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# Density

The density compares the mass of a matter or object to its volume. A heavy object in a certain amount of volume has high density. An object with a little mass in the same amount of volume has a low density.

Temperature has an effect on density. When the temperature of a substance increases, density decreases because the atoms are moving all around the place having less matter in one space. When the temperature of a substance decreases, the density increases. Many substances follow this rule, but there are exceptions. When most substances freeze, they make crystals that are more dense than the liquid. When water freezes, the water molecules are actually farther apart instead of being closer together. This is because the molecules in an ice crystal are arranged in a way that makes it less dense than liquid water.

Fresh water is often used as a standard of density.

The unit for density in the International System of Units is kilogram by cubic meter (kg/m3).

A formula to find out density is:

${\displaystyle density=mass/volume}$

## Other uses of the word "density"

Sometimes people use the idea of density to talk about other properties instead of mass. They're trying to describe how much of a property can be found in a specific piece of what they are looking at. For example, population density is how many people live within the same amount of space. The population density in the city is higher than the country side because there are more people living closer to each other in the city. In computers, storage density is how much data can fit on a data storage device in relation to its physical size. A Blu-ray disc has a higher storage density than a DVD which has a higher storage density than a CD, even though they all have almost exactly the same volume.