Food Processing Unit Operations Multiple Choice Questions on “ Crystallization – Nucleation Mechanism”.
1. What is nucleation in crystallization?
a) It is the initial step in crystallization
b) It is the final step in crystallization
c) It is the step where nucleus is added
d) It is the sub category crystallization
Answer: a
Clarification: Nucleation is the initial process that occurs in the formation of a crystal from a solution, a liquid, or a vapor, in which a small number of molecules arranged in a pattern.
2. How to initiate crystallization?
a) Adding seed crystals
b) Adjusting temperature
c) Adjusting pressure
d) Adjusting concentration
Answer: a
Clarification: Crystallization can be spontaneous or can be done by addition of small amount of the seed crystal or the pure compound to the saturated solution.
3. How does temperature affect nucleation?
a) Nucleation is directly proportional to temperature
b) Nucleation is indirectly proportional to temperature
c) Nucleation and temperature are always at equilibrium
d) Nucleation and temperature does not depend on each other
Answer: b
Clarification: The decrease in nucleation centers at high temperature interprets that decrease in supersaturation, would increase in critical size of nucleus and free energy of nucleus formation.
4. What is nucleation in freezing?
a) Process of gathering of molecules into tiny clusters
b) It is the final step in freezing
c) It is the step where nucleus is added
d) It is the sub category freezing
Answer: a
Clarification: Nucleation is a process where the molecules or the atoms in a liquid start to gather into tiny clusters, arranging to form the crystal structure of the solid.
5. Which of the following statements about primary nucleation are correct?
Statement 1:The appearance of the first crystal is called primary nucleation time.
Statement 2:It describes the transition to a new phase.
a) True, False
b) True, True
c) False, True
d) False, False
Answer: b
Clarification: The time until the presence of the first crystal is called primary nucleation time, it refers to the first nucleus to form. It describes the transition to a new phase which does not rely on the new phase which is already present.
6. What is homogeneous nucleation?
a) Spontaneously generated nuclei
b) Randomly generated nuclei
c) Orderly generated nuclei
d) Randomly and spontaneously generated nuclei
Answer: d
Clarification: Homogeneous nucleation is the process by which nuclei that are randomly and spontaneously generated grow irreversibly to form a new phase.
7. What is seeding in crystallization?
a) It is the initial step in crystallization
b) It is the final step in crystallization
c) It is the step where nucleus is added
d) It is the sub category crystallization
Answer: c
Clarification: Crystal seeding is the process of adding homogeneous or heterogeneous crystals to a crystallizing solution to nucleate and grow more crystals.
8. What are the factors that affect crystallization seedling?
a) Temperature and pressure
b) Polymorphism of the crystals and pressure
c) Crystal size distribution and temperature
d) Crystal size distribution and polymorphism of the crystals
Answer: d
Clarification: Crystal seeding is the technique to add homogeneous or heterogeneous crystals to a crystallizing solution for nucleating and growing of more crystals. This is done by controlling the polymorphism of the crystals that are formed and crystal size distribution.
9. Which of the following statements about secondary crystallization are correct?
Statement 1:Secondary nucleation is the formation of nuclei from pre-existing crystals.
Statement 2: Only secondary nucleation increases the number of crystals in the system.
a) True, False
b) True, True
c) False, True
d) False, False
Answer: a
Clarification: Secondary nuclei are crystal nuclei that are produced from a pre-existing crystal. Primary and secondary nucleation increases the number of crystals in the system and depends on crystals which are already present.
10. Is supersaturation the driving force for all solution crystallization processes?
a) True
b) False
Answer: a
Clarification: Supersaturation is the driving force for all solution crystallization processes where the supersaturated solution spontaneously separate into pure crystals of the solute and a less concentrated solution.