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Crystallisation Questions

Crystallisation is a process in which solid crystals are formed from a liquid phase. An atom or molecule arranges itself in a well-defined three-dimensional lattice known as a crystal. It also minimises the system’s overall energy.

We can typically classify crystallisation into two classes, i.e. evaporative and cooling crystallisation.

Definition: Crystallisation is a purification technique under which an atom or molecule arranges itself in a well-defined three-dimensional lattice and minimises the system’s overall energy.

Crystallisation Chemistry Questions with Solutions

Q1. Which of the following facilitates crystallisation?

(a) Adiabatic evaporation and cooling

(b) Solution is concentrated by evaporation

(c ) Hot concentrated solution is cooled for crystals to form

(d) All of the above

Answer: (d) Adiabatic evaporation, cooling and cooling of hot concentrated solution to form crystals facilitates crystallisation.

Q2. What is the name of the substance containing water of crystallisation?

(a) Anion

(b) Hydrate

(c ) Buffer

(d) None of the above

Answer: (b) Hydrate contains water of crystallisation.

Q3. Which of the following is not an essential criterion for crystal formation?

(a) Moisture content

(b) Caking characteristics of the crystal

(c ) Shape

(d) None of the above

Answer: (d) Moisture content, caking characteristics of the crystal and shape are the essential criterion for crystal formation.

Q4. Which of the following salts does not contain water of crystallisation?

(a) Blue vitriol

(b) Washing soda

(c ) Baking soda

(d) All of the above

Answer: (c ) Washing soda does not contain water of crystallisation.

Q5. Which of the following parameter affects the rate of crystallisation?

(a) Purity of solution

(b) Agitator speed

(c ) Population density

(d) All of the above

Answer: (d) Purity of solution, agitator speed and population density affect the rate of crystallisation.

Q6. Which of the following tells the significance of crystallisation in food processing?

(a) Physical stability of the food

(b) Consistency of food emulsions

(c ) A good feel of food inside the mouth

(d) All of the above

Answer: (d) Physical stability of the food, consistency of food emulsions and a good feel of food inside the mouth tells about the significance of crystallisation in food processing.

Q7. What is crystallisation? Why is it important?

Answer: Crystallisation is a purification technique under which an atom or molecule arranges itself in a well-defined three-dimensional lattice and minimises the system’s overall energy.

It is essential because it enables us to prepare substances of the highest purity.

Q8. What is the water of crystallisation?

Answer: Water of crystallisation is the substantial number of water molecules present in a loose combination with a formula unit of the compound.

crystallisation

Q9. What are the different steps involved in crystallisation?

Answer: Crystallisation is a purification technique under which an atom or molecule arranges itself in a well-defined three-dimensional lattice and minimises the system’s overall energy. Various steps are involved in the process of crystallisation. They are

1. Preparation of the solution

2. Filtration of the solution

3. The concentration of the solution (by heating)

4. Slow cooling of the solution (to form crystals, i.e. crystallisation)

5. Separation and drying of the crystals

process of crystallisation

Q10. What happens when the following crystals are heated individually?

(a) Benzoic acid

(b) Potash alum

(c ) Blue vitriol

Answer: (a)When the crystals of benzoic acid are heated, they will undergo sublimation.

(b) When the crystals of potash alum are heated, they turn into a fluffy white mass.

(c ) When the crystals of blue vitriol are heated, they will change into a white powder due to the loss of water of crystallisation.

Q11. What are the applications of crystallisation?

Answer: Crystallisation is an essential purification technique under which an atom or molecule arranges itself in a well-defined three-dimensional lattice and minimises the system’s overall energy. It plays a vital role in

1. Separation of salt from seawater

2. Separation of alum crystal from an impure alum sample

3. Synthesis and isolation of co-crystals, controlled release pulmonary drug delivery, pure active pharmaceutical ingredients (API), and separating chiral isomers.

Q12. What are the benefits of crystallisation?

Answer: Crystallisation is an essential purification technique under which an atom or molecule arranges itself in a well-defined three-dimensional lattice and minimises the system’s overall energy. It has a lot of advantages. A few of them are enlisted below.

1. It enables us to obtain a high purity compound in a single step.

2. The energy requirement and the operating temperature of crystallisation are fairly low.

3. The parched products obtained from crystallisation can be directly packaged and stored.

Q13. Match the following.

Column I

Column II

Alloying

Exothermic change

Crystallisation

Periodic change

Soap powder + Water

Mixing of molten solids

Seasonal changes

Pure solid

Answer:

Column I

Column II

Alloying

Mixing of molten solids

Crystallisation

Pure solid

Soap powder + Water

Exothermic change

Seasonal changes

Periodic change

Q14. What are the two types of crystallisation?

Answer: Crystallisation is an essential purification technique under which an atom or molecule arranges itself in a well-defined three-dimensional lattice and minimises the system’s overall energy. We can classify crystallisation into two classes, namely.

1. Evaporative crystallisation

2. Cooling crystallisation

Q15. Differentiate between evaporation and crystallisation.

Answer:

S. No.

Evaporation

Crystallisation

1.

Evaporation converts a liquid into its gaseous phase at a specific high temperature.

Crystallisation is the formation of crystals and may occur as either natural or artificial processes.

2.

Evaporation is the formation of vapour from a liquid.

Crystallisation is the formation of solid crystals from a liquid.

3.

It can remove a more volatile substance from a liquid mixture.

It can remove a solid substance from a liquid.

4.

It is less efficient because some solid decomposes on heating.

It is used in separating pure solid from liquid with high efficiency.

Practise Questions on Crystallisation

Q1. Why is the solution not heated to dryness to get crystals?

Q2. Why is the hot saturated solution not cooled suddenly?

Q3. What are three factors that affect the rate of crystallisation?

Q4. How does temperature affect the rate of crystallisation?

Q5. Distinguish between precipitation and crystallisation.

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