not change as we change the direction due to the unordered arrangement of constituent particles. They melt over a range of temperatures.

Classification of crystalline solids

The classification of crystalline solids can be done into four types based on the nature of chemical bonding:

• Ionic crystalline solids: The constituent particles are ions. They are hard, brittle, and conduct electricity in a molten and aqueous state. Examples: NaCl, CaCl2, etc.
• Molecular solids: They are categorised into polar, non-polar, and H-bonded molecular solids.
  1. Polar solids constituents have polar bonds and strong dipole-dipole interactions. They are mostly gases or liquids at room temperature. Examples: HCl and ammonia. 
  2. Non-polar solids are insulators with low melting points in which constituents have non-polar covalent bonds. Examples are Cl2 and I2. 
  3. H-bonded molecular solids have polar hydrogen bonds with electronegative elements. Example: ice.
• Metallic solids: Cations of metals are held together by kernels, which is the sea of valence electrons. 
• Covalent solids: Also called network solids, they have non-metallic covalent bonds. Carbon forms such solids. Example: diamond (a hard, poor conductor of electricity and heat) and graphite (a soft and good electricity and heat conductor).

Conclusion

The intermolecular forces and thermal energy determine which state of matter would the compound be in. In a solid state, the matter has high intermolecular forces and low thermal energy. The general characteristics of the solid state of matter are defined by the fact that particles of the solid can only vibrate about their mean position to a small extent. They have high density and a fixed shape. Amorphous and crystalline solids are two types of solids based on the difference in particle geometry. Crystalline solids can further be divided into four categories based on the nature of intermolecular bonding.