Why does 1mL of pure water weigh exactly 1gram?

Question

I thought the MSI unit lengths were now based on a particular wavelength of light. So why the perfect correlation between 1000 cubic mm and 1.000g of pure water? Can't be luck... can it?

Answer 1

The metre was 1/40000 of the Earths circumference and 1 cubic metre of pure water has a mass of 1 metric tonne.However in later years it was found that light travels very nearly 300,000,000 million metres in 1 second in a vacuum.They are now talking about redefining the meter eg shortening it so that it correlates to the exact distance light travels in one second divided by3x10(8).they will then have to redefine the litre and the kilogram by a very tiny amount.The second is derived from the vibration of a caesium atom .Sorry not 1/40000 but 1/4000000

Answer 2

No, the equivalence between ml, cm3 and g is by definition. As you say, 1ml of water weighs exactly 1g, and takes up exactly 1cm3. This is because they were originally defined as such.

The wavelength of light comes into play because, having taken these as given, the definitions of a second and a metre have been redefined as using a certain wavelength of light, and a certain mass of caesium, that ends up with the above values. The choice was arbitrary, but don't forget that litres and kilogrammes were first defined shortly after the French revolution, many years before we could measure the speed of light (or even had any idea that it moved at all).

Answer 3

Originally, the units were defined to make what you say perfectly true at, I think,maximum density (4 C) and1 atm, although expansion and compression effects for water are very small.. And when they were redefined, units were chosen to keep this true to a large number of decimal places.

Actually, water of different origin will have slightly different amounts of deuterium in it, a fact used in studies of ancient climates,so the density of water depends toa measurable extent on where that water came from.

Answer 4

Apparently it was luck. I wondered that myself.

The mass unit is based on a prototype made from platinum and iridium, and is kept under controlled conditions. The measure for length, and hence volume, was originally based on the circumference of Earth, later on the speed of light (but that was derived from the second).

So yes, it is actually an amazing coincidence.

Answer 5

1 mL of pure water does NOT weigh exactly 1 g. The density of water is temperature dependent - there are tables you look up to determine density at various temperatures.

Answer 6

I'm no scientist, but I thought the whole metric system of volume and weight was based on pure water.