# one Gram

#### Voltboy

Joined Jan 10, 2007
197
One gram is the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre, and at the temperature of melting ice (4°C). That's 1 milliliter of pure water at 4°C.
Why does it have to be at 4°C, if 1ml of pure water at 4°C still will be 1ml of pure water at 5°C, and will have the same mass (because its still 1ml or pure water).

#### Elektronster

Joined Oct 19, 2007
2
Water changes volume as it warms up or cools down. At 4° C (actually, 3.98° C) pure water is at its densest state, when one milliliter equals exactly one gram. Water any warmer or cooler than 3.98° C is not as dense, so one milliliter of pure water would exhibit less mass than exactly one gram.

#### Papabravo

Joined Feb 24, 2006
16,830
So the answer is that the sum of the masses of the water atoms is the same but the volume of those atoms changes as a function of temperature. To get the standard gram you have to have the right number of water molecules, each having a mass that can be computed from the atomic weight and Avagodro's number, occupying a volume of one cubic centimeter.

In short one cubic centimeter of water at 4 degrees C with the right number of atoms will have a mass of 1 gram. If you change the temperature, the number of atoms stays the same, the mass stays the same, but the volume is no longer 1 cubic centimeter so that situation is no good for a standard.

#### Voltboy

Joined Jan 10, 2007
197

#### recca02

Joined Apr 2, 2007
1,214
what about pressure isnt that defined for density?
i think density also changes with pressure (though negligibly with liquid and even lesser with solids)

#### Papabravo

Joined Feb 24, 2006
16,830
what about pressure isnt that defined for density?
i think density also changes with pressure (though negligibly with liquid and even lesser with solids)
I don't think so. Density is mass per unit volume. Pressure has units of force per unit area. By NSL force has units of mass times distance per second squared. This leaves mass per distance times seconds squared. I can't see any relation between density and pressure, at least with respect to a cubic centimeter of water sitting on a table in a room a 4 degrees C.

#### recca02

Joined Apr 2, 2007
1,214
units aside, pressure greatly influences density. so much so that at criticle point the density of steam and water become equal (though this time the temperature equals boiling point).

the critical pressure is at abt 221 bars or thereabout.
and temp abt 374 deg centigrade(not sure).

let me elaborate a little.
even solids have something called bulk modulus.
there is a change in volume for a pressure applied.
mass does not change with pressure.
thus density shud change.what say?

#### Voltboy

Joined Jan 10, 2007
197

#### Papabravo

Joined Feb 24, 2006
16,830
Hardly a significant effect on a solid or a liquid at 1 atm( ~ 1000 mB). How many angels can dance on the head of a pin? If I go from 960 mB during Hurricane Katrina to 1020 mB on a beautifal fall day in Michigan what is the change in volume of my cubic centimeter of water? Assume the temperature is a constant 68 degrees F.

#### recca02

Joined Apr 2, 2007
1,214
true,
like i said before the compressibility of fluid is ideally zero,
but for a standard definition pressure like STP should be mentioned since pressure also changes boiling and freezing points.in any case is the above definition accurate?
does the density fo water exactly equal 1000 Kg/cu.m

#### GS3

Joined Sep 21, 2007
408
One gram is the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre, and at the temperature of melting ice (4°C). That's 1 milliliter of pure water at 4°C.
Why does it have to be at 4°C, if 1ml of pure water at 4°C still will be 1ml of pure water at 5°C, and will have the same mass (because its still 1ml or pure water).
The gram *was* originally defined like that but no longer.
http://en.wikipedia.org/wiki/Kilogram

The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK), which is almost exactly equal to the mass of one liter of water.
...
On 7 April 1795, the gram was decreed in France to be equal to the absolute weight of a volume of pure water equal to a cube of one hundredth of a meter, and to the temperature of the melting ice. The regulation of trade and commerce required a practical reference standard in addition to the definition based on fundamental physical properties. Accordingly, a provisional kilogram standard was made as a single-piece, metallic reference standard one thousand times more massive than the gram.

In addition to this provisional kilogram standard, work was commissioned to determine precisely how massive a cubic decimeter (now defined as one liter) of water is. Although the decreed definition of the kilogram specified water at 0 °C  a highly stable temperature point  the scientists chose to redefine the standard and perform their measurements at the most stable density point: the temperature at which water reaches maximum density, which was measured at the time as 4 °C. They concluded that one cubic decimeter of water at its maximum density was equal to 99.92072% of the mass of the provisional kilogram made earlier that year. Four years later in 1799, an all-platinum prototype kilogram, the Kilogramme des Archive (Kilogram of the Archives), was fabricated with the objective that it would equal, as close as was scientifically feasible for the day, the mass of a cubic decimeter of water at 4 °C. The kilogram was defined to be equal to the mass of the Kilogram of the Archives and this standard stood for the next ninety years.

Since 1889, the SI system defines the magnitude of the kilogram to be equal to the mass of the International Prototype Kilogram  often referred to in the professional metrology world as the IPK. The IPK is made of an alloy of 90% platinum and 10% iridium (by weight) and is machined into a right-circular cylinder (height = diameter) of 39.17 mm to minimize its surface area.
...
The kilogram underpins the entire SI system of measurement as it is currently defined and structured, so its stability is crucial. For instance, the newton  the SI unit of force  is defined as the force necessary to accelerate the kilogram by one meter per second². Accordingly, if the mass of the IPK were to change slightly, so too must the newton by a proportional degree so that the acceleration remains at precisely one meter/second². In turn, the pascal  the SI unit of pressure  is defined in terms of the newton. This chain of dependency follows to all the electrical units. For instance, the joule, which is the electrical and mechanical unit of energy, is defined as the energy expended when a force of one newton acts through one meter. The ampere is also defined relative to the kilogram. With two of the primary units of electricity thus defined in terms of the kilogram, so too follow all the rest, including the watt, volt, ohm, coulomb, farad, and weber.

Clearly, having the magnitude of many of the units comprising the SI system of measurement ultimately defined by the mass of a 128-year-old piece of metal is a tenuous state of affairs.
Very interesting article.