We have learnt that most of the natural substances are not chemically pure. Different methods of separation are used to get individual components from a mixture. Separation makes it possible to study and use the individual components of a mixture.
Heterogeneous mixtures can be separated into their respective constituents by simple physical methods like handpicking, sieving, filtration that we use in our day-to-day life. Sometimes special techniques have to be used for the separation of the components of a mixture.
Fill half a beaker with water.
Put a watch glass on the mouth of the beaker
Put few drops of ink on the watch glass.
Now start heating the beaker. We do not want to heat the ink directly. You will see that evaporation is taking place from the watch glass.
Continue heating as the evaporation goes on and stop heating when you do not see any further change on the watch glass.
Observe carefully and record your observations.
We find that ink is a mixture of a dye in water. Thus, we can separate the volatile component (solvent) from its non-volatile solute by the method of evaporation.
Take some full-cream milk in a test tube.
Centrifuge it by using a centrifuging machine for two minutes. If a centrifuging machine is not available in the school, you can do this activity at home by using a milk churner, used in the kitchen.
If you have a milk dairy nearby, visit it
and ask (i) how they separate cream from milk and (ii) how they make cheese (paneer) from milk.
Sometimes the solid particles in a liquid are very small and pass through a filter paper. For such particles the filtration technique cannot be used for separation. Such mixtures are separated by centrifugation.
The principle is that the denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly.
Used in diagnostic laboratories for blood and urine tests.
Used in dairies and home to separate butter from cream.
Used in washing machines to squeeze out water from wet clothes.
Pour the mixture of kerosene oil and water in a separating funnel
Let it stand undisturbed for sometime so that separate layers of oil and water are formed.
Open the stopcock of the separating funnel and pour out the lower layer of water carefully.
Close the stopcock of the separating funnel as the oil reaches the stop-cock.
To separate mixture of oil and water.
In the extraction of iron from its ore, the lighter slag is removed from the top by this method to leave the molten iron at the bottom in the furnace.
The principle of separating funnel is that immiscible liquids separate out in layers depending on their densities.
We have learnt in previous chapter that ammonium chloride changes directly from solid to gaseous state on heating. So, to separate such mixtures that contain a sublimable volatile component from a non-sublimable impurity (salt in this case), the sublimation process is used . Some examples of solids which sublime are ammonium chloride, camphor, naphthalene and anthracene.
Take a thin strip of filter paper.
Draw a line on it using a pencil, approximately 3 cm above the lower edge
Put a small drop of ink (water soluble, that is, from a sketch pen or fountain pen) at the centre of the line. Let it dry.
Lower the filter paper into a jar/glass/beaker/test tube containing water so that the drop of ink on the paper is just above the water level, as shown in Fig.4 (b) and leave it undisturbed.
Watch carefully, as the water rises up on the filter paper. Record your observations.
The ink that we use has water as the solvent and the dye is soluble in it. As the water rises on the filter paper it takes along with it the dye particles. Usually, a dye is a mixture of two or more colours. The coloured component that is more soluble in water, rises faster and in this way the colours get separated.
This process of separation of components of a mixture is known as chromatography. Kroma in Greek means colour. This technique was first used for separation of colours, so this name was given.
Chromatography is the technique used for separation of those solutes that dissolve in the same solvent.
With the advancement in technology, newer techniques of chromatography have been developed. You will study about chromatography in higher classes.
Take the mixture in a distillation flask. Fit it with a thermometer.
Arrange the apparatus as shown in Fig.5
Heat the mixture slowly keeping a close watch at the thermometer.
The acetone vaporizes, condenses in the condenser and can be collected from the condenser outlet.
Water is left behind in the distillation flask.
This method is called distillation. It is used for the separation of components of a mixture containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling points.
To separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K, fractional distillation process is used, for example, for the separation of different gases from air, different fractions from petroleum products etc. The apparatus is similar to that for simple distillation, except that a fractionating column is fitted in between the distillation flask and the condenser.
A simple fractionating column is a tube packed with glass beads. The beads provide surface for the vapours to cool and condense repeatedly, as shown in Fig.6.
Air is a homogeneous mixture and can be separated into its components by fractional distillation. The flow diagram (Fig.7) shows the steps of the process.
If we want oxygen gas from air (Fig.8), we have to separate out all the other gases present in the air. The air is compressed by increasing the pressure and is then cooled by decreasing the temperature to get liquid air. This liquid air is allowed to warm-up slowly in a fractional distillation column, where gases get separated at different heights depending upon their boiling points.
Take some (approximately 5 g) impure sample of copper sulphate in a china dish.
Dissolve it in minimum amount of water.
Filter the impurities out.
Evaporate water from the copper sulphate solution so as to get a saturated solution.
Cover the solution with a filter paper and leave it undisturbed at room temperature to cool slowly for a day.
You will obtain the crystals of copper sulphate in the china dish.
This process is called crystallisation.The crystallisation method is used to purify solids. For example, the salt we get from sea water can have many impurities in it. To remove these impurities, the process of crystallisation is used.
Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. Crystallisation technique is better than simple evaporation technique as –
Thus, by choosing one of the above methods according to the nature of the components of a mixture, we get a pure substance. With advancements in technology many more methods of separation techniques have been devised.
In cities, drinking water is supplied from water works. A flow diagram of a typical water works is shown in Fig.9.
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