When an object is placed in water, it either sinks or floats. This behavior is not random but governed by fundamental scientific principles. The answer to what makes something “heavier than water” and thus causes it to sink lies in understanding how matter is arranged within an object.
The Science of Sinking and Floating
The behavior of an object in water is primarily determined by a property called density. Density is a measure of how much mass is packed into a given volume. A denser object contains more material within the same space than a less dense one. This property can be calculated by dividing an object’s mass by its volume.
Water has a density of approximately 1 gram per cubic centimeter (g/cm³), which serves as a benchmark for comparison. When an object is placed in water, its density is compared to this value.
If an object is denser than water, it will sink. Conversely, an object with a density less than that of water will float. The interaction between the object’s weight pulling it down and the water’s upward push, known as buoyancy, dictates whether it sinks or floats.
Common Materials and Their Water Relationship
Many everyday materials demonstrate the principles of sinking and floating based on their inherent densities. Metals like iron, steel, and copper are significantly denser than water, which is why a small nail or a rock will quickly sink when dropped into a pond.
In contrast, substances such as wood and oil are less dense than water. A log floats on a lake because the wood’s density is lower than water’s, even for a large piece. Similarly, oil creates a distinct layer on top of water because it is less dense. Ice, surprisingly, also floats in liquid water because, unlike most substances, water expands as it freezes, making solid ice less dense than liquid water.
How Objects Can Defy Simple Density Rules
While material density is a factor, objects can sometimes appear to defy simple density rules due to the concept of overall or average density. A prime example is a large steel ship, which floats despite being constructed from steel, a material much denser than water. The ship floats because its design incorporates a vast amount of enclosed air space.
This hollow construction means that the ship’s total mass is distributed over a very large volume, including all the air within its hull. Consequently, the ship’s average density—its total mass divided by its total volume (steel plus air)—becomes less than the density of water. The water exerts an upward force, called the buoyant force, equal to the weight of the water the ship displaces.
A ship sinks into the water until it displaces a volume of water whose weight is equal to the ship’s total weight. As long as the ship displaces this amount of water before being fully submerged, it will float. This principle, known as Archimedes’ Principle, explains how even massive objects made of dense materials can remain afloat by strategically managing their overall density through shape and displaced volume.