Among the three states of matter,solids, liquids and gas, solids have shape of its own, while liquidsand gases take the shape of the container. Solids are of two types-amorphous solids, which do not have a definite ordered structure andcrystalline solids – which have a well defined structure.Crystalline solids can exist in more than one crystalline form.
Polymorphism is a property by which onesubstance can exist in more than one crystalline form and in eachform, has different physical properties. Polymorphism is seen in manyelements and compounds.
Allotropy, by which elements exist inmore than one form, is a type of polymorphism.
Each polymorphic form of a compound orelement is considered to be a separate phase.
In the phase diagram, each of thesecrystalline forms is taken as separate phase, and the change from oneform to another is taken as a phase change.
Each phase change point is called thetransition point.
There are two types of polymorphism.
Enantiotropic polymorphism – Inthis kind of polymorphism, one allotropic form can be converted intoanother at the transition point, but can be reversed again to thefirst form. So, a polymorphic substance which can convert from oneform to another is said to be enantiomorphic polymorphism.
Monotropic polymorphism – Somepolymorphic forms, are not reversible, once converted to anotherform is said to be monotropic polymorphism.
Polymorphic formswhich do not undergo reversible transformation into one another aresaid to be monotropic polymorphism.
Thus, polymorphismcan be of two different types, based on their interconvertibility.
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Since allotropic forms are polymorphismin elements, Sulfur can be taken as an example for polymorphism.
Sulfur exists as monoclinic sulfur andrhombic sulfur.
These forms can interchange into eachother, at a certain point called the transition point. Thetemperature is said to be transition temperature.
Sulfur is an example of suchpolymorphic forms, which undergo reversible transformations attransition point and are said to be enantiotropic.
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Another example of polymorphism is thatof carbon. The element carbon can exist in many forms. Two such formsare diamond and graphite.
Diamond can be converted into graphite,at a phase change point or transition point. But, even after cooling,graphite cannot be converted back to Diamond.
Thus, phase change in the case ofcarbon is irreversible. Carbon, exhibits monotropy or monotropicpolymorphism.
Other examples of polymorphism incompounds are SiO2, which exists in two crystalline forms,
Zinc sulphide also has two polymorphicforms: Sphalerite and wurtzite structures.