Float or sink, and what density means

Wash rice and the husk floats while the rice sinks. Pour oil into water and the oil floats. People usually say things that float are "lighter" and things that sink are "heavier". That's only partly right.

A wooden stick and an iron rod of the same size don't weigh the same: the iron feels much heavier. When we say iron is heavier than wood, we really mean a property called density. (Density isn't the only thing that decides floating and sinking, but it's the main one here.)

Picture a bus packed with people: high density. The same bus with just a few passengers: low density. A forest where trees grow close together is a dense forest. Or fill two boxes of the same size, one with cotton and one with stones: the stone box is far heavier.

Density is the mass in each unit of volume:

Density = Mass ÷ Volume

A substance's density doesn't depend on its shape or size. It does depend on temperature and pressure, and pressure mainly matters for gases (its effect on solids and liquids is negligible).

One mL of water has a mass of very nearly 1 g at room temperature, so 100 mL of water is about 100 g.

An aluminium block has a mass of 27 g and a volume of 10 cm³. Its density is 27 ÷ 10 = 2.7 g/cm³. Water is about 1 g/cm³, so aluminium is 2.7 times as dense as water. That number is its relative density: the density of a substance divided by the density of water at the same temperature. It has no units.

Long before big ships, people crossed rivers and seas on bamboo rafts and hollowed-out logs. Bamboo is light and hollow, so it floats easily. Similar boats are still used in some places today, for transport and for tourists.