6.8 - Salts, Crystals and Their Uses in Daily Life

Meaning of Salt

• A salt is an ionic compound formed when the ${\mathrm{H}}^{+}$ ion of an acid is replaced by a metal ion or $\mathrm{N}{\mathrm{H}}_4^{+}$.
• Salts can form through reactions of acids with alkalis, reactive metals, metal carbonates or aqueous ammonia.
  • $\mathrm{HCl}+\mathrm{NaOH}\to \mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}$
  • $2HCl+\mathrm{Zn}\to \mathrm{ZnC}{\mathrm{l}}_2+{\mathrm{H}}_2$
  • $2HCl+\mathrm{CaC}{\mathrm{O}}_3\to \mathrm{CaC}{\mathrm{l}}_2+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$
  • $\mathrm{HCl}+\mathrm{N}{\mathrm{H}}_3\to \mathrm{N}{\mathrm{H}}_4\mathrm{Cl}$

Physical Properties of Salt Crystals

• Salt crystals have flat surfaces, straight sides and sharp vertices.
• Each salt has a fixed geometrical shape such as cube, cuboid, rhombus or prism; different salts may have different crystal shapes.
• The angle between adjacent surfaces is fixed; crystals of the same salt retain the same geometrical shape at different sizes.
• These properties arise from particles packed in a compact, orderly and specific arrangement.

Uses of Salts

Field	Examples
Agriculture	$\mathrm{N}{\mathrm{H}}_4\mathrm{N}{\mathrm{O}}_3$ as fertiliser; $\mathrm{FeS}{\mathrm{O}}_4$ in pesticides for pests and grass
Medicine	$\mathrm{CaS}{\mathrm{O}}_4$ as plaster of Paris to support broken bones; potassium manganate(VII) as an antiseptic
Food preparation	$\mathrm{NaCl}$ for flavour; $\mathrm{NaHC}{\mathrm{O}}_3$ raises dough
Preservation	Sodium benzoate preserves sauces; $\mathrm{NaN}{\mathrm{O}}_3$ preserves processed meat

• Salt is needed for body-fluid balance and prevention of muscle cramps, but excessive intake is associated with high blood pressure, stroke, kidney failure and other diseases.