Three States Of Matter Examples In Physics

Three States Of Matter 

If two molecules are kept at a separation r = r。,  they will stay in equilibrium. If they are slightly pulled apart so that r > r。 , an attractive force will operate between them. If they are slightly pushed so that r < r。, a repulsive force will operate. Thus, if a molecule is slightly displaced from its equilibrium position, it will oscillate about its mean position. This is the situation in a solid. The molecules are close to each other, very nearly at the equilibrium separations. The amplitude of vibration is very small and the molecules remain almost fixed at their positions. This explains why a solid has a fixed shape if no external force act to deform it.

In liquids, the average separation between the molecules is somewhat large. The attractive force is weak and the molecules are more free to move inside the whole mass of the liquid. In gases, the separation is much larger and the molecular force is very weak.

(1). Solid State

In solids, the intermolecular forces are so strong that the molecules or ions remain  almost fixed at their equilibrium positions. Quite often these equilibrium positions have a very regular three-dimensional arrangement which we call crystal. The positions occupied by the molecules or the ions are called lattice points. Because of this long range ordering, the molecules or ions combine to form large rigid solids.

The crystalline solids are divided into four categories depending on the nature of nature of the bonding between the basic units.

(2). Molecular Solid 

In a molecular solid, the molecules are formed due to covalent bonds between the atoms. The bonding between the molecules depends on whether the molecules are polar or nonpolar as discussed below. If the centre of negative charge in a molecule coincides with the centre of the positive charge, the molecule is called nonpolar. Molecules of hydrogen, oxygen, chlorine, etc., are of this type. Otherwise, the molecule is called a polar molecule. Water molecule is polar. The bond between polar molecules is called a dipole-dipole bond. The bond between nonpolar molecules is called a van der Waals bond. Molecular solids are usually soft and have low melting point. They are poor conductors of electricity.

(a). Ionic Solid

In an ionic solid, the lattice points are occupied by positive and negative ions. The electrostatic attraction between these ions binds the solid. These attraction force are quite strong so that the material is usually hard and has fairly high melting point. they are poor conductors of electricity.

(b).Covalent Solid 

In a covalent solid, atoms are arranged in the crystalline form. The neighbouring atoms are bound by shared electrons. Such covalent bonds extend in space so as to form a large solid structure. Diamond, silicon, etc., are examples of covalent solids. Each carbon atom is bonded to four neighbouring carbon atoms in a diamond structure. They are quite hard, have high melting point and are poor conductors of electricity.

(c). Metallic Solid

In a metallic solid, positive ions are situated at the lattice points. These ions are formed by detaching one or more electrons from the constituent atoms. These electrons are highly mobile and move throughout the solid just like a gas. They are very good conductors of electricity.

(3). Amorphous or Glassy State

There are several solids which do not exhibit a long range ordering. However, they still show a local ordering so that some molecules (say 4-5) are bonded together to form a structure. Such independent units are randomly arranged to form the extended solid. In this respect the amorphous solid is similar to a liquid which also lacks any long range ordering. However, the intermolecular force in amorphous solids are much stronger than those in liquids. This prevents the amorphous solids to flow like a fluid. A typical example is glass made of silicon and oxygen together with some other elements like calcium and sodium. The structure contains strong Si-O-Si bonds, but the structure does not extend too far in space.

The amorphous solids do not have a well-defined melting point. Different bonds have different strengths and as the material is heated the weaker bonds break earlier starting the melting process. The stronger bonds break at higher temperatures to complete the melting process.