Ask A Scientist Chemistry Archive

name         Gunda
status       student
age          15

Question -   I was wondering how one would know when all the water in a crystal (made of an 
anhydrate + water) has been evaporated.
---------------------------------------------------------------------------- --------
There are many ways to determine if a solid contains any residual water by infrared 
spectroscopy which gives a characteristic "finger print" of water. However, the way that 
is commonly done in the lab is to heat the substance to an elevated temperature and measure 
the weight loss. When the substance reaches a constant weight it is generally assumed that 
all the water has evaporated, but obviously there could be cases where that criterion is 
misleading.

Vince Calder
=====================================================
By checking for weight loss when you subject it to conditions known to drive the water out of 
the crystal.

Greg Bradburn
=====================================================