Basic science: understanding numbers
Basic science: understanding numbers

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2.4 Density of water

In the last two sections you have considered areas and volumes, using the Greenland ice sheet as an example. Our estimate of the volume of ice contained in the ice sheet was 2,565,000 km3. You’re now going to look at density.

What is density?

Density is the amount of matter within a given volume. It is, if you like, a measure of how tightly packed a substance is. A cubic metre of lead weighs more than a cubic metre of water, because the mass of the lead atoms is greater than that of the water molecules. The SI unit for density is kilograms per cubic metre, or kg/m3.

But density is not just about different substances. The same substance can have different densities under different conditions. When discussing SI units in Week 1, you saw that heat is the measure of the motion of atoms in a substance. The hotter the substance, the more the atoms are moving about. Hot air has faster atoms in it than cold air, moving about more rapidly, so they become more spread out, and take up more room. For any given volume, hot air has fewer molecules than cold air. That volume of hot air, having fewer molecules in it, weighs less than the cold air and floats up through it. We say that hot air is less dense than cold air. This is why hot-air balloons rise into the sky.

When water cools enough to freeze solid, it forms ice. You might expect that, being colder, it would be denser, and for most substances you would be right, but water has a very peculiar property. Due to the special way the molecules of water stick together as they freeze, they are spaced further apart as a solid than as a liquid. This makes ice less dense than water, and is why ice cubes float on the top of your drink. It is also the main reason why icebergs float in the ocean when ice breaks off an ice sheet (the other reasons being that they contain air bubbles trapped in the ice, and salty seawater is denser than the fresh water from which the land ice forms).

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