Q.(a) Differentiate crystalline solids and amorphous solids. OR

(a) Crystalline solids have an ordered, definite geometric structure and sharp melting point (anisotropic), while amorphous solids lack long-range order and soften gradually (isotropic); (b) pH is $-\log_{10}[H^+]$, and the common ion effect suppresses ionisation of a weak electrolyte via a common-ion strong electrolyte.

(a) Crystalline solids vs amorphous solids:

• Shape: Crystalline solids have a definite geometric shape, with constituent particles arranged in a regular, repeating (long-range ordered) three-dimensional pattern; amorphous solids have no definite geometric shape, and their particles have only short-range order.
• Melting point: Crystalline solids have a sharp, definite melting point; amorphous solids melt/soften gradually over a range of temperature.
• Nature: Crystalline solids are 'true solids'; amorphous solids are often called 'pseudo-solids' or 'super-cooled liquids'.
• Anisotropy: Crystalline solids are anisotropic - physical properties (refractive index, electrical conductivity, etc.) differ with the direction of measurement; amorphous solids are isotropic - properties are the same in all directions.
• Cleavage: A crystalline solid, when cut with a sharp edge, splits into two pieces with smooth, flat faces; an amorphous solid gives an irregular cut/break.
• Heat of fusion: Crystalline solids have a definite and characteristic heat of fusion; amorphous solids have no definite heat of fusion.
• Examples: Crystalline - sodium chloride, quartz, diamond, potassium nitrate. Amorphous - glass, rubber, plastics, amorphous silica.

(b)(i) Definition of pH:

pH is defined as the negative logarithm (base 10) of the molar hydrogen ion (hydronium ion) concentration of a solution:

$pH = -\log_{10}[H^+] = -\log_{10}[H_3O^+]$

It is a convenient scale (usually running 0 to 14 in aqueous solutions at 298 K) for expressing the acidity/basicity of a solution: pH $<7$ is acidic, pH $=7$ is neutral, pH $>7$ is basic.

(b)(ii) Common ion effect:

The common ion effect is the suppression of the degree of ionisation (dissociation) of a weak electrolyte when a strong electrolyte containing an ion common to the weak electrolyte is added to the solution. This follows from Le Chatelier's principle: the added common ion increases the concentration of that ion in solution, which shifts the weak electrolyte's ionisation equilibrium backward (toward the un-ionised form), thereby suppressing further ionisation.

Example: Acetic acid ionises as:

$CH_3COOH \rightleftharpoons CH_3COO^- + H^+$ …