Unit 15: Basic Separation Techniques — Short Questions

Ans. Usually during a chemical reaction when products are obtained they are mixed with some amount of reactant and side products, such a product is called crude product. As the crude product is impure therefore crystallization must be carried out to purify it.

Ans. Filter paper is folded in accordian shape to get the fluted filter paper which has alternative elevations and depressions. Rate of filtration through conical funnel is considerably increased using fluted filter paper.

Ans. Gooch crucible | Sintered glass crucible
It is made of porcelain with perforated bottom. | It is made up of glass with a porous glass disc sealed into the bottom.
Pores in bottom are covered with paper pulp, filter paper cut to its size or asbestos mat. | No such preparation is needed.
Useful for precipitate need to be ignited at high temperature. | Convenient to use as no preparation is needed.

Ans.
(i) Gooch crucible is useful in separating mixtures need to be ignited at high temperature.
(ii) If its perforations are covered with asbestos mat, it may be used to filter solutions that react with paper e.g. concentrated HCl and KMnO4 solutions.

Ans. For fractional distillation of petroleum, a fractionating tower is used. Different fractions are obtained at different sites along the column depending on their boiling points. The fraction with lowest boiling point is collected at the top of tower.

Ans. The stationary phase may be a solid or a liquid supported as a thin film on the surface of an inert solid. Water entrapped in cellulose fibers act as stationary phase

Ans. If the components of mixtures have a comparable attraction for stationary phase the will travels equal distances along the chromatographic paper and appear as a single dot and will not be separated.

Ans. Developing the chromatogram means immersing the paper in mobile phase. Moving the mobile phase over the stationary phase due to capillary action, drying the paper and identifying the compounds.

Ans. Chromatography is based on the principle that distance travelled by components of mixture depends on the distribution between stationary and mobile phase.

Ans.
(i) Water has high polarity comparable to that of cellulose fibers.
(ii) Usually the focus of chromatography is organic compounds which are usually insoluble in water.

Ans. Adsorption chromatography | Partition chromatography
Stationary phase in this type of chromatography is solid. | Stationary phase in this type of chromatography is solid.
Substance leaves the mobile phase to get adsorbed on the surface of stationary phase. | Substance being separated spread throughput stationary and mobile phase.
Example Thin layer chromatography (TLC) Column chromatography. | Example Paper chromatography Gas liquid chromatography.

Ans. Paper chromatography is a convenient way of checking if a substance formed in a chemical reaction is pure or impure using a suitable solvent in which the substance dissolves. A pure substance produces one spot on the chromatogram while an impure substance produces more than one spot.

Ans. Glucosazone is a yellow colored crystal its purity can be checked by
(i) Chromatography: Only one spot will be produced by pure glucosazone
(ii) Melting point: If sample has a sharp and fixed melting point of 204°C than it pure. If it gives any other temperatures or range of temperatures it is an impure sample.

Ans. A complete quantitative determination generally consists of four major steps.

• Obtaining a sample for analysis
• Separation of the desired constituent
• Measurement, and calculation of results
• Drawing conclusion from the analysis.

Ans. The components of a mixture can be separated by any of the following physical methods.

• Filtration
• Crystallization
• Simple distillation
• Fractional distillation
• Paper Chromatography

Ans. Solute The substance that is present in lesser amount in a solution is called solute.
Example: Salt in solution is solute.
Solvent: The substance that is present in a larger amount in a solution is called solvent.
Example: In salt solution, water is called solvent.

Ans. Solution A homogeneous mixture of two or more substances.
Example: solution of salt and water.
Residue: The precipitates (sand) left behind on the funnel and collected at filter paper after the filtration is called residue.
Example: Sand remains on the filter paper after filtration. When mixture of sand and water is filtered, sand is residue.
Filtrate: The purified substance that is obtained after filtration is called filtrate.
Example: Water is filtrate after mixture of sand and water is filtered.

Ans. Separation techniques are methods used to separate different components of a mixture.
They work on differences in physical properties such as boiling point, density, or solubility.
These techniques help in obtaining pure substances.
Examples: Filtration, distillation, and chromatography etc.

Ans. The process that is used to separate one of the components of a two-component heterogeneous mixture from a solution is called filtration.
Following are the methods to increase rate of filtration.

• Using fluted filter paper
• Using suction filtration

Ans. Add water to dissolve the salt. Filter to remove the sand. Evaporate the water from the salt solution. Pure salt crystals are obtained.

Ans.

• It removes suspended solids from water.
• It makes water clearer and safer to drink.
• It is usually done after sedimentation.
• It prevents clogging in later treatment stages.

Ans. If the perforations of gooch crucible are covered with asbestos mat, it may be used to filter solutions that react with paper e.g. concentrated HCl and KMnO4 solutions.

Ans. Quick filtration can be done by placing such crucible in a suction filtering apparatus. It is useful for the filtration of precipitates, which need to be ignited at high temperature. If its perforations are covered with asbestos mat, it may be used to filter solutions that react with paper e.g. concentrated HCl and KMnO4 solutions.

Ans. Filter papers are available in a variety of porosities (pore sizes). Which pore size is to be used, depends upon the size of particles in the precipitate.

Ans. Whatmann's filter paper which are used in paper chromatography are made from specially selected cotton cellulose. Whatmann's filter paper No. 1 is used for routine applications and it has medium retention and medium flow rate. Its pore size is 11 μm.

Ans. Sometimes during the preparation of a crude substance, the coloring matter or resinous products affect the appearance of product. Such impurities are conveniently removed by boiling the substance in the solvent with the sufficient quantity of finely powdered animal charcoal and then filtering the hot solution. The colored impurities are adsorbed by animal charcoal and the pure decolorized substance crystallizes out from the filtrate on cooling.

Ans. Crystallization is the process of forming pure solid crystals from a solution. A hot saturated solution is cooled to allow crystals to form.
It gives more pure solids than evaporation.
It is used to purify sugar, salt, and copper sulfate.

Ans. If the solvent is inflammable, then precaution should be taken while heating the solution so that it does not catch fire. In such cases, water bath is used for heating purpose.

Ans. Vacuum desiccator is used for temperatures sensitive crystals. In this process the crystals are spread over a watch glass and kept in a vacuum desiccator for several hours. The drying agents used in a desiccator are
CaCl2,
Silica gel
Phosphorus pentaoxide

Ans. During crystallization, sometimes the crystals start to appear during the process of filtration. This result in choking the filter paper or the runnel. To avoid this a hot water funnel may be used.

Ans. Petroleum in natural form is a mixture of many compounds which are commonly used as fuels. Examples of such fuels are petrol, diesel, kerosene oil and furnace oil. These components have boiling points not very different from one another. These components are separated as fractions by fractional distillation using a fractionating or distillation column.

Ans. It produces pure nitrogen, oxygen, and argon.
Liquid air is fractionally distilled at low temperature.
Nitrogen boils off first due to lower boiling point.
Oxygen and argon are collected afterwards.

Ans. It has a fractionating column with surfaces for repeated condensation. This allows better separation of liquids with close boiling points.
It produces purer fractions.
It is used in petroleum and air separation.

Ans. Evaporation removes solvent and leaves behind solute.
Distillation recovers both solvent and solute.
Evaporation is simpler but less pure.
Distillation is more precise and controlled.

Ans. Thermometer reading

100°C ----
78°C ---- Ethanol distils
Water distils

time

When a mixture of ethyl alcohol and water is heated first ethyl alcohol will be distilled over because its boiling point is 78°C less than that of water whose boiling point is 100°C. As long as the alcohol is being distilled over, the temperature of the thermometer will remain at 78°C.

Ans. Distillation separates liquids based on differences in boiling points.
The mixture is heated until one component boils and vapourizes.
The vapour is then condensed back into liquid form.
It is used to get pure water from seawater.

Ans. Petroleum is separated by fractional distillation in large towers.
It is heated and vapours rise through the column.
Fractions condense at different heights based on boiling point.
Products include petrol, diesel, kerosene, and bitumen.

Ans. A very convenient way of visualizing the colorless chromatogram is to see it under the light of a UV lamp. The spots look colored under UV light. Alternatively, a locating agent like ninhydrin can be used to locale the spots. A locating agent is generally a chemical that reacts with the colorless substances like amino acids to give colored products that are visible for inspection.

Ans. It identifies substances in crime evidence.
Ink from documents can be analysed.
Blood and drug samples can be tested.
It helps in criminal investigations.

Ans. It is the solvent that moves up the paper.
It carries the mixture along with it.
Components travel at different rates.
This causes separation of the mixture.

Ans. It is the paper on which the mixture is placed.
It holds substances based on adsorption.
Different substances stick differently to the paper.
This helps in their separation.

Ans. The stationary phase is the filter paper.
The mobile phase is the solvent used.
Substances travel at different speeds based on solubility.
This results in separation of components.

Ans. Rf is the ratio of distance moved by the substance to distance moved by the solvent front. It is always less than 1
It helps identify unknown substances.
Different compounds have different Rf values.

Ans. Chromatography separates substances based on solubility and adsorption.
Paper chromatography uses paper as the stationary phase.
The solvent carries different substances at different rates.
It is used to separate pigments in ink.

Ans. It is the ratio of distance travelled by a component from the original to distance travelled by solvent from the original spot.
Rf = Distance travelled by a component from the original spot / Distance travelled by solvent from the original spot
Rf (A) = x/z Rf (B) = y/z

Ans. Separation is needed to remove unwanted materials from useful substances.
It helps in purifying chemicals for industrial and laboratory use.
It is important for safety, health, and quality control.

Ans. Air is cooled until it liquefies.
Fractional distillation is used at very low temperatures.
Nitrogen boils off first, followed by oxygen and argon.
It is used for industrial gases supply.

Ans. See Q4. from theory.

Ans. See Q3. from theory.

Ans. See Q5. from theory.