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Separation of Substances

Nature of matter

Matter is anything that has weight and occupies space. This definition implies that substances found around us are all forms of matter. Substances exist either as solids, liquids or gases, each with its distinctive local or scientific name. Examples of solids are tables, books, ice block and iron filings; water, kerosene and petrol are liquids while oxygen, hydrogen and water vapour are gases. Substances also exist as elements, compounds or mixtures.

Elements
Elements are substances that cannot be split into simpler substances by chemical or physical methods. Chlorine, iron and carbon are elements.

Compounds
Compounds are formed by chemical combination of two or more elements and can only be split into simpler substances by chemical methods. Water, kerosene, limestone are examples of compounds.

Mixtures
A mixture is a substance that contains two or more compounds, which can be separated by simple physical methods. The different substances in the mixture retain their individual composition and properties within the mixtures. Examples of mixtures include air, seawater, crude oil and soil. A mixture could be homogeneous or heterogeneous. A homogeneous mixture is the one whose individual components cannot be distinguished from each other. Examples of such mixtures include water and alcohol, air, kerosene, diesel and seawater. A mixture is said to be heterogeneous when the individual components can be distinguished from each other. Examples include sand and salt, iron filings and sulphur powder, benzene and water.

The components of a mixture can be separated using a technique that depends on the characteristics of the mixture.

Characteristics of a mixture

1. The compositions a mixture varies widely.
2. The properties are the same as those of its constituents.
3. The process of mixing is a physical change.
4. A mixture can be separated into its constituents using suitable physical methods.

Separation techniques

There are various physical methods employed in separating mixtures into their individual constituents. These methods depend on the physical properties of the constituents of the mixtures. The physical properties include colour, size, shape, solubility in various solvents, melting points, boiling points, magnetic properties and absorption. The following techniques can be used in separating various types of mixtures:

1. Picking: A mixture whose constituents differ in colour, size or shapes can be easily separated by making use of the sense of sight. This can be applied to mixtures in the solid state. A mixture of fruits can be separated by picking the various fruits, making use of the sizes or shapes of the different fruits.

Examples include a mixture of oranges, mangoes, apples or laboratory apparatus.

In Rivers State where most families depend on marketing palm produce for a living, palm kernels are cracked manually leaving both the palm nuts and the shells together as a mixture. After cracking, the palm nuts are carefully separated from the shells by picking. The process is often laborious and time-wasting.

2. Sieving: This method is used in separating solid constituents that have different sizes in a mixture. The coarse mixture is poured into a sieve or sifter made up of a mesh of a particular size which allows only solids with smaller sizes than the mesh to pass through when shaken into another container. Solids of larger sizes than the mesh are retained in the sieve.

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Sieving

Miners of gold and diamond use this method extensively to grade their minerals into various sizes for sale. A visit to the quarry sites in Old Netim, Akamkpa shows how this method is used in classifying the crushed rocks or chippings for sale to construction engineers. Producers of garri in Rivers State also use the sieve to get the fine particles, which are then fried for sale. The chaff is discarded. Fine particles of garri can also be separated from the bigger ones using a sieve.

3. Decantation: This method is used for separating undissolved solids from a liquid. The liquid is poured into another container very slowly so that the solid or sediment is left behind and discarded. It could be used as a quick method of separating a mixture of sand and water although it is not as efficient as filtration. Palm wine can be separated from the dregs using the decantation method. Substances. which float on liquids, can be separated from the liquid by decantation.

4. Evaporation: When a solvent dissolves in a solid, a clear solution is formed. This is an example of a homogeneous mixture. It becomes impossible to separate the solute from the solvent by filtration. The solution is put on an evaporating dish, which is carefully placed on a steam bath (or sand bath) over a period of time. The solvent escapes to the atmosphere while the solute is left in the evaporating dish when all the solvent has completely vaporised.

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Evaporation

5. Sedimentation: This method is used in separating tiny undissolved substances that are suspended in a particular solvent. The solid particles suspended in the solvent fall to the bottom of the solution because of the density of the particles. The accumulation of the suspended particles at the bottom of the solution is known as sedimentation. The liquid thus forms at the top of the sediment and can either be decanted or filtered.

Examples include muddy water and starch solution. In the preparation of ogi(pap) from corn this method is used.

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Sedimentation

After the corn is ground, mixed with water, washed and strained to remove the chaff, the suspension is left to stand overnight. The starch sediments as shown in the diagram can be decanted slowly. Pap is a common meal in Rivers State.

6. Filtration: Filtration is a method used in separating insoluble solids from a solution. A mixture of sand and water can be separated using this method.

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Filtration

This mixture is carefully poured into the funnel as shown in the diagram. The insoluble part retained in the funnel is called the residue while the liquid in the beaker is called the filtrate. To ensure efficient filtration, a filter paper is appropriately folded and placed in the funnel before the mixture is poured into it. In our homes, drinking water can be filtered using fine wire gauze, a piece of clean white cloth or very fine sand in place of the filter paper. In fact, in the State public water supply, water is made to pass through many layers of fine sand bed to filter the water by removing suspended particles before it is chemically treated to kill any bacteria. The treated water is then discharged to our homes for domestic use.

7. Simple distillation: This method is used in separating a mixture of two solvents whose boiling points differ by at least 20 degrees Celsius from each other. It can be used to separate a mixture of water and local gin (ogogoro). The boiling point (b.p) of water is 100 degrees Celsius while that of ogogoro is 80 degrees Celsius. A simple distillation set up is shown below:

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Simple distillation

As the mixture is heated, molecules of ogogoro with a lower b.p. of 80 degree Celsius begin to vaporise and separate from the mixture. The vapour passes through the delivery tube and is condensed by circulating cold water through the Liebig condenser. The condensed liquid is collected as the distillate. The first distillate which is ogogoro is removed and another container is brought in. As the temperature of the mixture gets up to 100 degrees Celsius, water boils off and condenses through the same process and is collected as the second distillate. By this process, both liquids have been separated, as a result of the difference in their boiling points. This technique is used in the production of local gin (ogogoro) and to obtain pure water from sea water.

8. Fractional distillation: This method is used for the separation of liquids with close boiling points. A fractionating column is introduced between the round bottomed flask and the Liebig condenser as shown below:

 

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Fractional distillation

As the mixture is heated, the vapours of the liquid move into the fractionating column and are cooled by the glass surface. The liquid with the higher boiling point condenses and falls back into the flask, while the more volatile vapour (low b.p.) distills over the condenser and is collected as the first distillate.

Fractional distillation is used in refining crude oil into its various fractions like petrol, kerosene, diesel oil, etc. In Nigeria, refineries are sited at Port Harcourt, Warri and Kaduna, where crude oil is separated into various fractions by this process.

9. Separating funnel method: A mixture containing two liquids that do not form a homogeneous mixture e.g. kerosene in water can be separated using the separating funnel. Of the two layers formed, one will be denser than the other and thus will be the lower layer as illustrated in Fig. 1.7.

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Separating funnel method

Water, which is denser, forms the lower layer in the separating funnel and is tapped off first leaving kerosene in the funnel. Another example is the separation of petrol and water.

10. Crystallisation: Crystallisation can be used for separating salts that can decompose easily on heating. An example is copper(ll) tetraoxosulphate (Vl) pentahydrate. The solution of the salt is heated to drive away some of the solvent until a saturated solution is obtained. When the saturated solution is cooled, crystals of the solute begin to form. In order to facilitate crystal formation, a few crystals of the same salt is introduced into the solution. This process is called seeding, also a scratch can be made inside the reaction vessel to facilitate crystal formation.

11. Recrystallisation: Where the crystals obtained are not very pure, the process can be repeated until the desired level of purity is attained. This latter process is known as recrystallisation and is an important step in obtaining pure crystals in a sample. Crystallisation and recrystallisation are very useful in pharmaceutical and food processing industries where the purity of products is very important.

12. Magnetisation: When a mixture contains magnetic and non-magnetic substances, a magnet can be used in separating them. For example, a mixture of iron filings and sulphur can be easily separated by using a magnet. Magnetic impurities can be removed from ores in tin and other nonmagnetic substances, in the iron and steel industry. Other examples also include separation of office pins from sand.

13. Sublimation: This is a process where certain solid substances when heated can change from solid to gaseous states without passing through the liquid state. Common examples of substances that sublime are iodine, ammonium chloride, benzoic acid and anhydrous iron(lll) chloride. Sublimation is a reversible process. This means that as soon as the substance changes its state from solid to gaseous state, the gas can condense back to the solid form on getting to a cooler environment. A mixture of iodine and black copper(ll) oxide can be separated using the sublimation technique. On heating the mixture, the iodine turns to gas and condenses to iodine solid near the top of the inverted filter funnel leaving copper(ll) oxide in the evaporating dish.

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Chromatography

14. Chromatography: Chromatography is a technique used for separation of components between stationary and mobile phases according to their
differences in mobility rates.

There are several types of Chromatography which include:

i) Paper Chromatography.
ii) Thin layer Chromatography.
iii) Gas-liquid Chromatography.
iv) Column Chromatography.

Only paper and column Chromatography will be treated in this chapter.

i) Paper Chromatography

In this technique, the mixture to be separated is dissolved in a suitable solvent and a spot of the mixture is made on the strip of filter paper, which acts as the stationary phase (absorbent) just above the solvent level as shown in the diagram. The paper is dipped into an airtight chamber containing the mobile phase. As the solvent moves up, it dissolves the mixture and the components of the mixture begin to separate according to their different rates of mobility (speed). The paper is removed, dried and the distances travelled by each spot to the solvent front and the solvent front are measured. It is these distances that enable the chemist to identify the components since their values are known for various substances. Chromatography is used to separate the components in a plant pigment, various constituents in black ink and complex materials such as proteins and enzymes.

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Paper Chromatography

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Column chromatography

ii) Column chromatography
In this technique, the column is packed with an adsorbent, for example alumina, which is the stationary phase while the mixture to be separated is mixed with a suitable solvent to form a solution. This is placed on top of the adsorbent and as more solvent is introduced into the column, different constituents of the mixture are adsorbed on the solid adsorbent according to their affinity for the solid. Strongly adsorbed components move down the column slowly while the weakly adsorbed do so quickly. In this way separation is effected. The advantage of column chromatography is that it is possible to collect the different constituents of the mixture by using the tap. The constituents are known as eluants.

Chromatography is by far the most valuable method for the separation, isolation, purification and identification of the constituents in chemical products. It is used extensively in industry, biomedical analysis as well as in scientific research. For instance, in hospitals, the presence of certain substances in the blood and urine of patients can be determined using chromatography and the result so obtained help doctors in their diagnosis and prescription.

Criteria for purity

The various techniques outlined above are used to separate mixtures into their various constituents. The aim of the separation is to obtain pure constituents from these mixtures. Since the human senses have proved inadequate for deciding the purity of these substances despite the use of these techniques, it becomes necessary to apply further scientific tests to confirm the purity of these components so separated. A pure substance possesses certain properties which are unique to that particular substance and therefore can be used when testing its purity. These properties include melting points, boiling points, density, specific gravity, refractive index, etc. The most common properties used to test the purity of substances in the laboratory are sharp melting points for solids and boiling points for liquids. If the value agrees with that of known values, the substance is probably pure.

Determination of melting and boiling points of a substance

A melting point tube is filled with a little quantity of the solid substance and the tube is attached to a mercury thermometer in such a way that the substance is at the level of the mercury bulb which has previously been wetted with paraffin. The thermometer is immersed in liquid paraffin contained in a beaker. As heat is applied, the paraffin is stirred slowly to distribute the heat energy evenly. As soon as the substance starts to melt, the temperature is recorded from the thermometer reading. It is noted as T1. As soon as it is completely melted, another temperature reading is taken as T2.

The difference between the two temperatures, i.e. T2 – T1 should not be more than 0.5 degrees Celsius if the substance is pure. This shows that the melting point is sharp. If the difference is wider, the substance is not very pure and should be further purified. In fact, the wider the temperature range, the more impure the substance. The same applies to the boiling point of substances.

However, with the recent advances in technology, modern melting and boiling points determination equipment are used to save time and guarantee precision. The criteria remains the same.

Summary

i) Substances can be elements, compounds or mixtures.
ii) Compounds are formed by the chemical combination of two or more elements. They can be separated by chemical means only,
iii) Mixtures contain two or more components which can be separated by simple physical methods.
iv) Methods used in separating constituents of a mixture depend on the physical properties of each constituent.
v) Simple separation techniques include picking, sieving, decantation, evaporation, sedimentation, filtration, simple distillation, fractional distillation, sublimation, crystallisation, recrystallization, magnetisation and chromatography.
vi) Separating techniques are applied extensively in scientific research, manufacturing industries and in the medical field.
vii) The criteria for purity of a substance is that it must have a sharp melting or boiling point.

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