An insoluble salt is a salt that not dissolves in water at room temperature.
Insoluble salts in water include:
(a) all carbonates except ammonium carbonate [(NH4)2CO3], sodium carbonate (Na2CO3) and potassium carbonate (K2CO3)
(b) all oxides except calcium oxide (CaO), barium oxide (BaO), sodium oxide (Na2O) and potassium oxide (K2O)
(c) all hydroxides except ammonium hydroxide (NH4OH), sodium hydroxide (NaOH), potassium hydroxide (KOH) and barium
(c) silver chloride (AgCl), lead (II) chloride (PbCl2), and mercury chloride (HgCl2)
* Lead (II) chloride dissolves in hot water but not in cold water
(d) barium sulphate (BaSO4), lead (II) sulphate (PbSO4), and calcium sulphate (CaSO4)
In precipitation method two aqueous and soluble solutions are mixed to form the precipitation of an insoluble salt. The first soluble solution will contain cation (metallic ion) for the insoluble salt while the second soluble solution will contain anion (nonmetallic ion) for the wanted insoluble salt.
The double decomposition reaction means the reaction two aqueous solutions are mixed to formed an insoluble as well as soluble salt.
The reaction also called double decomposition because the ions from two different salts interchange to form an insoluble and a soluble salt.
These can be simplified as below with some examples:
(a) Pb(NO3)2 (aq) + H2SO4 (aq) PbSO4 (s) + 2HNO3(aq)
lead (II) nitrate sulfuric acid lead (II) sulphate nitric acid
(b) CaCl2 (aq) + Na2CO3 (aq) PbSO4 (s) + 2NaCl (aq)
calsium chloride sodium carbonate lead (II) sulphate sodium chloride
Salts are ionic compounds composed of ions which are arranged in a close, orderly manner at fixed positions. Each cell unit (one unit of the compound for example in NaCl, one unit is Na+Cl- is arranged repeatedly many times until the geometric shape is formed. This is called a crystal.
A crystal is homogeneous solid with fixed shapes.
All crystal have the following physical features:
(a) Similar geometric shapes, for example cuboid, tetragonal, monoclinic, and hexagonal.
(b) They have even surfaces, straight edges and sharp tips.
(c) The angles between two corresponding surfaces are fixed and equal.
Even though the size of the crystals varies, the shape remain the same.
The size of the crystal vary according to the rate of crystallization. The lower the rate of crystallization (slower), the bigger the crystals which are formed.
Crystals are hard but brittle. They may be cut into certain shapes as their particles (ions) are arranged in a close, orderly manner.
The preparation of salt crystals in the laboratory depends on:
(a) whether the salt is soluble or not in water
(b) if soluble, whether it is a sodium, potassium or ammonium salt or not