By the end of the sub topic learners should be able to:
State the methods of purification.
Describe and explain the methods of purification.
Design the arrangement of apparatus, given information about the substances involved.
Describe the use of volumetric apparatus and indicators.
Substances that have been identified to be impure can be purified by removing and separating the unwanted substances from the substance of interest.
Impure substances are mixtures as they contain two or more substances.
Various methods are used in the laboratory to purify substances.
The method used for purification depends on the properties of the substance to be purified and those of the impurities.
Methods used for purification include filtration, solvent extraction, crystallisation, simple distillation and fractional distillation. These methods are discussed in this section.
2.3.1 Filtration
The method is used to separate an insoluble substance from a liquid.
An insoluble substance does not dissolve in a solvent.
Examples of substances that are insoluble in water are sand, chalk and glass.
The process is carried out using the set up of apparatus below.
A mixture is poured into the filter paper in a filter funnel. The insoluble substance has large particles and therefore it will not pass through the filter paper, the liquid has smaller particles so it easily passes through the filter paper.
The insoluble substance which remains in the filter paper is called a residue.
The liquid which passes through the filter paper and is collected below the filter funnel is called the filtrate.
The following experiment demonstrates the use of filtration to separate substances.
Experiment 2.3.1: Separation of calcium carbonate (chalk) and water
Materials
filter paper
filter funnel
conical flask
conical flask
beaker with solution to be separated.
Procedure
The set up for the experiment is shown in fig 2.3.1
Clamp the filter funnel to the clamp stand.
Fold the filter paper and place it in the filter funnel. The filter paper may be moistened to ensure that it sticks to the filter funnel and prevents any solid from escaping.
Place the conical flask below the filter funnel.
Pour the mixture in the beaker into the filter paper. Ensure that it does not overflow the filter paper. The mixture has to be poured in intervals after the level in the filter paper has gone down.
Observe the substance in the filter paper and that in the conical flask after the filtration process is complete.
Expected observations
A solid remains in the filter paper.
A clear liquid collects into the conical flask.
The calcium carbonate is insoluble in water and it has large particles which are not able to pass through the filter paper, they remain in the filter paper as the residue. The water particles pass through the filter paper and are collected as filtrate.
The two substances have been separated through filtration.
2.3.2 Crystallisation.
The method is used to separate a solid from a solution by crystallising it.
A solution is made up of a solute and a solvent.
A solute is a substance that dissolves in a solvent.
A solvent is a substance in which a solute dissolves.
The process of crystallisation is carried out by gently heating the solution in an evaporating dish to concentrate it by removing the solvent.
When a little amount of the solvent is left in the evaporating dish, the solute starts to crystallise. The concentrated solution can then be left to cool and crystals will form.
Crystallisation occurs because as the solvent evaporates, the amount of solvent left will not be able to hold all of the solute so the solution becomes saturated with the solute and the solute separates out and crystallises.
The crystals formed can then be filtered off and left to dry.
The following experiment demonstrates the process of crystallisation in the laboratory.
Experiment 2.3.2: Crystallisation of copper sulphate
Materials
Bunsen burner
tripod stand
wire gauze
evaporating dish
white tile
dropper
copper sulphate solution.
Procedure
Put the copper sulphate solution in the evaporating dish.
Set up the apparatus as shown in fig 2.3.2
Heat the solution gently until only a small amount of the liquid remains.
To confirm that crystals are about to form, use the dropper to take a small amount of the solution and place a few drops on the white tile. If crystals are quickly formed it means the crystallisation point has been reached.
Switch off the burner and allow the solution to cool.
Observe the evaporating dish after a few hours.
Expected observations
After several hours, crystals are observed at the bottom of the evaporating dish.
2.3.3 Solvent extraction
The method is used to separate two solutes dissolved in a solvent.
A second solvent into which only one of the solutes dissolves is then used to extract one of the solutes from the first solvent.
The second solvent used must not mix with the first one. Liquids that do not mix together are said to be immiscible. They have different densities and when mixed they form layer with the heavier liquid collecting at the bottom and the liquid with the smaller density forming the upper layer.
A solution of iodine and salt dissolved in water can be separated using solvent extraction.
Hexane is used as the second solvent; it will dissolve the iodine and not the salt.
After adding hexane and shaking two liquid layers will be formed, the solution of salt and water collects at the bottom as water is denser than hexane while the solution of iodine and hexane collects on the upper layer.
A separating funnel can then be used to separate the two solutions.
The funnel is left to stand for several minutes and the two solutions will separate out.
The tap at the bottom of the separating funnel is then opened to release the heavier solution leaving the less dense solution.
Fig 2.3.3 shows how a separating funnel can be used to separate two solutions which are immiscible.
The two solutes are therefore separated.
The solvents can be evaporated to obtain the two solutes.
2.3.4 Simple distillation
The method is used to separate and collect a solvent from a solution.
The process of simple distillation can be demonstrated by an experiment on the separation of water from a salt solution.
This concept is used to obtain drinking water from seawater.
Experiment 2.3.3: Separation of water from a salt solution using simple distillation
Materials
Bunsen burner
tripod stand
round bottomed flask with side arm
rubber stopper
Condenser
Beaker
water source
rubber tubes
salt solution.
Procedure
Set up the apparatus as in fig 2.3.4.
Heat the solution in the round bottomed flask.
Connect the rubber tube at the bottom of the condenser to a water source. Put the tube at the top of the condenser into a sink.
Observe the round bottomed flask, condenser and beaker after several minutes.
Expected observations
The solution in the round bottomed flask boils when it is heated, the water escapes as steam.
The amount of solution in the flask will decrease and with time all of the liquid vapours leaving crystals of the salt in the flask.
In the condenser the steam is cooled and the steam turns into liquid. The water entering the condenser absorbs the heat from the steam; the particles in the steam lose their kinetic energy and move closer together forming the liquid, the steam condenses to a liquid. The water leaving the condenser will be warmer because of the heat absorbed.
The condensed liquid will be collected in the beaker. The liquid collected is termed a distillate.
The water in the condenser has to flow continuously so that the water which has absorbed heat is removed and cooler water comes in to absorb more heat promoting the condensation of the steam into liquid water.
Water has to enter from the bottom of the condenser so that the condenser gets completely filled with water and the water stays for a longer time in the condenser and absorb heat before it leaves.
When water enters from the top, the condenser will not be completely filled and the water will quickly leave the condenser from the bottom, this will slow down the condensation process.
Water entering from the bottom provides an effective cooling system.
Note
The round bottomed flask should not be filled by more than two thirds of its volume with the mixture to be distilled to ensure that there is enough space above the surface of the mixture so that the mixture is not spilled into the condenser when it boils, this will compromise the quality of the distillate collected.
Boiling chips or stones may be added into the round bottomed flask to prevent bumping of the liquid mixture thereby promoting a smooth boiling process and to also prevent overheating of the mixture during the distillation process.
2.3.5 Fractional distillation
The method is used to separate a mixture of liquids with different boiling points.
Liquids which mix together can be separated using this method. Liquids which mix together are said to be miscible.
In fractional distillation, an additional apparatus called a fractionating column is added. A thermometer is also added for the measurement of the vapours of the solvents passing through fractionating column.
The fractionating column is tall and packed with glass beads and continuous evaporation and condensing of the liquid mixture occurs in it.
The method is used in industry to separate crude oil into several fractions which have various uses.
The process is also used to separate and collect the gases in air through fractional distillation of liquid air.
Fractional distillation is also used in the distillation of fermented liquor to concentrate it thereby increasing its alcohol content. In the laboratory this can be demonstrated by the distillation of a mixture of ethanol and water.
Experiment 2.3.4: Fractional distillation of a mixture of ethanol and water.
Materials
Bunsen burner
Tripod stand
Round bottomed flask with side arm
rubber stopper with one hole
Fractionating column
Thermometer
Condenser
Beaker,
Water source
Rubber tubes
Solution of ethanol and water.
Procedure
Set up the apparatus as shown in fig 2.3.5
Heat the mixture in the flask.
Observe what happens as the temperature rises to about 78 0C.
Observe the liquid collected in the beaker.
Expected observations
At about 78 0C, some liquid in the flask starts to boil and some of it vapours. The temperature remains constant for some minutes, this is the boiling point temperature for one of the liquids, ethanol.
The ethanol vapour condenses in the condenser and is collected in the beaker as the distillate.
After some time, the temperature starts to rise again, this means that there is no longer ethanol in the flask. The two liquids have therefore been separated, the water remains in the flask and the ethanol is now in the beaker.
During the process of fractional distillation, at about 78 0C, the ethanol starts to boil into a vapour; some of the water evaporates with it too. The vapours move up the fractionating column.
In the column the vapours condense and release heat onto the glass beads thereby making them hot, when the beads reach about 780C the ethanol becomes a vapour while the water continues to condense into a liquid which drips back into the flask.
The ethanol vapour continues up the column and goes through the condenser where it is condensed into liquid and collected as the distillate.
During the process of fractional distillation the liquid with the lowest boiling point is always collected first. The liquid with the low boiling point is said to be more volatile and it always moves up the fractionating column first as it is first to vapour out of the liquid mixture.
Ethanol has a boiling point of 78 0C and water has a boiling point of 100 0C, ethanol is more volatile and therefore it vapours first and is collected.
When there are more than two liquids, they will be collected in order of increasing volatility.
Combining methods of purification
Some impure substances have to be purified by combining more than one method of purification.
The following example demonstrates the combination of methods.
Example: Explain how a mixture of salt and chalk can be separated.
Answer: The chalk is insoluble in water while the salt is soluble in water. Water is added to the mixture, filtration is done to separate the chalk. The salt is obtained through crystallisation to remove the water.
You should be able to identify the method(s) of purification required to purify an impure substance using the information given.
IN SUMMARY:
Filtration is used to separate an insoluble solid from a liquid.
Crystallisation is used to separate a soluble solute from a solvent
Solvent extraction is used to separate two solutes that are dissolved in a solvent.
Simple distillation is used to obtain the solvent from a solution.
Fractional distillation is used to separate a mixture of miscible liquids.
