Final answer:
For boilover to occur, the two immiscible layers are oil and water. Oil floats on top of water due to its lower density, leading to challenges in extinguishing fires involving crude oil.
Step-by-step explanation:
The key condition for boilover in an open container of a combustible material is the presence of two immiscible layers, which are oil and water. When these two substances are mixed, they form two separate layers due to their immiscibility, with the less dense oil floating on top of the denser water.
Various liquids such as gasoline, oil, benzene, carbon tetrachloride, and some paints are immiscible with water. They form separate layers in a container because the attractive forces between the polar molecules of water and the nonpolar molecules of these liquids are insufficient to overcome the hydrogen bonds between water molecules. This phenomenon can lead to complexities in situations like crude oil fires, where the oil floats on water and is difficult to extinguish due to the inefficiency of water in absorbing the heat, as well as the energy-intensive nature of burning crude oil.