19. The process that occurs when heat is generated by a chemical reaction within a substance and continues to a point of ignition is known as __________.

Spontaneous combustion is the correct term; it describes ignition resulting from heat generated by internal chemical reactions without an external ignition source. This process occurs when heat buildup exceeds dissipation, leading to ignition temperature. The term distinguishes itself from other options by specifically denoting self-heating and ignition without external factors, unlike chemical combustion, radiation ignition, or chemical ignition.