Q.(a) Describe the nature of glass.

(a) Glass is a non-crystalline, isotropic 'super-cooled liquid' formed by rapid cooling of a molten silicate mixture, lacking the long-range order and sharp melting point of a true crystalline solid. (b) Physical and chemical adsorption differ mainly in the strength/nature of the surface bonding, reversibility, layer thickness, specificity, temperature dependence, and heat evolved.

(a) Nature of glass

Glass is an amorphous solid, not a true crystalline solid. It is best described as a super-cooled liquid of very high viscosity — a fused mixture of silicates (and sometimes borates/phosphates) that has been cooled rapidly enough that its constituent particles could not arrange themselves into the regular, long-range repeating lattice of a crystal; instead they are frozen in a random, liquid-like (short-range-ordered only) arrangement.

Characteristic features:

• No sharp melting point — glass softens gradually over a range of temperature (unlike a crystalline solid, which melts sharply at a fixed temperature) because there is no ordered lattice to break down all at once.
• Isotropic — its physical properties (refractive index, electrical conductivity, mechanical strength, etc.) are the same in all directions, since there is no directional long-range order (crystalline solids are anisotropic).
• Metastable — over extremely long periods, or on reheating, glass can slowly crystallise ('devitrify').
• Composition — ordinary soda-lime glass is manufactured by fusing sand ($SiO_2$), sodium carbonate ($Na_2CO_3$) and limestone ($CaCO_3$) and can be represented approximately as $Na_2O\cdot CaO\cdot 6SiO_2$; it is essentially a mixture/solid solution of metal silicates in excess silica.

(b) Physical adsorption vs chemical adsorption