Polymorphism is the ability of a solid material to form different crystalline structures (synonyms: forms, modifications).
Although different modifications of a polymorph have the same chemical structure, they differ in the physical properties such as:
This influences the processability of drug substances and the performance of drug products, such as:
- Absorption into the body
- Dissolution (rate)
That is why polymorphism is an important topic for the pharmaceutical and also the food field.
The different modifications of a polymorph can be characterized with differential scanning calorimetry (DSC).
The figure depicts the two heatings of a paracetamol sample (initial mass: 2.6 mg). The heating rates as well as the cooling rate of the segment between both heatings amount to 10 K/min.
In the first heating, a peak at 169°C (onset temperature) is detected. This melting temperature is typical for the monoclinic form I of paracetamol. 
During cooling at 10 K/min, no crystallization occurs. Crystallization takes place during the second heating at 72°C (onset temperature) to form another modification with melting point at 157°C. This is typical for the orthorhombic form II of paracetamol .