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).
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.
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.
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.
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?
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.
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