CBSE Chemistry Experiment Viva Questions with Answers Basic Laboratory Techniques

The chemistry laboratory is the first place you should familiarise yourself with before learning scientific procedures and experimental skills. Some basic laboratory procedures frequently required during chemistry laboratory experiments include heating, filtration, crystallisation, decantation, volume measurements, and weighing solids and liquids. The process of crystallisation is one of the methods for purifying impure compounds, particularly if the original crude material obtained by the reaction is extremely impure. 

Filtration involves passing a liquid through a porous material to separate solids from liquids. This article covers both filtration and crystallisation along with chemistry lab viva questions with answers.

Table of Content

1. Aim

2. Theory

3. Apparatus required

4. Procedure

5. Observation

6. Result

7. Precautions

Aim

To purify the sample of a compound (copper sulphate, potash alum, or benzoic acid) by crystallisation.

Theory

As the first step in the process of crystallization, the crude material is dissolved readily when hot, but to a limited extent when cold, using a single or a mixture of solvents. A saturated solution is then obtained by dissolving the crude substance in a minimum amount of boiling solvent. Filtration removes insoluble impurities from hot solutions. Once the solution reaches its crystallisation point, it is cooled slowly, and the impurities are left behind.

Apparatus Required

• Beaker (250 ml)

• Glass funnel

• Tripod stand

• Porcelain dish

• Glass rod

• Sand bath

• Copper sulphate, potash alum, and benzoic acid

Procedure

• A saturated solution of potash alum/copper sulphate is prepared in a beaker with 30-50 ml of distilled water by stirring in small amounts of the impure solid sample. 

• When the solid has not dissolved further, stop adding it. 

• Hot water is used to prepare saturated solutions of benzoic acid. 

• Transfer the filtrate from the saturated solution into a porcelain dish. 

• In a sand bath, heat it until almost three fourth of the solvent has evaporated. 

• Blow air from your mouth as you take a glass rod out of the solution. 

• Stop heating the rod if a solid film forms. 

• Keep the porcelain dish undisturbed while cooling, covered with a watch glass. 

• After crystallisation, decant the mother liquor (the liquid left over after crystallization).

• Remove adhering mother liquor from the crystals of potash alum and copper sulphate obtained by washing them in ethanol containing a small amount of water, followed by washing them in alcohol to remove moisture. 

• Wash benzoic acid crystals with cold water. 

• The crystals should be dried between the folds of filter paper and stored at a safe and dry location after they have been dried.

• Repeat the following steps to get the maximum quantity of pure substance.

Observation

The shape of copper sulphate, potash alum, and benzoic acid are observed and noted down.

Result

The crystals of copper sulfate, benzoic acid, and potash alum are blue, greenish-white, and white, respectively.

Precautions

• The solution should not be evaporated completely while being concentrated.

• The solution should not be disturbed when it is getting cooled. 

Aim

To separate a solid from a liquid by filtration.

Theory

Filters may have different pore sizes. When a filter paper has large pores, liquid can pass through it more easily, resulting in a faster filtering process. It is also possible for solid particles of smaller sizes to pass through the filter. A filtering method and filtering material must be chosen according to the particle size of the material that needs to be retained on the filter paper.

Apparatus Required

• Funnel

• Beaker  

• Funnel stand

• Glass rod

• Filter paper

Procedure

• Once the circular filter paper has been folded in half, tear off a small piece from its corner and fold it once more. 

• Make a cone by folding the filter paper in half, three folds on one side and one on the other, so that the torn corner is on the outside. 

• Make sure the filter paper cone fits within one cm of the funnel's rim when fitting it into the funnel. 

• Apply the solvent, usually water, to the paper and adjust it so that it fits tightly onto the glass funnel's inner surface, with no air gaps. 

• Pour more water into the funnel until the stem is filled. 

• The filter paper in the funnel stem can support a column of water if fitted appropriately. 

• A mild suction is created by the weight of this column of water, which speeds up the filtration process.

• Suction Filtration: The above method of filtration can be sped up by using suction, which can be applied by using a vacuum pump or aspirator.

Filtration

Suction Filtration 

Observation

Residues are observed on the filter paper.

Result

Filtration results in the separation of solid materials from liquid through a porous material.

Precautions

• To avoid splashing, the stem of the funnel should contact the side of the beaker in which the filtrate is collected. 

• A portion of the filtered liquid may pass through the funnel into the beaker beneath the funnel to collect the filtrate if the liquid level rises above the cone.

Lab Manual Questions

1. What is the purpose of preparing a saturated solution before crystallising?

Ans: As a saturated hot solution cools, the solute becomes insoluble in the solvent and crystallises into pure compounds. So, a saturated solution is prepared before crystallisation.

2. What is the advantage of suction filtration over normal filtration?

Ans: The filtration process can be done at high speed by using the suction filtration method, which can be applied by using a vacuum pump or aspirator.

3. What are the processes of crystallization?

Ans: Crystallization starts with nucleation, the appearance of crystals from either supercooled liquids or supersaturated solvents. Secondly, crystal growth occurs, which is the process of increasing particle size and resulting in crystals.

4. What is seeding in crystallisation?

Ans: To facilitate crystallization, a small amount of the solid (which is being purified) is sometimes added to the solution during cooling, which is called seeding. As a result of the addition of a tiny crystal, new crystals grow around it.

Viva Questions

1. What is mother liquor?

Ans: Mother liquor is the liquid left behind after a substance crystallises.

2. What is a saturated solution?

Ans: A solution that has dissolved all the solutes it can dissolve is a saturated solution.

3. Which funnel is used in suction filtration?

Ans: Buchner funnel is used in suction filtration.

4. What is the role of glass rod during filtration?

Ans: Glass rod directs the flow of liquid into the funnel and prevents small amounts of liquid from running down the outside of the beaker when the transfer process is interrupted.

5. What is a sand bath?

Ans: The sand bath is a vessel of hot sand in a laboratory that partially immerses vessels that need to be heated.

6. Define filtration.

Ans: Filtration involves passing a solution through a filter paper to separate insoluble substances.

7. What is the colour of the benzoic acid crystals?

Ans: Benzoic acid crystals are an opaque white colour.

8. Why the hot saturated solution doesn't cool down suddenly?

Ans: During the cooling process of a saturated solution, crystals expand in size. To minimise the chances of small or disfigured crystals, rapid cooling should be avoided.

9. What is crystallisation?

Ans: Crystallization refers to the arrangement of the atoms/molecules of a substance in a well-defined three-dimensional lattice, which minimises the system's overall energy.

10. What is a funnel?

Ans: A funnel is a pipe or tube with a wide top and narrow bottom used to pour liquid or powder through a narrow opening and to hold filter papers.

Practical-Based Questions

1. Which of the following factor affects filtration?

1. Density

2. Viscosity

3. Temperature

4. All the above

Ans: D) All the above

2. For initial crystallisation, minute quantities of the substance are added to the solution during cooling. This is called

1. Seeding

2. Seedling

3. Filtration

4. Separation

Ans: A) Seeding

3. Which of the following can be used as a porous material during filtration?

1. Sintered glass

2. Paper

3. Asbestos

4. All the above

Ans: D) All the above

4. Insoluble particles are separated from liquids by which of the following processes??

1. Evaporation

2. Filtration

3. Crystallization

4. None of the above

Ans: B) Filtration

5. A filtering method and filtering material must be chosen according to the __.

1. Particle size

2. Color

3. Density

4. pH

Ans: A) Particle size

6. Copper sulphate crystals are ___ in colour?

1. Green

2. White

3. Blue

4. Orange

Ans: C) Blue

7. Benzoic acid crystals are washed with ___?

1. Alcohol

2. Cold water

3. Hot water

4. Methanol

Ans: B) Cold water

8. Which of the following is used for the evaporation of excess solvents to make a concentrated solution?

1. China dish

2. Measuring cylinder

3. Beaker

4. Burette

Ans: A) China dish

9. The porcelain dish is covered with a ___?

1. Filter paper

2. Asbestos sheet

3. Watch glass

4. Sintered glass

Ans: C) Watch glass

10. What is the remaining liquid after crystallization?

1. Saturated solution

2. Unsaturated solution

3. Mother liquor

4. None of the above

Ans: C) Mother liquor

Conclusion

The process of crystallisation is one of the methods for purifying impure compounds, particularly if the original crude material obtained by the reaction is extremely impure. Filtration involves passing a liquid through a porous material to separate solids from liquids. When a filter paper has large pores, liquid can pass through it more easily, resulting in a faster filtering process. Benzoic acid crystals are monoclinic, copper sulfate crystals and potash alum crystals are triclinic in geometry.