During differential scanning calorimetry (DSC) of an unknown solid
CHEMISTRY · Single correct · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
During differential scanning calorimetry (DSC) of an unknown solid, the following thermal events are recorded:
• A broad endothermic event beginning near 350 °C and ending near 450 °C.
• A sharp, intense endothermic peak at 793 °C.
Which interpretation best explains both observations?
(a) The broad event is due to crystallization of the sample on heating, and the sharp peak is the subsequent melting of the freshly formed crystals.
(b) The sample is completely crystalline but occurs in two polymorphic forms; the lower-temperature polymorph melts gradually (broad event), whereas the higher-temperature polymorph melts sharply at 793 °C.
(c) The sample contains both amorphous and crystalline regions; the broad event is the glass transition of the amorphous fraction, and the sharp peak is melting of the crystalline fraction.
(d) The broad event represents evaporation of absorbed water, and the sharp peak corresponds to a second-order phase transition within the amorphous phase.