Final answer:
A loaded oil tanker sits lower in the water than an empty one because as oil is loaded, the total mass increases, requiring the displacement of more water to balance the weight, according to Archimedes' principle.
Step-by-step explanation:
The density of oil is indeed lower than that of water, which explains why oil floats on water's surface. However, the situation described in the question relates to the physics of floating objects and buoyancy. When an oil tanker is empty, it displaces a certain volume of water which is equivalent to its own weight. As oil is loaded onto the tanker, the total mass of the tanker increases. According to Archimedes' principle, a floating object displaces a volume of fluid equal to the weight of the object. Therefore, a loaded tanker must displace more water to balance the added weight of the oil, which results in the tanker sitting lower in the water compared to when it is empty. This principle is crucial for understanding how objects float and why even though oil itself may be less dense than water, the steel structure of a loaded tanker still leads to increased displacement and a deeper draft in the water.