Final answer:
A substance with a density less than the density of water (1 g/cm³) will float. Ice cubes, with a relative density of about 0.91, and oils are examples of substances that float due to being less dense than water.
Step-by-step explanation:
Density is a critical property that determines whether a substance will float on water. The density of water is 1 g/cm³ (or 1000 kg/m³), and substances with densities less than this will float. An object made of substance 4 would need to have a density less than 1 g/cm³ to float. In the case of an ice cube, which has a relative density of approximately 0.91, it floats, since its density is less than that of water. Similarly, since oils typically have a lower density than water, they will float. A substance with a higher density than water, such as many metals, will sink when placed in water.
The relationship between an object's average density and its ability to float is straightforward: if the average density is less than that of the fluid, the buoyant force will be greater than the weight of the object, allowing it to float. The specific gravity of a substance, which is the ratio of its density to that of water at 4°C, can also determine if an object will float: a specific gravity less than one indicates that the substance will float. This can be useful for divers who aim for neutral buoyancy. Temperature can affect density as well, with density generally decreasing as temperature rises.