POLYMORPHISM-ALLOTROPY-ISOMORPHISM

POLYMORPHISM

 
Existence of substance into more than one crystalline forms is known as "POLYMORPHISM".
In other words: Under different conditions of temperature and pressure, a substance can form more than one type of crystals. This phenomenon is called Polymorphism and different crystalline forms are known as ‘POLYMORPHICS’
 Example:

1) Mercuric iodide (HgI2) forms two types of crystals.

       a. Orthorhombic

       b. Trigonal

2) Calcium carbonate (CaCO3) exists in two types of crystalline forms.

       a. Orthorhombic (Aragonite)

       b. Trigonal

       Polymorphous substances have similar chemical properties but different physical properties.

ALLOTROPY
 
    "Existence of an element into more than one physical forms is known as ALLOTROPY "
    Under different conditions of temperature and pressure an element can exist in more than one
     physical forms. This phenomenon is known as Allotropy and different forms are known as "Allotropes"

    Example:
    Coal, lamp black, coke, Diamond, graphite etc. are all allotropic forms of carbon.
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ISOMORPHISM
 

    Existence of different substances in one crystalline form is known as "ISOMORPHISM"

Or

    Different substances may exist in identical crystalline forms. This phenomenon is called as     
     Isomorphism and these substances are known as ‘Isomorphous’.

    Examples:

1) Na2SO4 & Ag2SO4 both exist in Hexagonal crystalline form.

2) KBF4 & BaSO4 both exist in Orthorhombic

3) ZnSO4 & NiSO4 both exist in Orthorhombic

4) CaCO3 & NaNO3 both exist in Trigonal

Properties of Isomorphic Substances

1) Isomorphic substances have same atomic ratio

2) Empirical formula of isomorphic substances is same

For example

CaCO3 NaNO3

1:1:3 1:1:3

NaF MgO

1:1 1:1

3) They have different chemical & physical properties.

4) When their solutions are mixed, they form mixed type of crystals.

5) They show property over growth.

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