Final answer:
The term used to describe the lowest temperature at which spontaneous combustion occurs is the flash point. It is a measure of a substance's flammability and can be influenced by the temperature dependence of reaction spontaneity.
Step-by-step explanation:
The term used to describe the lowest temperature at which spontaneous combustion occurs is known as the flash point. This is not to be confused with the ignition point, which is the temperature required to cause a substance to ignite in the absence of a spark or flame.
The flash point is a chemical property that indicates the flammability of a substance. For instance, materials with a flash point below 73 °F are rated as 4, indicating they are highly flammable, whereas those that will not burn have a rating of 0.
Considering the temperature dependence of spontaneity in reactions, such as the incomplete combustion of carbon (2C(s) + O₂(g) → 2CO(g)), the process's spontaneity can vary with temperature changes.
The free energy change (ΔG) equation reflects this, as it includes temperature as part of its calculation (ΔG = ΔH - TΔS), showing that reactions may shift from nonspontaneous to spontaneous with the increase or decrease in temperature.