Experimental approach of equilibriums between water vapor and hydrated salts

Experimental approach of equilibriums between water vapor and hydrated salts 

Equilibrium between two hydrated forms of a salt and water vapor can be represented by the reaction (n and q being two positive numbers, n can be null)  

When equilibrium states of such a system are studied, we generally proceed by isothermal and isobaric thermogravimetiy under controlled water vapor pressure. 

If we indicate by s the total amount of water contained in the sohd per mole of salt, two kinds of curve are obtained at equilibrium. 

The first experimental result thus obtained is in fact the variance of the system. Indeed, in case a), we see that if temperature is fixed, we have equilibrium between two hydrated forms for only one determined pressure therefore, the variance of the system is 1. On the other hand, in case (Jf), the water content of the solid varies continuously with pressure and each point depends on temperature thus, the variance of the system is 2. 

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