Consider the following minerals: Anhydrite (CaSO4), Bassanite (CaSO4·0.5H2O, the
ID: 802591 • Letter: C
Question
Consider the following minerals: Anhydrite (CaSO4), Bassanite (CaSO4·0.5H2O, the stuff of which plaster of Paris is made), and Gypsum (CaSO4·2H2O), answer the following questions: a) If water and water vapor are possible phases of pure water in the system, coexisting with some of the three minerals, how many phases and components are there in this system? b) How many phases are present at invariant points in such a system?c) If these three minerals coexist only with watervapor, how many univariant reactions are possible? Write all univariant reactions, labeling each according to the phase that does not participate in the reaction.
Explanation / Answer
a. A phase is defined as the physically distinct material in a system that is mechanically separable from the rest. It could be a mineral, a liquid. a gas or an amorphous solid such as a glass. Whereas component is the minumum number of chemical species used to define the system eg SiO2, H2O, O2, NaAlSi3O8.
There are 4 phases in the system i.e. Anhydrite( CaSO4), Gypsum( CaSO4.2H2O), water vapour and liquid( saturated solution) and 2 components in the system namely anhydrite, and gypsum.
b. Invarient points are those at which all the phases coexist at equilibrium and the system is completely defined. At the invarient point all the phases namely 4 phases liquid(saturated solution), water vapour, gypsum and anhydrite all coexist.
c. Univarient reactions are those which are represented by one dimensional line or curve on a pressure temperature plot.
Univarient reactions possible if these 3 minerals coexist only with water vapour. 3 reactions are possible
gypsum+anhydrite+water vapour
gypsum+liquid+water vapor
gypsum+liquid+anhydrite