SEPARATION OF HETEROGENEOUS MIXTURES

SEPARATION OF HETEROGENEOUS MIXTURES

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.

Source: This topic is taken from NCERT TEXTBOOK

SEPARATION OF COLOURED COMPONENTS IN A DYE

SEPARATION OF COLOURED COMPONENTS IN A DYE

Activity 2.4: (Separation by evaporation)

*  Fill half a beaker with water.

*  Put a watch glass on the mouth of the beaker (Fig. 2.5).

*  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.

Figure 2.5: Evaporation

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.

Questions

1.  What do you think has got evaporated from the watch glass?

2.  Is there a residue on the watch glass?

3.  What is your interpretation? Is ink a single substance (pure) or is it a mixture?

Source: This topic is taken from NCERT TEXTBOOK

SEPARATION OF CREAM FROM MILK

SEPARATION OF CREAM FROM MILK

Now-a-days, we get full-cream, toned and double-toned varieties of milk packed in poly- packs or tetra packs in the market. These varieties of milk contain different amounts of fat.

Activity 2.5: (Centrifugation)

*  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.

Applications

*  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

Questions

1.  What do you observe on churning the milk?

2.  Explain how the separation of cream from milk takes place.

Source: This topic is taken from NCERT TEXTBOOK

SEPARATION OF MIXTURES OF TWO IMMISCIBLE LIQUIDS

SEPARATION OF MIXTURES OF TWO IMMISCIBLE LIQUIDS

Now-a-days, we get full-cream, toned, and double-toned varieties of milk packed in poly- packs or tetra packs in the market. These varieties of milk contain different amounts of fat.

Activity 2.5: (Separation of immiscible liquid mixture)

*  Let us try to separate kerosene oil from water using a separating funnel.

*  Pour the mixture of kerosene oil and water into a separating funnel (Fig. 2.6).

*  Let it stand undisturbed for some time 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.

Figure 2.6: Separation of immiscible liquids

The principle is that immiscible liquids separate out in layers depending on their densities.

Applications

*  To separate a 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.

Source: This topic is taken from NCERT TEXTBOOK

SEPARATION OF MIXTURES OF SALT AND CAMPHOR

SEPARATION OF MIXTURES OF SALT AND CAMPHOR

We have learned in chapter 1 that camphor 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, the sublimation process is used (Fig. 2.7). Some examples of solids that sublime are ammonium chloride, naphthalene, and anthracene.

Activity: Separating salt and camphor mixture

Figure 2.7: Separation of camphor and salt by sublimation

Questions

1.  Name the technique to separate

        (i)  butter from curd,

       (ii)  salt from sea-water,

     (iii)  camphor from salt.

Source: This topic is taken from NCERT TEXTBOOK

SEPARATION OF DYES USING CHROMATOGRAPHY

SEPARATION OF DYES USING CHROMATOGRAPHY

*   Take a thin strip of filter paper.

*   Draw a line on it using a pencil, approximately 3 cm above the lower edge [Fig. 2.8 (a)].

*   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. 2.8(b) and leave it undisturbed.

*   Watch carefully, as the water rises up on the filter paper. Record your observations

Figure 2.8: Separation of dyes in black ink using chromatography

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.

Applications

To separate

*   colours in a dye

*   pigments from natural colours

*   drugs from blood

Questions

1.  What do you observe on the filter paper as the water rises on it?

2.  Do you obtain different colours on the filter paper strip?

3.  What according to you, can be the reason for the rise of the coloured spot on the paper strip

Source: This topic is taken from NCERT TEXTBOOK

SEPARATION OF MIXTURES OF TWO MISCIBLE LIQUIDS

SEPARATION OF MIXTURES OF TWO MISCIBLE LIQUIDS
 

Activity: (Separating Miscible liquid mixtures)

*   Let us try to separate acetone and water from their mixture.

*   Take the mixture in a distillation flask. Fit it with a thermometer.

*   Arrange the apparatus as shown in Fig. 2.9.

*   Heat the mixture slowly keeping a close watch on 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.

Figure 2.9: Separation of two miscible liquids by distillation

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 factions 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. 2.10.

Figure 2.10: Fractional distillation

Questions

1.  What do you observe as you start heating the mixture?

2.  At what temperature does the thermometer reading become constant for some time?

3.  What is the boiling point of acetone?

4.  Why do the two components separate?

5.  How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25ºC), which are miscible with each other?

Source: This topic is taken from NCERT TEXTBOOK

SEPARATION OF DIFFERENT GASES FROM AIR

SEPARATION OF DIFFERENT GASES FROM AIR

Separation of components of air

Air is a homogeneous mixture and can be separated into its components by fractional distillation. The flow diagram (Fig. 2.11) shows the steps of the process.

Figure 2.11: Flow diagram shows the process of obtaining gases from the air

If we want oxygen gas from air (Fig. 2.12), 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.

Figure 2.12: Separation of components of air

Questions

1.  Arrange the gases present in air in increasing order of their boiling points.

2.  Which gas forms the liquid first as the air is cooled?

Source: This topic is taken from NCERT TEXTBOOK

SEPARATION OF PURE COPPER SULPHATE FROM IMPURE SAMPLE – CRYSTALLISATION

SEPARATION OF PURE COPPER SULPHATE FROM IMPURE SAMPLE – CRYSTALLISATION

Activity 2.9: (Crystallisation)

*  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 –

*  some solids decompose or some, like sugar, may get charred on heating to dryness.

*  some impurities may remain dissolved in the solution even after filtration. On evaporation these contaminate the solid.

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.

Applications

*  Purification of salt that we get from sea water.

*  Separation of crystals of alum (phitkari) from impure samples

Questions

1.  What do you observe in the china dish?

2.  Do the crystals look alike?

3.  How will you separate the crystals from the liquid in the china dish?

4.  What type of mixtures are separated by the technique of crystallisation?

Source: This topic is taken from NCERT TEXTBOOK

WATER PURIFICATION SYSTEM

WATER PURIFICATION SYSTEM

In cities, drinking water is supplied from water works. A flow diagram of a typical water works is shown in Fig. 2.13. From this figure write down the processes involved to get the supply of drinking water to your home from the water works and discuss it in your class.

Figure 2.13: Water purification system in water works

Source: This topic is taken from NCERT TEXTBOOK