History of Density

	Revision 6 . . October 14, 2001 9:22 am by Stokerm [density of water at 4C]
	Revision 5 . . September 13, 2001 5:07 pm by Simon J Kissane [*moving to more consistent unit spelling]

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Difference (from prior major revision) (no other diffs)

Changed: 4c4

Formerly mass and volume were linked by defining the gram to be the mass of one cubic centimetre of water which meant that water had density 1 kg/litre. Now length (that is the metre), and hence volume, and mass (that is the kilogram), are defined independently of water so the density of water is not quite exactly 1 but it is still very close. A cubic metre of water thus weighs a ton.

Formerly mass and volume were linked by defining the gram to be the mass of one cubic centimetre of water at 4°C which meant that water had density 1 kg/litre. However, using one cubic centimeter of water as a standard for one gram is problematic due to the possibility of mass loss from evaporation as well as changes in density with temperature. For this reason alternate definitions of the metre and kilogram have been developed, which can be reproduced more reliably in a laboratory. Because of slight changes in the metre and kilogram due to these new definitions, the density of water at 4°C is not quite exactly 1, but 0.99995 kg/litre. A cubic metre of water thus weighs approximately one metric ton.