Unit – I C
Terminology and Explanations
- The ability of crystalline solids to change their physical properties when measured in different directions is called anisotropy.
- Some of physical properties of such solids like electrical resistance or refractive index show different values when measured along different directions in the same crystals. This arises from different arrangement of particles in different directions.
- The ability of amorphous solids to exhibit identical physical properties when measured in different directions is called isotropy.
- This property is due to no long range order of regular pattern arrangement in them. Thus the arrangement is irregular.
- A phenomenon in which two or more crystalline substances show same crystalline structure is called isomorphism and the crystals showing isomorphism are called isomorphous.
- Examples: a) K2SO4 and K2SeO4 and b) NaCl and KCl
- A phenomenon in which when a single substance crystallises in two or more forms under different conditions of solidification is called polymorphism and the crystals showing polymorphism are called polymorphous.
- Examples: Silica forms many polymorphs such as ∝-quartz, ß-quartz, tridymite, cristobalite, coesite, and stishovite.
- When a substance exists in two or more forms then they are called allotropes. They are polymorphous.
- Example: Diamond and graphite are allotropes of carbon.
- Due to short range order of regular pattern in amorphous solids, the small part of it behaves like crystalline solid. These small parts of amorphous solid behaving like crystalline solid are called crystallites.
Sharp Melting Point of Crystalline Solids:
- In a crystal, the arrangement of constituent particles (atoms, molecules or ions) is ordered. It has long range order which means that there is a regular pattern of arrangement of particles which repeats itself periodically over the entire crystal.
- Due to regular pattern the interatomic and intermolecular forces are identical. Hence the thermal energy required to break these bonds (i.e. regular structure) is uniform throughout.
- Hence the heat and temperature required to melt crystalline solid is uniform. Hence crystalline solids have sharp melting points.
No Sharp Melting Point of Amorphous Solids:
- The arrangement of constituent particles (atoms, molecules or ions) in such a solid has only short range order. In such an arrangement, a regular and periodically repeating pattern is observed over short distances only. Such portions are scattered and in between the arrangement is disordered.
- Due to irregualr pattern the interatomic and intermolecular forces are different at different places. Hence the thermal energy required to break these bonds (i.e. regular structure) is not uniform throuhout.
- Hence the heat and temperature required to melt crystalline solid is not uniform. Hence amorphous solids do not have sharp melting points.
Ice is lighter than water:
- The structure of liquid water and solid ice is identical.
- Ice has hexagonal three-dimensional crystal structure formed due to intermolecular hydrogen bonding. It leaves about 50 % space vacant.
- On melting the hydrogen bond between the molecules break and free molecule occupy the empty space which is not utilised in solid ice. Thus the volume of formed liquid is quiet less than the volume of the ice.
- Thus the density of liquid water is more than that of solid ice. i.e. solid ice is lighter than liquid water.