# Phase Transformation Questions and Answers – Equilibrium in Heterogeneous Systems

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This set of Phase Transformation Interview Questions and Answers focuses on “Equilibrium in Heterogeneous Systems”.

1. Which rule gives the relative number of moles of α and β that must be present, if the overall composition is given as some X? (α, β are stable forms of 2 substance)
a) Richard’s Rule
b) Raoul’s Rule
c) Henry’s Rule
d) Lever rule

Explanation: If the overall composition of the phase mixture is X, the lever rule gives the relative number of moles of α and β that must be present and the Gibbs free energy can be calculated from the molar energy curve for the phase.

2. Properties of metallic material primarily depends on _____________
a) Processing
b) Type of crystal structure
c) Microstructure
d) Grain size

Explanation: The properties of metallic material primarily depends on chemical composition and processing. This is the property that decides the microstructure that gets developed into the material.

3. The driving force for the phase transformation increases in proportion to__________
a) Microstructure
b) Change in composition
c) Degree of undercooling below critical temperature
d) Melting point

Explanation: It primarily depends on undercooling and that provides for the energy to create surface, enveloping the product phase. Similarly as you cool a single phase material below a solvus the driving force of precipitation increases so cooling is basically the driving force of many processes.
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4. In a general sense it is not possible to obtain any information about kinetics or the morphology of the phase mixture from the phase diagram.
a) True
b) False

Explanation: It is not possible to obtain any information about kinetics or the morphology of the phase mixture from the phase diagram. The diagram only gives information about the phases that can be in equilibrium under certain composition–temperature combinations. Although there are three different species present in a system, there are times when the phase diagram is presented as a binary phase diagram.

5. The first step in drawing the free energy curve of the FCC α phase is, to convert the stable BCC arrangement of β atoms into ____________
a) An unstable FCC arrangement
b) A stable FCC arrangement
c) A metastable BCC arrangement
d) An unstable BCC arrangement

Explanation: The first step is to convert it into an unstable FCC arrangement, since was in a stable state it requires the optimum energy to continue in that state but to convert it into an unstable FCC arrangement, we require an increase in free energy.

6. Which among the following condition the most stable state of a system should satisfy so that it can be defined as a state in which pure component has unit activity?
a) Highest free energy
b) Lowest free energy
c) Change in Gibbs energy 0
d) Positive free energy

Explanation: The most stable state, with the lowest free energy, is usually defined as the state in which the pure component has unit activity. The definition of activity can be shown graphically.

7. In heterogeneous systems containing more than one phase the pure components can, at least theoretically, exist in different crystal structures.
a) True
b) False

Explanation: In a heterogeneous system containing more than one phase the pure substance can exist in different crystal structures, the definition of heterogeneous system itself gives an idea about the statement.

8. Calculate the number of phases present in a system in equilibrium, if the degree of freedom is 1, the number of components and non-compositional variables are 1 and 2 respectively?
a) 1
b) 3
c) 0
d) 2

Explanation: The Gibbs phase rule explains the number of phases that will be present in a system in equilibrium and is expressed as F+P = N+C. So here the F value is given as 1 and the N, C as 1 and 2 respectively, hence the P = 2.

9. What happens when the two Gibbs energy curves for the α- and β-phases have a common tangent and the equilibrium has been established?
a) Driving force for diffusion increases
b) Driving force for diffusion decreases
c) Driving force for diffusion remains constant
d) Driving force first decreases then increases

Explanation: As the diffusion proceeds, the driving force for diffusion gradually decreases and vanishes when the chemical potentials of the components (A and B) become equal in both phases.

10. The condition for equilibrium in a heterogeneous system containing two phases can also be expressed using the_____________
a) Lever’s concept
b) Raoults rule
c) Henrys rule
d) Activity concept

Explanation: The same activity concept defined for homogeneous system can be used to state the above mentioned condition. From the definition of activity coefficient, for an ideal solution, its value is unity for all the species. For a non-ideal solution, however, it may be either greater or less than unity, the larger the departure from unity the greater the non-ideality of the solution.

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