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).
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).