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In the solid state the molecules are closely packed together and are not free to move about randomly; they vibrate about their fixed position (known as the mean position)
The ability of a solid to maintain its definite shape and fixed volume is the result of the strong intermolecular forces of attraction between the molecules.
These are known as cohesive forces.
Solids are classified into two groups: crystalline and non-crystalline (amorphous)
The difference between the two groups is the result of the way in which the molecules are arranged.
This molecular arrangement is called the lattice structure of the molecules
Crystalline solids
These are solids whose lattice structure has a definite regular pattern.
E.g. sodium chloride (rock salt)
Amorphous solids
The lattice structure of an amorphous solid is an irregular, disordered pattern. E.g.: charcoal and sulphur.
Molecules are arranged differently in different solids and have varying magnitudes of intermolecular forces of attraction.
These factors are responsible for some of the mechanical properties of solids (especially metals)
Physical Properties of Solids
Elasticity: It is the ability of a material to return to its original state or its original shape after it has been stretched or compressed by an external force.
Plasticity: It is the property of a material by which some permanent deformation remains even after the force producing it has been removed.
Ductility: it is the property of a material which governs whether it can be stretched and drawn into wires of small cross-section where there will be considerable deformation without fracture.
Brittleness: This is the property of a material which is fragile and breaks suddenly when a force is applied to it.
Malleability: It is the property which allows a material to be rolled into thin sheets without breaking. E.g. Iron, copper and aluminium can be rolled into thin sheets
Change of solid state in Solids:
When a solid is heated, the vibrations of molecules and their kinetic energy increase.
If the temperature goes to a certain level, the intermolecular forces are broken apart and molecules are released from each other.
This results in the material changing its state from a solid to a liquid, the process known as melting or fusion.
The opposite phenomenon is called solidification or freezing - change of liquid into solid.
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