Final answer:
When water is added to diesel fuel, they form two distinct layers because diesel is a nonpolar liquid and water is polar, which makes them immiscible. This immiscibility is due to inadequate attractive forces to overcome water's hydrogen bonds.
Step-by-step explanation:
Adding water to diesel fuel results in the formation of two separate layers, which indicates that these two liquids do not mix well with each other. This means that diesel fuel and water do not have the ability to dissolve in each other, and such liquids are described as immiscible. The concept of immiscibility is due to the fact that diesel fuel is a nonpolar liquid with relatively weak attractive forces between its molecules and the polar water molecules. These weak forces are not strong enough to overcome the much stronger hydrogen bonding between water molecules. Therefore, when water is added to diesel, the two substances do not create a homogeneous (single-phase) mixture, but rather form two distinct layers due to their immiscibility.
Learn more about immiscible liquids