Certain crystalline substances which have similar chemical composition, e.g., sodium phosphate and sodium arsenate (i.e. Na₃ PO₄ and N₃AsO₄ are found to posses the same crystal shape. This phenomenon is know as isomorphism (Greek: same shape) and the substances are said to be isomorphism.

There are many substances (elements or compounds) which exist in more than one crystalline form which are formed under different conditions of temperature and pressure. This phenomenon is known as polymorphism. For example, carbon exist in two crystalline forms which are graphite and diamond; calcium carbonate exist in two crystalline forms called calcite and argotic. The polymorphism exhibited by the elements is commonly called allotropy. The temperature at which one crystalline form changes into another is called transiting temperature.

For more help in Isomorphism and polymorphism click the button below to submit your homework assignment