This set of Solid State Chemistry Interview Questions and Answers focuses on “Crystallization of Solutions, Melts, Glasses and Gels”.
1. What are the starting materials for crystallization?
a) Gas, aqueous solution, foam
b) Liquid, aqueous solution, emulsion
c) Solid, gas, melt, solid aerosol
d) Aqueous solution, melt, glass or gel
Explanation: Aqueous solution, melt, glass or gel, is usually homogenous, single phase and amorphous. This may greatly facilitate formation of crystalline product, since long range diffusion of ions may not be necessary and the product my form at lower temperature.
2. What is the range of temperature for the crystallization process in ◦C?
Explanation: Crystalline phases are prepared at low temperature range such as 25-100 degree Celsius whereas much high temperatures would be required for a normal solid state synthesis. Crystals usually form at low temperatures but due to the release of heat of fusion during crystallization, the entropy of the universe increases thus the second law of thermodynamics is not altered.
3. What are molecular sieve zeolites?
a) Aluminosilicate framework structures
b) Aromatic hydrocarbons
c) Aliphatic hydrocarbons
d) Silicate structures
Explanation: Zeolites are hydrated Aluminosilicate framework structures which contain large channels and cavities such that a variety of organic and inorganic substances may enter the zeolite structure.
4. What are the starting materials for zeolite synthesis?
a) Aqueous solution of silicate and aluminate cations
b) Aqueous solution of silicate and aluminate anions
c) Powdered silicon and aluminium metal
d) Non-nano silicon and aluminium metal
Explanation: Starting materials for zeolite structures are aqueous solution of silicate and aluminate anions which are mixed together with alkali. A gel forms by a process of copolymerization of the silicate and aluminate anions and on subsequent heat treatment, under a high water vapour pressure, crystals of zeolite are produced.
5. Crystallization of melts is very similar to crystallization of solutions, what is the factor that differentiates the crystallization of melt and solutions?
a) Size of the particles
b) Environmental factors
Explanation: Melts are high temperature liquids whereas solutions are liquids at low temperatures. Thus on melting together the solid starting materials, complete homogenization occurs and recrystallization takes place on subsequent cooling of the melt.
6. Crystallization of melts is a valuable method for which of the following number of crystals?
d) Polycrystals(more than 3)
Explanation: Crystallization of melts is a valuable method for growing single crystals since in the presence of liquid phase and in the absence of too many crystal nuclei, large crystals readily grow.
7. Lithium disilicate Li2Si2O5, can be called which of the following materials?
Explanation: Li2Si2O5 is a glass ceramic material, which is a crystalline material of glassy intermediates and an important component of several commercial glass-ceramics. Crystalline Li2Si2O5 melts at 1032oC.
8. What is the transition temperature region required during the preparation of glass-ceramic material, lithium disilicate in oC?
a) -25 to 0
b) 0 to 25
c) 25 100
d) 300 to 500
Explanation: Transition temperature is the temperature region where the polymer transition occurs from a hard, glassy material to a soft, rubbery material. Transition temperature required during the preparation of lithium disilicate is 450oC and on heating at 500 to 700oC crystallization of LI2Si2O5 occurs readily after few hours.
9. Which of the following is the type of reaction for the formation of gaseous PtO2 at 1200oC or higher?
a) Exothermic reaction
b) Endothermic reaction
c) Both exothermic and endothermic reaction
d) Neither exothermic nor endothermic reaction
Explanation: The formation of gaseous PtO2 at 1200oC or higher is an endothermic reaction which can be used for chemical transport. The PtO2 diffuses at lower temperatures where it may deposit well-formed crystals of platinum metal.
10. Which method is used for the purification of certain metals which uses metal and iodine to form a gaseous iodide?
a) Van Arkel method
b) Zone refining
Explanation: Van Arkel method is used for the purification of certain metals makes uses of an exothermic reaction between metal and iodine to form a gaseous iodide, example CrI2. Since the formation of metal iodide is exothermic, and so the metal is redeposited at a higher temperature.
11. An elegant variation of Van Arkel purification method is to transport two substances in__________
a) Same direction along the temperature gradient
b) Opposite direction along the temperature gradient
c) Same direction along the concentration gradient
d) Opposite direction along the concentration gradient
Explanation: Van Arkel method is a purification method which uses metal and iodine to form gaseous iodide, it transports two substances in opposite directions along the temperature gradient and this is possible when one reaction is exothermic and the other is endothermic.
12. During the preparation of Ca2SnO4, Cao and SnO2 react slowly according to the equation
Thus, in the presence of CO, the reaction may be greatly ______________
a) Speeded up
b) Slowed down
c) Affected but speed remains the same
Explanation: In the presence of CO the reaction is greatly speeded up since SnO2 converts to gaseous SnO and can be chemically transported according to the following equation –
SnO2(s) +CO ⇄ SnO(g) + CO2
The gaseous SnO subsequently reacts with CaO and CO2.
13. During the preparation of aluminium sulphide, Al2S3 what is the reaction rate at which aluminium and sulphur react together at 800oC?
a) React very slowly
b) React rapidly
c) Reacts and reaches equilibrium
d) Doesn’t react
Explanation: Aluminium and sulphur react very slowly during the preparation of aluminium sulphide since liquid Al becomes coated with a skin of Al2S3, which acts as a diffusion barrier to further reaction.
14. What will be the product formation when Metallic niobium and silica, SiO2 are heated under vacuum at 1100oC?
d) No product
Explanation: Metallic niobium and silica when heated at vacuum at 1100oC, they do not react together at all hence no product will be formed. But in the presence of traces of H2 gaseous SiO forms and migrates to the niobium.
15. What is the appropriate relation between the reactions of gases and solids?
a) Gases react slower than the solids
b) Gases react faster than the solids
c) Gases do not react at all while solids react faster
d) Both gases and solid reacts at a same rate
Explanation: Gases react much more quickly than do solids because mobilities are increased. In addition, the gaseous phase is often important in normal solid state reactions under isothermal conditions where it may act as a rapid means of transporting matter from one crystal to another.
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