Density is a measurement that compares the amount, ratio of matter an object has to its volume. An object with a lot of matter in a certain amount of volume has high density. An object with a little matter 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 because the atoms move slower making the atoms stay in one place. Many substances follow this rule, but there are exceptions. Water is one such exception. When water freezes, it makes ice that is less dense than liquid water.
Fresh water is often used as a standard of relative density. This is called specific gravity.
The most common SI units for density are g/cm3 and kg/m3. When the numerator is much larger than the denominator, that means the substance has a higher density. When the denominator is much larger than the numerator, the substance has a lower density.

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