The property possessed by certain elements to exist in
two or more distinct forms that are chemically identical
but have different physical properties. In the case of iron
the crystal structure has one form at room temperature
and another at high temperature. When heated above
910 deg C the atomic structure changes from body centered
cubic to face centered cubic but reverts again when
cooled. The allotropy of iron modifies the solubility of
carbon, and it is because of this that steel can be
hardened.