Final answer:
A cube of solid ethanol sinks in liquid ethanol because the density of the solid form is higher than that of its liquid form. This is explained by the molecules in the solid state being packed more closely together, unlike water, where the solid form (ice) is less dense and floats.
Step-by-step explanation:
A cube of solid ethanol sinks in liquid ethanol because its density is higher than that of its liquid form. The solid state of a substance is typically more dense due to the molecules being packed more closely together, which is true for many substances including ethanol. However, a notable exception to this general rule is water, which is less dense as a solid (ice) due to the lattice structure enforced by hydrogen bonding, allowing it to float on liquid water.
The concept can further be understood by considering the density of the materials involved. The density of solid ethanol will be greater than the density of liquid ethanol. If a substance (or an object) has a higher density than the fluid it is in, it will sink. This is a direct result of Archimedes' Principle, which relates buoyant force to the amount of fluid displaced by the object.
When the conditions of solid and liquid phases for the same material are identical, like temperature and pressure, and the solid phase is more dense, the solid form sinks in its liquid form. Solid ethanol is an example of such a substance. However, this is not a universal property for all substances, as the structure of the molecular bonds can also influence density, such as in the case of water.