Density

Density tells you how much matter is packed into a given amount of space, defined as the mass per unit volume of a substance. It is written with the Greek symbol ρ (rho), and its SI unit is the kilogram per cubic metre (kg/m³), though grams per cubic centimetre are common in everyday use. Because it depends only on the material and not on how much of it you have, a tiny gold nugget and a large gold bar share exactly the same density. Objects denser than a fluid sink in it while less dense ones float, which is why oil sits on top of water and ice floats. Density usually changes with temperature and pressure, since heating most materials makes them expand and become slightly less dense.

This simple idea underpins a surprising range of real-world tasks. Engineers use it to choose lightweight yet strong materials, shipbuilders rely on it to keep steel hulls afloat, and jewellers use it to tell real gold from cheaper imitations. It also explains natural phenomena, from why warm air rises to how ocean currents form as saltier, colder water sinks beneath lighter layers.