Density

What density means

Density is how much mass is packed into a certain volume. A dense material has a lot of matter crammed into a small space. Steel is dense. Polystyrene foam is not.

Density is a physical property: you can measure it without turning the substance into something new. Every pure substance has its own density, which is why it can be used to identify materials, as you'll see in the gold crown story below.

Density explained with particles

Two things decide how dense a substance is.

1. How closely the particles are packed. Gas particles are far apart, so gases have very low densities. One litre of air has a mass of only about 1.2 g, while one litre of water has a mass of 1000 g.
2. How heavy each particle is. Lead and aluminium are both tightly packed metals, but a lead atom has about eight times the mass of an aluminium atom. So a block of lead is far denser than a block of aluminium the same size.

Calculating density

Mass is usually in grams (g). Volume is in cubic centimetres (cm³) for solids or millilitres (mL) for liquids. These two volume units are the same size: 1 mL = 1 cm³. So density comes out in g/cm³ or g/mL.

You can rearrange the formula when you know the density and need something else: mass = density × volume and volume = mass ÷ density.

Measuring mass and volume in the lab

• Mass: use an electronic balance. For a liquid, weigh the empty measuring cylinder first (or press "tare"), then weigh it again with the liquid in it.
• Volume of a regular block: measure it and multiply: length × width × height.
• Volume of an irregular object such as a rock or a key: use the displacement method. Half-fill a measuring cylinder with water and read the level. Gently lower the object in. The water rises by exactly the object's volume.

Floating and sinking

An object floats on a liquid if it is less dense than the liquid, and sinks if it is more dense. The same rule works for liquids and gases stacked on each other: oil floats on vinegar, and a helium balloon rises because helium is less dense than air.

The odd one out: ice

For almost every substance, the solid is denser than the liquid, because solid particles are packed tighter. Water breaks this rule. When water freezes, its particles lock into an open, honeycomb-like pattern called a lattice that has more space in it than liquid water. The mass stays the same but the volume grows by about 9%, so ice is less dense than water and floats. That's also why ice cubes bulge out of the tray and why frozen water pipes can burst.