This is fine for quiz purposes, but if we want to be strictly accurate it's not as simple as that.
The gram was first defined in 1795, as the mass of one cubic centimetre of water at the melting point of ice. Four years later it was redefined as the same amount of water, but at 4 degrees Centigrade, which is the temperature at which water is densest. (The new gram was therefore slightly heavier than under the previous definition.)
At this point, a cylindrical platinum prototype was presented to the Archive of the (French) Republic; it became known as the Kilogramme des Archives, and the kilogram was redefined as being equal to its mass.
The Kilogramme des Archives was replaced in 1885 by the International Prototype of the Kilogram (IPK). This was made from an alloy consisting of 90% platinum and 10% iridium, which is harder than pure platinum.
There are several reference copies of the IPK, including many "national prototypes" in various countries around the world. Eventually these were found to vary from each other over time, so in 2005 a search began for a fundamental constant of nature that could be used in the definition (as with other basic SI units). In 2011 the International Committee for Weights and Measures (CIPM) agreed in principle that the kilogram should be redefined in terms of the Planck constant.
Max Planck (1858–1947) was the German physicist who discovered the concept of quanta of energy, and as such he is regarded as the originator of quantum theory. In 1900 he postulated that energy could only be emitted in quantities that were multiples of an elementary unit. For radiation of frequency v, that amount of energy (E) is equal to v multiplied by h, where h is a constant – which became known as Planck's constant.
Planck's constant has an exact value, which is a little over 6.626 x 10-34 joule seconds. The kilogram is involved in the definition of the joule (which can be expressed as kg.m2.s2 – kilogram metre squared second squared); so by this definition the kilogram can be said to have a constant, fundamental value.
The CIPM voted in November 2018 to adopt this definition.
After all this, however – and as I said at the top of this piece – for quiz purposes, we can say without much fear of contradiction that a litre of water weighs one kilogram.
© Macclesfield Quiz League 2019