Salt: A salt is the product of reaction between an acid and a base. It can be organic as well as inorganic. They are abundant on the earth’s crust as well as in the oceans e.g. NaCl or Sodium Chloride is deposited in large amounts on earth and ocean water.

Double Salt: These are the combination of two simple salts. Unlike simple salts, Double salts contain two different kinds of positive and one negative ion or one positive and two different kinds of negative ions.  E.g. Dolomite is a double salt of Calcium Magnesium Carbonate [Ca Mg (CO₃)₂]; Apatite is Calcium Flurophosphate [Ca₅F(PO₄)₃].

Salts can appear to be clear and transparent (sodium chloride), opaque, metallic and lustrous (iron disulfide). In many cases the apparent opacity or transparency are only related to the difference in size of the individual monocrystals. Since light reflects from the grain boundaries (boundaries between crystallites), larger crystals tend to be transparent, while polycrystalline aggregates look like white powders.

Few examples:

Color	Salt
Yellow	Sodium chromate
Orange	Otassium dichromate
Red	Mercury sulfide
Mauve	Cobalt chloride hexahydrate
Blue	Copper sulfate pentahydrate Ferric hexacyanoferrate
Purple	Potassium permanganate
Green	Nickel chloride hexahydrate
White	Sodium chloride
Black	Manganese dioxide

Isomers:

Isomers are molecules with the same molecular formula but different chemical structures. Isomers contain the same number of atoms of each element but have different arrangements of their atoms in space.

To illustrate: carbon- carbon single bonds permit rotation of the atoms or group of toms linked to the carbon atoms; whereas the carbon- carbon double bonds completely restricts the motion. Alkene hydrocarbons are much more chemically reactive than the alkanes since double bonds can be easily oxidized or reduced.

e.g.:  Propanol: It has the formula C₃H₈O (or C₃H₇OH) and occurs as two isomers: propan-1-ol (n-propyl alcohol)(I) and propan-2-ol (isopropyl alcohol)(II).

An example of Structural Isomers