Why Is Mass Measured As Dry Mass?

Because if it's wet, then you aren't measuring the mass of the stuff you're interested in, you're measuring the mass of the water contained within it, too.

Sometimes that's okay, if the substance you're handling always has that amount of water in it, or if you need to know the mass at its usual saturation level. But often, to understand the physical properties of something you need to get dry mass.

Getting truly dry mass can be tricky as the air almost always has some water vapour in it, and this will be absorbed into any material you're working with. The rate at which it gets absorbed depends partly on the surface properties of the material under investigation. To get a true dry mass measurement, either the material may need to be measured in a vacuum or in other laboratory conditions where humidity has been minimised.

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