Separating The Components of A Mixture

Many of the materials around us are mixtures. These mixtures have two or more than two constituents mixed in them. It may not be possible to use a mixture as such in homes and in industries. We may require only one or two separate constituents of a mixture for our use. So, we have to separate the various mixtures into their individual constituents to make them useful in our daily life.

The various constituents of a mixture have different physical properties such as density, solubility, size of particles, volatility, boiling points, etc. and these properties form the basis of separation.

1.0To Obtain Coloured Component (dye) From Blue/Black Ink - Evaporation

This method is applied when the solvent is volatile liquid and the solute is non-volatile solid. For example, Sodium chloride can be separated from its aqueous solution by evaporation. Similarly, the coloured dye present in the blue or black ink can be separated by evaporation. The volatile component evaporates leaving behind non-volatile components.

Ink (blue or black) is a mixture of dyes in water. When the ink is heated, the volatile component of the mixture i.e. water gets evaporated while the non-volatile components of the mixture i.e. dyes remain as the residue.

The method of evaporation can be used to separate the volatile component (solvent) of the mixture from its non-volatile solute.

On direct heating, ink may undergo decomposition. Therefore, it is heated by steam on a watch glass.

2.0To Separate Cream From Milk - Centrifugation

Sometimes the solid particles in a liquid are very small and hence they pass through a filter paper. For such mixtures, the filtration technique cannot be used for separation. Such mixtures are separated by centrifugation.

Definition

The method of separating finely suspended particles in a liquid, by whirling the liquid at a very high speed is called centrifugation. 

Principle of centrifugation

When a very fine suspension or a colloidal solution is whirled rapidly, the heavier particles are forced towards the bottom of liquid and the lighter stay at the top.

Application of centrifugation 

•  It is employed in diagnostic laboratories in testing urine and blood samples.

• It is employed in blood banks to separate different constituents of blood.

• It is used in drying machines to squeeze out water from the wet clothes.

3.  To separate a mixture of two immiscible liquids - Using a separating funnel

The separation of two immiscible liquids is based on the difference in their densities. The apparatus used for separation is the separating funnel.

Kerosene oil and water being insoluble form two separate layers. Kerosene oil being lighter than water forms the upper layer while water forms the lower layer. On opening the stop-cork of separating funnel, the lower layer of water comes out first and is collected in a beaker. When the water layer has completely run off, then the stop-cork is closed.

The kerosene is left behind in the separating funnel. It can be removed in a separate beaker by opening the top-cork again.

Immiscible liquid separates out in different layers depending upon their densities and hence can be separated using a separating funnel.

Application

• 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.

3.0Separation of A Mixture of Salt and Ammonium Chloride- Sublimation

This method is used in the separation of such solid-solid mixtures where one of the components sublimes on heating. However, it is useful only if the components of the mixture do not react chemically on heating. So, we can separate ammonium chloride from a mixture of common salt and ammonium chloride by this process.

Method

The mixture of common salt and ammonium chloride is taken in a china dish and placed on a tripod stand. The china dish is covered with an inverted glass funnel. A loose cotton plug is put in the upper open end of the funnel to prevent the ammonium chloride vapours from escaping into the atmosphere. The china dish is heated by using a low Bunsen flame. On heating the mixture, ammonium chloride changes into white vapour. These vapours rise up and get converted into solid ammonium chloride on coming in contact with the cold inner walls of the funnel.

When the mixture gives off no more white fumes, lift the funnel, scrap the fine white powder from its sides on a piece of paper. This is pure ammonium chloride. The residue left behind in the china dish is sodium chloride.

Some examples of solids which sublime are camphor, naphthalene and anthracene. 

4.0Separation of Different Colours From Black Ink-Chromatography

Separation of different components of a mixture based upon their different solubilities in the same solvent is known as chromatography.

This separation is based on the fact that the different constituents of a mixture get adsorbed differently on the same adsorbent material, because they have different rates of movement. The rate of movement of each adsorbed material depends upon

• The relative solubility of the constituent of mixture in a given solvent.
• The relative affinity of the constituents of mixture for the adsorbent medium. 

Adsorption :  The process by which molecules of a substance, such as a gas or a liquid, collect on the surface of another substance such as a solid. The molecules are attracted to the surface but do not enter the solid's minute spaces as in absorption.

The adsorbent material is generally magnesium oxide, alumina or filter paper.

Separation of coloured constituents present in a mixture of ink and water: 

Paper chromatography is very useful in separating various constituents of coloured solutes present in a mixture like ink, dyes, etc.

Kroma in Greek means colour. There are many types of chromatography like gas chromatography, liquid chromatography, etc. but the simplest form is the paper chromatography.

Method

The different coloured dyes present in black ink can be separated by performing a paper chromatography.

Separation of dyes in black ink by paper chromatography

(i) As solvent rises on the filter paper, it takes along with itself different components of the ink.

(ii) Since a dye usually consists of two or more-coloured substances, therefore, different colours are obtained on the filter paper strip. The dye which is more soluble in solvent dissolves first and rises faster and produces a coloured spot on the paper at a higher position. The less soluble dyes dissolve a little later, rise slower and form coloured spots at lower heights. In this way, all the dyes present in black ink get separated. 

(iii)Furthermore, due to different solubility (in solvent) of coloured components present in the ink, they rise to different heights. The paper with its separate coloured spots is called a chromatogram.

Separation of different coloured components present in ink occurs on a chromatographic paper due to their different solubilities in solvent.

Application

To separate:

• Colours in a dye. 

• Pigments from natural colours.

• Drugs from blood.

5.0Separation of Two Miscible Liquids-Distillation

Simple distillation

This method is applied for the separation of a mixture containing two miscible liquids having large differences in their boiling points or a mixture containing solid dissolved in liquid. Distillation involves vaporization followed by condensation. In this method, both the components are recovered.

Separation of mixture of acetone and water

(i) When the mixture is heated, the vapours of the low boiling liquid, i.e., acetone are first formed. These travel upwards. On passing through the condenser, they get condensed to form liquid acetone which is collected in the beaker.

(ii) The boiling point of acetone is 329 K(56°C), therefore, the thermometer reading becomes constant at 329 K when acetone is distilled.

(iii) Water is left behind in the flask.

Separation of the components of a mixture containing two miscible liquids which boil without decomposition and have difference (30–50 K) in their boiling points can be carried out by simple distillation. This is because at the boiling point of each liquid, the vapours almost entirely consist of that liquid.

Simple distillation is applicable in the following cases:

1. Two miscible liquids that boil without decomposition and have large differences (greater than 25 K) in their boiling points.

2. A solution that contains a volatile solvent and a non-volatile solute.

Fractional distillation

If the boiling points of the two liquids which form a solution differ only by a small temperature difference (less than 25 K), the fractional distillation is carried out using a fractionating column.

A fractionating column is a long tube provided with obstruction to the passage of vapours upwards and that to liquid downwards. A simple fractionating column is a long vertical glass tube filled with glass beads. The glass beads provide a large surface area for hot vapours to cool and condense. 

Example

Ethyl alcohol and water are miscible liquids. The boiling point of ethyl alcohol is 78°C and the boiling point of water is 100°C, a mixture of ethyl alcohol and water can be separated by fractional distillation.

Method 

(a) The mixture of ethyl alcohol and water is heated in a distillation flask fitted with a fractionating column.

(b) When the mixture is heated, both ethyl alcohol and water form vapours as their boiling point approaches, respectively. 

(c) The ethyl alcohol vapour and water vapour rise up in the fractionating column. 

(d) The upper part of the fractionating column is cooler, so as the hot vapours rise up in the column, they get cooled, condense and trickle back into the distillation flask.

(e) As the experiment goes on, the fractionating column warms up by the heat released by the condensed vapours. 

(f) When the temperature at the top of the fractionating column reaches 78°C, ethyl alcohol vapour passes into the condenser, gets cooled and collects in a beaker kept at the end of the condenser.

(g) In this way, all the ethyl alcohol distils over and gets separated.   

Petroleum Products Are Also Separated by Fractional Distillation

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