Form 1:
Classification of substances

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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/vaporize 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 broken porcelain/porous pot/glass/sand.

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 solvents evaporate/vaporize.

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 vaporize /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

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/vaporize first.e.g. Pure ethanol has a boiling point of 78oC.

Pure water has a boiling point of 100 oC at sea level/one atmosphere pressure. When a miscible mixture of ethanol and water is heated, ethanol vaporizes /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 vapors 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 condensed 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 at78oC 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 of 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 in a different receiver as the second/subsequent fraction.

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

Example:

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.

Set up of apparatus
Industrial application of Fractional distillation

On a large scale,fractional distillation is used:

(i)In fractional 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 cooling to very low temperature (-200oC) to form a liquid. The liquid is then heated.

Nitrogen is the most volatile (-196 oC) and thus comes out as the first fraction. Argon (at - 186 oC) is the second fraction. Oxygen ( at -183 oC) is the last fraction. The three gases are very useful industrial gases.

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