1st Year Chemistry Chapter 2

CHAPTER NO.
2
EXPERIMENTAL TECHNIQUES IN CHEMISTRY
Q.1. Mention the major steps involved in crystallization?
Ans: The major steps involved in crystallization are:
1. Preparation of saturated solution.
2. Choice of solvent.
3. Filtration of impurities.
4. Cooling of hot filtered solution.
5. Collecting of crystals.
6. Drying of crystals.
7. Decolourization of crystals.
Q.2: Mention various experimental techniques which are used for the purification of
substances?
Ans: The experimental techniques which are used for the purification of substances
are:
1. Filtration 4. Solvent Extraction
2. Crystallization 5. Chromatography
3. Sublimation
Q.3: Why is there a need to crystallize the crude products?
Ans: The crude product is impure, therefore, there is a need to purify it by
crystallization from a suitable solvent.
Q.4: Which solvents are mostly used in crystallization?
Ans: The solvents which are mostly use for crystallization are:
1. Water 2. Rectified spirit 3. Absolute alcohol
4. Ether 5. Acetone 6. CCl4
7. Chloroform 8. Acetic acid 9. Petroleum ether
Q.5: Why solid iodine sublimes? Give reason.
Ans: Solid Iodine has only dispersion force, which are the weakest force. The vapour
pressure of iodine is also low. Therefore, solid iodine sublimes.
Q.6: What is the role of stationary phase in chromatography?
Ans: The role of stationary phase in chromatography is to attract the components of
the mixture and allow then pass over it with different strengths of attraction?
Q.7: What is chromatography? Give its two types.
Ans: Chromatography:
Chromatography is a technique which is used for the separation of components
of a mixture. It involves the distribution of a solute between a stationary phase
and a mobile phase.
Types:
These are two types of chromatography. These are:
1. Adsorption chromatography
2. Partition chromatography
Q.8: How the decolourization of undesirable colours is carried out for freshly
prepared crystalline substances?
Ans: The decolourization of undesired crystals are carried out by boiling the
substance with sufficient amount of animal charcoal in the solution and then
XI Chemistry Chapter 2 - By: Muhammad Ashfaq - For: www.notespk.com

filtering the hot solution coloured impurities are adsorbed by the charcoal and
pure substance crystal are out on cooling.
Q.9: What is Rf Value? Give its formula?
Ans; Rf Value:
Rf stands for retardation factor. Each component in the mixture has a specific
retardation factor called Rf value. The Rf value is related to the distribution
coefficient and is given by:
Formula:
Rf = Distance traveled by a component from the original spot
Distance traveled by the solvent from the original spot
Q.10: What is solvent extraction?
Ans: A technique in which solute is separated from a solution by shaking the solution
with another solvent in which the solute is more soluble and the added solvent
does not mix with the solution is called solvent extraction.
Q.11: Define sublimation with an example?
Ans: Sublimation:
The process is which a solid, when heated, vapourizes directly without passing
through the liquid state is called sublimation.
Example:
Naphthalene, NH4Cl, Iodine and camphor undergo sublimation.
Q.12: What is crystallization?
Ans: The process in which a dissolved solute comes out of solution and form a
crystalline solid is called crystallization.
Q.13: Rf value is always less than 1.0 comment on it?
Ans: The Rf value is related to the distribution coefficient and is given by:
Rf = Distance traveled by a component from the original spot
Distance traveled by the solvent from the original spot
Since the distance traveled by solvent from the original spot is greater and is in
the denominator than the distance traveled by a component which is in the
numerator, therefore Rf value is always less than 1.0.
Q.14: How vacuum desicator is used to dry the crystals?
Ans: The crystals are spread over a watch glass and kept in a vacuum desicator for
several hours. The drying agents used in a desicator are CaCl2, silica gel or P2O5.
Q.15: What is adsorption chromatography?
Ans: Chromatography in which stationary phase is solid, is called adsorption
chromatography. In adsorption chromatography, a substance leaves the mobile
phase to become adsorbed on the surface of solid phase. e.g. column chromatography.
Q.16: What should be the size of stem of the funnel used for filtration?
Ans: The stem of funnel should be long enough to extend in a few centimeters down
into receiving beaker and the tip should touch the side of the beaker to avoid
splashing.
Q.17: Why Rf Value has no units?
Ans: Rf value has no unit, because it is a ratio and the units in numerator and the
denominator cancel each other.
Q.18: Give the man characteristics of the solvent used for crystallization.
Ans: An ideal solvent should have the following characteristics.

1. The solvent should dissolve a large amount of the substance at its boiling
pint but only a small amount at room temperature.
2. It should not react chemically with the solute.
3. It should not dissolve the impurities.
4. It should be cheap.
5. On cooking it should form well-developed crystals of the pure compound.
6. It should be easy to remove.
7. it should be save to use.
Q.19: How we can run the process of filtration smoothly?
Ans: To run the process of filtration smoothly, the stem of the funnel should remain
continuously full of liquid as long as there is liquid in the conical portion. The
stem of the funnel should be long enough and the tip should touch the side of the
beaker to avoid splashing.
Q.20: Differentiate between stationary phase and mobile phase?
Ans: Stationary Phase:The phase over which the mobile phase flows is called
stationary phase.
Mobile Phase: The solvent or mixture of solvents used for the separation of
components of a mixture in chromatography is called mobile phase.
Q.21: Differentiate between sublimation and condensation?
Ans: Sublimation: The process in which solid is directly converted into vapour
without passing through a liquid state is called sublimation.
Condensation: The change of gas to either the liquid or the solid state is called
condensation.
Q.22: How does Gooch crucible increase rate of filtration?
Ans: It is made of porcelain having a perforated bottom which is covered with paper
pulp. For quick filtration, Gooch crucible is placed in a suction filtering
apparatus to increase the rate of reaction.
Q.23: Define adsorption and portion chromatography?
Ans: Adsorption Chromatography: Chromatography in which stationary phase is solid
is called adsorption chromatography. e.g. column chromatography.
Partition Chromatography: Chromatography in which stationary phase is
liquid is called partition chromatography. e.g. paper chromatography.
Q.24: How naphthalene can be purified?
Ans: Naphthalene can be purified by sublimation process. Impure naphthalene is
placed in watch glass covered with an inverted funnel and open and is closed
with a cotton plug. Naphthalene is then heated slowly over a sand bath and the
funnel is cooled with wet cotton. The pure naphthalene deposits on the inner
side of the funnel and thus naphthalene is purified.

1st Year Chemistry Chapter 2

Uploaded by

Copyright:

Available Formats

1st Year Chemistry Chapter 2

Uploaded by

Document Information

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

1st Year Chemistry Chapter 2

Uploaded by

Copyright:

Available Formats

CHAPTER NO.

XI Chemistry Chapter 2 - By: Muhammad Ashfaq - For: www.notespk.com

XI Chemistry Chapter 2 - By: Muhammad Ashfaq - For: www.notespk.com

XI Chemistry Chapter 2 - By: Muhammad Ashfaq - For: www.notespk.com

You might also like