Density

Density describes how much mass is contained within a given volume of a substance. It is calculated by dividing mass by volume and is commonly expressed in kilograms per cubic metre (kg/m³) or grams per cubic centimetre (g/cm³). A material with a high density contains more matter packed into the same space than a material with a lower density.

For example, iron is much denser than wood, so two objects of the same size made from these materials can have very different masses. Density is a characteristic physical property of a substance and helps identify materials because pure substances have a specific density under the same temperature and pressure. However, density can change if temperature or pressure changes, especially in gases, whose volume varies easily.

Density explains many familiar phenomena in everyday life. Objects that are less dense than a liquid tend to float, while those with a greater density usually sink. This is why ice floats on liquid water even though both are made of the same substance: when water freezes, its molecules arrange themselves into a structure that occupies more space, making ice less dense than liquid water.

Ships made of steel can also float because their overall average density, including the air inside them, is lower than that of the surrounding water. Scientists and engineers use density in fields such as geology, chemistry, medicine and engineering to identify materials, design structures and better understand how substances behave under different conditions.