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METHODS OF SEPARATING MIXTURES

METHODS OF SEPARATING MIXTURES

Mixtures can be separated from applying the following methods:

(A) DECANTATION

Sediments can be separated from a liquid by pouring out the liquid. This process is called decantation.

Experiment

Put some sand in a beaker. Add about 200cm3 of water. Allow sand to settle. Pour off water carefully into another beaker.

Observation

Sand settles at the bottom as sediments. Less clean water is poured out.

Explanation

Sand does not dissolve in water. Sand is denser than water and thus settles at the bottom as sediment. When poured out, the less dense water flows out.

 

(B) FILTRATION

Decantation leaves suspended particles in the liquid after separation. Filtration is thus improved decantation. Filtration is the method of separating insoluble mixtures/particles/solids from a liquid.

Experiment:

To separate soil and water using filtration. Fold a filter paper to fit well into a filter funnel. Place the funnel in an empty 250cm3 beaker. Put one spatula end full of soil into 50cm3 of water. Stir. Put the soil/water mixture into the filter funnel.

Observations

Clean water is collected below the filter funnel. Soil remains above the filter paper.

Explanation

A filter paper is porous which act like a fine sieve with very small holes. The holes allow smaller water particles to pass through but do not allow bigger soil particles. The liquid which passes through is called filtrate. The solid which do not pass through is called residue.

 

(C) EVAPORATION

Evaporation is a method of separating a solute/solid from its solution. This involves heating a solution (solvent and solute)to vapourize the solvent out of the solution mixture leaving pure solute/solid. If a mixture contain insoluble solid, they are filtered out.

Experiment: To separate a mixture of soil and salt (sodium chloride) .

Procedure:

Put one spatula end full of soil on a filter paper. Put one spatula full of common salt/sodium chloride into the same filter paper. Mix well using the spatula,. Place about 200cm3 of water into a beaker.

Put the contents of the filter paper into the water. Stir thoroughly using a glass/stirring rod for about one minute. Fold a filter paper into a filter funnel.

Pour half portion of the contents in the beaker into the filter funnel. Put the filtrate into an evaporating dish. Heat on a water bath.

 

Observation

(i) On mixing

Colourless crystals and brown soil particles appear on the filter paper.

(ii) On adding water

Common soil dissolves in water. Soil particles do not dissolve in water.

(iii) On filtration

Colourless liquid collected as filtrate below the filter funnel/paper.

Brown residue collected above the filter funnel/paper.

(iv) On evaporation

Colourless crystals crystals collected after evaporation

 

Explanation

Solid mixture of sand and common salt take the colours of the two. On adding water,common salt dissolve to form a solution .

Soil does not because it is insoluble in water and thus forms a suspension. On filtration, a residue of insoluble soil does not pass through the filter paper. It is collected as residue.

Common salt solution is collected as filtrate. On heating the filtrate, the solvent/water evaporate/vapourize out of the evaporating dish leaving common salt crystals. Vapourization/evaporation can take place even without heating.

This is the principle/process of drying wet clothes on the hanging line.

Set up of apparatus

 

(D) DISTILLATION

Distillation is an improved evaporation where both the solute and the solvent in the solution are separated /collected.

Distillation therefore is the process of separating a solution into constituent solid solute and the solvent.

It involves heating the solution to evaporate/vapourize the solvent out. The solvent vapour is then condensed back to a liquid.

 

Experiment: To obtain copper (II) sulphate(VI) crystals and water from copper(II)sulphate(VI) solution.

Procedure:

Put one spatula end full of copper (II) sulphate (VI) crystals into a 250cm3 beaker. Place about 200cm3 of water into the beaker.

Stir thoroughly using a glass/stirring rod for about one minute.

Pour half portion of the contents in the beaker into a round bottomed/flat/conical flask broken porcelain/sand/glass into the flask.

Put a few pieces of b Stopper the flask.

Connect the flask to a liebig condenser using delivery tube.

Place a 200cm3 clean empty beaker/conical flask as a receiver at the end of the liebig condenser.

Circulate water in the liebig condenser.

Heat the flask strongly on a tripod stand with wire mesh/gauze until there is no more visible boiling bubbles in the flask.

 

Observation

Copper(II) sulphate (VI) crystals dissolve in water to form a blue solution.

On heating, colourless liquid is collected in the receiver.

Blue crystals are left in the flask.

(if gently heated further, the blue crystals turn to white powder)

 

Explanation

On heating blue Copper(II) sulphate (VI) solution, the colourless liquid solvent evaporate/vapourize .

The liquid vapour/gas passes through the delivery tube to the liebig condenser.

The liebig condenser has a cold water inlet near the receiver and cold water out let.

This ensures efficient cooling. If the cold water outlet/inlet is reversed, the water circulation would be less efficient.

The water in the receiver would be warm.In the liebig condenser, the cold water,condenses the liquid vapour into liquid.

The condensed liquid collects in the receiver as distillate.

The solute of blue Copper(II)sulphate (VI) crystals is left in the flask as residue.

During simple distillation,therefore, the solution is heated to vapourize /evaporate the solvent/one component which is condensed at a different part of the apparatus.

 

The purpose of pieces of broken porcelain/porous pot/glass/sand/ is to:

(i) Prevent bumping of the solution during boiling.

(ii) Ensure smooth and even boiling.

Salty sea water can be made pure through simple distillation.

Any mixture with a large difference /40oC in boiling point can be separated using simple distillation.

Set up of apparatus

 

(E) FRACTIONAL DISTILLATION

Fractional distillation is an improved simple distillation used specifically to separate miscible mixtures with very close /near boiling points.

Fractional distillation involves:

(i) Heating the mixture in a conical/round bottomed/flat bottomed flask.

The pure substance with a lower boiling point and thus more volatile evaporates/boils/vapourizes first. e.g.

Pure ethanol has a boiling point of 78oC.Pure water has a boiling point of 100oC at sea level/one atmosphere pressure.

When a miscible mixture of ethanol and water is heated, ethanol vapourizes/boils/evaporates first because it is more volatile.

(ii) The conical/round bottomed /flat bottomed flask is connected to a long glass tube called fractionating column.

The purpose of the fractionating column is to offer areas of condensation for the less volatile pure mixture.

The fractionating column is packed with glass beads/broken glass/porcelain/shelves to increase the surface area of condensation of the less volatile pure mixture.

(iii) When the vapours rise they condense on the glass beads/broken glass /porcelain / shelves which become hot.

When the temperature of the glass beads/broken glass/porcelain/shelves is beyond the boiling point of the less volatile pure substance, the pure substance rise and condensation take place on the glass beads/broken glass/porcelain/shelves at a higher level on the fractionating column.

The less volatile pure substance trickles/drips back down the fractionating column or back into the conical/round bottomed /flat bottomed flask to be heated again. e.g.

If the temperature on glass beads/broken glass/porcelain/shelves is beyond 78oC, the more volatile pure ethanol rise to condense on the glass beads/broken glass/porcelain/shelves higher in the fractionating column.

Water condenses and then drip/trickle to the glass beads/broken glass /porcelain /shelves lower in the fractionating column because it is less volatile.

(iv) The fractionating column is connected to a liebig condenser.

The liebig condenser has a cold water inlet and outlet circulation.

The more volatile mixture that reach the top of the fractionating column is condenses by the liebig condenser into a receiver.

It is collected as the first fraction.

(v) At the top of the fractionating column, a thermometer is placed to note/monitor the temperature of the boiling mixtures.

Pure substances have constant/fixed boiling point. When one mixture is completely separated, the thermometer reading rises. E.g. the thermometer reading remains at 78oC when ethanol is being separated. When no more ethanol is being separated, the mercury/alcohol level in the thermometer rises.

(vi) The second /subsequent fractions are collected in the receiver after noting a rise the mercury/alcohol level in the thermometer. e.g.

The thermometer reading rises to 100oC when water is being separated.

It is passed through the liebig condenser with the cold water inlet and outlet circulation.

It is collected different receiver as the second/subsequent fraction.

(vii) Each fraction collected should be confirmed from known physical/chemical properties/characteristic. e.g.

 

Ethanol

Ethanol is a colourless liquid that has a characteristic smell .When it is put in a watch glass then ignited, it catches fire and burn with a blue flame.

Water

Water is a colourless liquid that has no smell/odour .When it is put in a watch glass then ignited, it does not catch fire.

 

Industrial application of Fractional distillation

On a large scale, fractional distillation is used:

(i) Infractional distillation of crude oil in an oil refinery.

Crude oil is a mixture of many fractions. When heated in a furnace, the different fractions separate out according to their boiling point.

In Kenya, fractional distillation takes place at Changamwe in Mombasa.

(ii) In fractional distillation of air.

Air contain a mixture of three main useful gases which are condensed by cool in to very low temperature (-200oC) to form a liquid.

The liquid is then heated. Nitrogen is the most volatile (-196oC) and thus comes out as the first fraction. Argon (at -186oC) is the second fraction.

Oxygen (at -183oC) is the last fraction. The three gases are very useful industrial gases.

 

(F) SEPARATION OF IN MISCIBLES (USING A SEPARATING FUNNEL)

Two or more liquids that form layers on mixing are immiscible. Immiscible mixture arranges them according to their densities i.e. The denser liquid sink to the bottom. The less dense liquid floats on the denser one. Immicible mixtures can be separated from each other by using a separating funnel.

Experiment: To separate an immiscible mixture of paraffin and water.

Procedure

Place about 100cmof water into a 250cm3 beaker. Add about 100cm3 of paraffin into the beaker. Stir.

Transfer the mixture into a separating funnel.

Allow to settle for about one minute. Open the tap, run out the lower layer out slowly into a clean beaker.

Close the tap when the upper layer is very close to the tap.

Run out the intermediate small amount of the mixture near the tap into a beaker.

Discard it. Run out the remaining upper layer into a fresh beaker.

Place a portion of upper and lower layer into a watch glass separately after separating each, Ignite.

 

Observation

Water and paraffin are both colourless liquids.

Two layers are formed on mixing.

Colourless odourless liquid collected first. It does not catch fire.

A colourless liquid with characteristic smell collected later/second. It catches fire and burn with a yellow smoky flame.

Explanation

Water and paraffin are immiscible. Water is denser than paraffin.When put in a separating funnel, paraffin float on water. On opening the tap, water runs out.

A mixture of water and paraffin at the junction of the two is discarded. It is not pure.

 

(G) SUBLIMATION/DEPOSITION

Some solids on heating do not melt to a liquid but change directly to a gas. The process by which a solid changes to a gas is called sublimation. The gas cools back and changes directly to a solid. The process by which a gas changes to a solid is called deposition.

Sublimation and deposition therefore are the same but opposite processes. Some common substances that undergo sublimation/ deposition include:

(i) Iodine (ii) Carbon(IV) oxide (iii) Camphor (iv) Ammonium chloride (v) Iron(III) chloride (vi) Aluminium (III) chloride

(vii) benzoic acid if a mixture has any of the above as a component, then on heating it will change to a gas and be deposited away from the source of heating.

 

Procedure

Place about one spatula full of ammonium chloride crystals into a clean dry 100cm3 beaker. Add equal amount of sodium chloride crystals into the beaker. Swirl to mix. Place the beaker on a tripod stand. Put about 100cm3 of water into another beaker. Place carefully the beaker containing water on top of the beaker containing the solid mixture. Light/ignite a burner and heat the solid.

Set up of apparatus:

 

Observation

(i) With ammonium chloride/common salt mixture

White fumes produced.

White sublimate deposited

Colourless residue left

(ii) With Iodine/common salt mixture

Purple fumes produced.

Dark grey sublimate deposited

Colourless residue left

Explanation

(i) On heating a mixture of ammonium chloride and common salt, a white fume of ammonium chloride is produced.

The white fumes solidify as white sublimate on the cooler parts. Common salt remains as residue.

Chemical equation:

Ammonium chloride solid Ammonium chloride gas

NH4Cl(s) NH4Cl(g)

(ii) On heating a mixture of Iodine and common salt, a purple fumes of Iodine vapour is produced. The purple fumes solidify as dark grey sublimate on the cooler parts.Common salt remains as residue.

Chemical equation:

Iodine solid Iodine gas

I2(s) I2 (g)

 

(H) CHROMATOGRAPHY

Chromatography is a method of separating components of a solution mixture by passing it through a medium where the different components move at different rates. The medium through which the solution mixture is passed is called absorbent material. Paper chromatography is a method of separating coloured dyes by using paper as the absorbent material.

 

Since dyes are insoluble/do not dissolve in water, ethanol and propanone are used as suitable solvents for dissolving the dye. Practically, a simple paper chromatography involve placing a dye/material on the absorbent material, adding slowly a suitable soluble solvent on the dye/material using a dropper, the solvent spread out on the absorbent material carrying the soluble dye away from the origin.

 

The spot on which the dye is initially/originally placed is called baseline. The farthest point the solvent spread is called solvent front.

The farthest a dye can be spread by the solvent depend on:

(i) Density of the dye-the denser the dye, the less it spread from the basely ne by the solvent.

(ii) Stickiness of the dye-some dyes sticks on the absorbent material more than other thus do not spread far from baseline.

 

Experiment: To investigate the colours in ink

Procedure

Method 1

Place a filter paper on an empty beaker. Put a drop of black/blue ink in the centre of the filter paper. Wait for about one minute for the ink drop to spread.

Using a clean teat pipette/dropper add one drop of ethanol/propanone.

Wait for about one minute for the ink drop to spread further.

Add about twenty other drops of ethanol waiting for about one minute before each addition. Allow the filter paper to dry.

 

Experiment: To investigate the colours in ink

Procedure

Method 2

Cut an8 centimeter thin strip of a filter paper. At about 3cm on the strip, place a drop of ink. Place the filter paper in a 10cm length boiling tube containing 5cm3 of ethanol. Ensure the cut strip of the filter paper just dips into the ethanol towards the ink mark.

Cover the boiling tube. Wait for about twenty minutes. Remove the boiling tube and allow the filter paper to dry.

 

Explanation

When a drop of ink is placed on an absorbent material it sticks. On adding an eluting solvent, it dissolves the dye spread out with it.

The denser and sticky pure dye move least.

The least dense/sticky pure dye move farthest.

A pure dye will produce the same chromatogram/spot if the same eluting solvent is used on the same absorbent material. Comparing the distance moved by a pure dye with a mixture, the coloured dyes in a mixture can be deduced as below:

Example 1

The chromatogram of pure dyes A, B , C and a dye mixture D is shown below Determine the pure dyes present in D. On the diagram show:

(i) The solvent front

(ii) Baseline

(iii) The most soluble pure dye

 

(I) SOLVENT EXTRACTION

Solvent extraction is a method of separating oil from nuts/seeds. Most nuts contain oil. First the nuts are crushed to reduce their size and increase the surface area. A suitable volatile solvent is added. The mixture is filtered. The filtrate solvent is then allowed to crystallize leaving the oil/fat.

If a filter paper is rubbed/smeared with the oil/fat, it becomes translucent. This is the test for the presence of oil/fat.

 

Experiment: To extract oil from Macadamia nut seeds

Procedure

Crush Macadamia nut seeds form the hard outer cover.

Place the inner soft seed into a mortar. Crush (add a little sand to assist in crushing).

Add a little propanone and continue crushing.

Continue crushing and adding a little propanone until there is more liquid mixture than the solid.

Decant/filter.

Put the filtrate into an evaporating dish.

Vapourize the solvent using solar energy /sunlight.

Smear/rub a portion of the residue left after evaporation on a clean dry filter paper.

Observation /Explanation

Propanone dissolve fat/oil in the macadamia nuts. Propanone is more volatile (lower boiling point) than oil/fat.

In sunlight/solar energy, propanone evaporate/vapourize leaving oil/fat (has a higher boiling point).

Any seed like corn, wheat, rice, soya bean may be used instead of macadamia seed. When oil/fat is rubbed/smeared on an opaque paper, it becomes translucent.

 

(J) CRYSTALLIZATION

Crystallization is the process of using solubility of a solute/solid to obtain the solute/solid crystals from a saturated solution by cooling or heating the solution.

A crystal is the smallest regular shaped particle of a solute. Every solute has unique shape of its crystals.

Some solutions form crystals when heated. This is because less solute dissolves at higher temperature. Some other solutions form crystals when cooled.

This is because less solute dissolves at lower temperature.

Experiment: To crystallize copper(II)sulphate(VI)solution

Procedure:

Place about one spatula full of hydrated copper sulphate (VI) crystals into 200cm3 of distilled water in a beaker. Stir.

Continue adding a little more of the hydrated copper sulphate(VI) crystals and stirring until no more dissolve. Decant/filter.

Cover the filtrate with a filter paper. Pierce and make small holes on the filter paper cover.

Preserve the experiment for about seven days.

 

Observation/Explanation

Large blue crystals formed

When hydrated copper(II)sulphate  crystals are placed in water, they dissolve to form copper(II)sulphate solution. After some days water slowly evaporate leaving large crystals of copper(II) sulphate.

If the mixture is heated to dryness, small crystals are formed.

 

See also:

CLASSIFICATION OF SUBSTANCES

CHEMISTRY LABORATORY AND APPARATUS

THE ROLE OF CHEMISTRY IN SOCIETY

INTRODUCTION TO CHEMISTRY

WATER ON EARTH

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