1. What is Crystallisation?
Answer:
Crystallisation is a process used to purify substances by separating solid particles from a solution. During this process, the particles arrange themselves into a structured lattice form, minimizing the system's energy.
2. What is water of Crystallisation?
Answer:
Water of crystallisation refers to the water molecules that become part of a crystalline structure during crystallisation. These molecules bond with the compound in a fixed ratio, helping the crystals form a specific shape and colour.
3. What are the steps of Crystallisation?
Answer:
The steps of crystallisation are:
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Heat a saturated solution of the substance to dissolve the solute.
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As the solvent evaporates, the solute will start to form.
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Once the solution cools, crystals of the solute will begin to form and accumulate.
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Filter out any remaining impurities, and the crystals are dried.
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The rate of cooling determines the size of the crystals; slower cooling yields larger crystals.
4. What are the methods used for purification?
Answer:
Some methods for purification include:
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Crystallisation
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Extraction
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Recrystallisation
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Filtration
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Chromatography
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Evaporation
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Distillation
5. What is the colour of benzoic acid crystals?
Answer:
Benzoic acid forms white, opaque crystals. The crystals are monoclinic, meaning they have three unequal axes.
6. What is the lattice and shape of a monoclinic crystal?
Answer:
Monoclinic crystals come in two forms: pinacoidal and prismatic. Their lattice consists of two unit cells: a primitive and a base-centred cell. The primitive cell has fractional lattice points at the corners, and the base-centred cell has a fractional lattice at the centre of the basal plane.
7. What is mother liquor?
Answer:
Mother liquor is the leftover solution after crystals have been removed during crystallisation. It is a saturated solution from which the desired component has crystallised out.
8. How to obtain crystals of alum?
Answer:
To obtain alum crystals, dissolve alum in warm water and keep adding more until no more can dissolve. Filter the solution, then let it cool. The rate of cooling determines the size of the alum crystals.
9. What is a saturated solution?
Answer:
A saturated solution is one where no more solute can dissolve in the solvent. Adding more solute will not result in dissolution. A saturated solution is created when enough solute is added to a solvent at a certain temperature, and no more can dissolve.
10. What is the application of Crystallisation?
Answer:
Crystallisation is used for:
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Purifying substances.
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Extracting pure alum from an impure solution.
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Purifying seawater.
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Manufacturing pharmaceutical products like co-crystals.
11. What is Kipp’s waste?
Answer:
Kipp’s waste is the mixture left after producing hydrogen sulphide (H₂S) gas, typically formed by the reaction between FeS and H₂SO₄. It consists mainly of FeSO₄ and unreacted diluted H₂SO₄.
12. How to form crystals of benzoic acid?
Answer:
To form crystals of benzoic acid, dissolve it in hot water until the solution becomes saturated. After filtering, allow the solution to cool, and the opaque white crystals will form.
13. What is seeding?
Answer:
Seeding is the process where a small crystal of the same substance is added to a supersaturated solution to initiate crystal formation. This speeds up the crystallisation process.
14. What is green vitriol?
Answer:
Green vitriol, or copperas, is the hydrated form of ferrous sulphate (FeSO₄·7H₂O). It is used for treating iron deficiency and appears as blue-green crystals.
15. What happens when blue vitriol crystals are heated?
Answer:
When blue vitriol (CuSO₄·5H₂O) is heated, it loses its water of crystallisation and becomes anhydrous copper sulphate (CuSO₄), as shown by the equation:
CuSO₄·5H₂O → CuSO₄ + 5H₂O.
16. What is sublimation?
Answer:
Sublimation is the process in which a solid directly transitions into a gas, bypassing the liquid phase. For example, when benzoic acid is heated, it undergoes sublimation.
17. What precautions should be taken during Crystallisation?
Answer:
The following precautions should be observed during crystallisation:
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Wash crystals to remove impurities.
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Stir the solution continuously to ensure proper dissolution of the solute.
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Allow the solution to cool naturally, avoiding artificial cooling methods.
Marks Distribution for CBSE Class 11 Chemistry Viva Questions
The marks distribution for CBSE Class 11 Chemistry Viva Questions generally follows a structure that evaluates students on their understanding and practical knowledge. Although the exact distribution can vary, a breakdown might look like this:
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Experiment-based Questions
- 2-3 marks
Questions related to the specific experiment conducted by the student. This tests their understanding of the experimental procedure, observations, and the conclusions drawn.
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Theoretical Knowledge
- 2-3 marks
Questions testing the student’s theoretical knowledge of the concepts behind the experiment or the chemistry involved. This could include concepts from physical, organic, or inorganic chemistry.
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Application of Concepts
- 1-2 marks
Questions that require students to apply their theoretical knowledge to real-life situations or other related chemical processes.
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Precautions and Errors
- 1 mark
Questions that focus on understanding the precautions to be taken during the experiment and the possible sources of error in the practical.
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Demonstration or Clarification
- 1 mark
Sometimes, students may be asked to demonstrate or clarify the steps of the experiment, showing their hands-on understanding.
