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DENSITY

What is density?

Density is how heavy something is for its size. It's how massive it is divided by how big it is. So, something that's very heavy and small has a high density. (Mass is like weight*? see note at end).

Density is Mass divided by Volume. It's usually measured in kilogrammes per cubic metre. To work out the density of something, divide mass (how much it "weighs"*), by volume (how much space it takes up).

A cubic metre of water weighs 1000 kilogrammes, so it's density is 1000 kilogrammes per cubic metre. This is the standard against which other substances are measured, so it's sometimes stated as "The density* of water is 1.0" , although in fact that's actually the Specific Gravity. (Specific gravities are really just densities expressed relative to the density of water). Oil is lighter than that at a density* (specific gravity - see note at end) of 0.8 and floats on top of water. The average density* of the Earth is 5.5 , but some metals have densities* in the region of 10. Liquid mercury has a density* of 13.6 . The density* of gold is 19.3 , and that of osmium is 22.

The density of air (at sea level) is 1.225 Kg/m3 (Note the decimal point. Air is almost a thousand times lighter than water)

Some gases, such as helium and hydrogen, have a density which is less than the density of air, so balloons filled with such gases float and rise in air, in the same sort of way that oil floats in water, or lead floats in mercury! See snooker ball floating in mercury

What if a balloon full of helium which floats in air is balanced with a weight which is exactly the right weight so the balloon neither floats nor sinks? See neutral buoyancy balloon. Also see the different densities of hydrogen and helium and the effect of this which is minimal on the buoyancy in air.

To see a LIST of different densities, see Densities; List

Plus, in a practical sense, How to measure density of an object, for example.

* Note: Where I've put "density*" , it's really a specific gravity. The true density of mercury is 13,600 kilogrammes per cubic metre, but it's often written that the density of mercury is 13.6 - even though it's actually the specific gravity of mercury that's 13.6 . Either way you look at it, mercury weighs 13.6 times as much as water, so a cubic metre of it weights 13.6 tonnes.