Separation of compounds in mixtures
Many substances essential for our daily affairs are available in the Earth's crust. Metals, mineral oils, salts, sand, clay, coal, minerals and rocks are some of them. These rarely exist in pure form in the Earth's crust. Naturally they occur mixed with other substances. Therefore, the essential components should be separated from those mixtures.
Some instances where components from mixtures should be separated, are given below.
- Removal of stones and sand from rice
- Separation of salt from sea water
- Separation of various minerals from mineral sands.
- Separation of various fuels by the mineral oil refinery
- Separation of sugar from molasses
- Separation of gases such as oxygen, nitrogen and argon from atmospheric air
- Obtaining distilled water from common well water or river water
- Preparing potable water from sea water
Many more such occasions can be given as examples. In this chapter we study about several methods of separating components in mixtures.
Mechanical separation
You know that rice is sifted to remove the sand mixed with it. In this, sand is removed from rice based on the difference of the densities of the components. The separation of components in a mixture using the difference of their physical properties such as density, particle size, particle shape, magnetic properties and electric properties is called mechanical separation. Study the examples given in below table and have a further understanding about mechanical separations.
The methods such as winnowing, sieving, sifting, floating and subjecting to magnetism that separate components of a mixture are called mechanical methods. Methods such as these are frequently used in day to day life.
Vapourisation/Evaporation
You may have observed the extraction of salt from sea water. What happens here is that the water evaporates due to solar heat. Water gets evaporated and the dissolved salts get precipitated. During vapourization, the unnecessary components are vaporized by supplying heat to a mixture and the essential component is isolated.
When metals are dissolved in mercury a special solution known as an amalgam is formed.
When impure gold is dissolved in mercury, a solution of pure gold is obtained. This is known as the gold amalgam. When gold amalgam is heated, mercury is evaporated and the pure gold is remained. The evaporated mercury is cooled and used again.
Filtration
Have you seen adding coconut milk to some curries when they are cooked at home? How is coconut milk made? Water is added to the coconut scraped by a coconut scraper, and then crushed and squeezed by hand. Parts of the white liming remain suspended in water without going into the solution. When the stuff is put into a milk-strainer the milky solution filters off leaving the pieces of solid coconut in the strainer.
Filtration can be used to separate from a mixture, the components that remain suspended in a liquid without going into the solution. A filter is required to filter a mixture. Milk-strainer is such a device. The filter paper used in laboratories is another such filter. Water purifying plants have sand filters.
A filter has small holes. Particles smaller than the holes can pass through. But particles larger than those holes cannot pass through them. This is the concept used in filtration. In filtration, the substance left in the filter is called the residue while the solution that gets filtered is known as the filtrate.
Activity 01
Materials required ;- Dry soil, salt, filter papers, a funnel, a beaker, glass rod, flask
Method ;- Mix well, about 10 g of dry soil and about 5 g of salt (NaCl). Take about 50 cm^3 of water to a beaker, add the above mixture into it and stir. Arrange the apparatus as shown in above figure and filter. After filtration is over, observe the filter paper. Add about 10 cm^3 of the filtrate to an evaporating dish and vaporize. See whether there is anything left in the dish.
Large clay particles in the sample of soil do not pass through the filter, and they are held back by the filter paper. Since water and salt are made up of smaller particles, they pass through the filter and get into the filtrate.
Crystallization
Let us consider instances where a solid dissolves in a solvent to form a homogeneous mixture. At a certain temperature, there is a maximum concentration of a substance that stays dissolved in a solution. Such solutions are said to be saturated with that substance.
If this saturated solution is vaporized, the concentration of that substance in the solution increases further. When the concentration of the solute exceeds the maximum possible concentration in the solution, the solute separates out forming crystals.
Crystallization is thus the method of separating solid substances by concentration when a solute that can turn into a solid is present in a solution. Manufacturing of sugar is an industry that utilities crystallization. Stems of Sugarcane are crushed and squeezed and the juice is purified. Its concentration is increased by vaporization. Then, sugar separates out from the juicy solution as crystals.
Production of salt from sea water is another industry that adopts crystallization. During the production of salt in salterns, several salts that are dissolved in sea water get crystallized.
Re-crystallization
Re-crystallization is used to separate pure substances from solid, crystalline substances carrying impurities. The process of dissolving a solid, crystalline substance and turning it again into crystals is called re-crystallization . Crystals of high quality without impurities can be obtained by re-crystallization.
In re-crystallization, the impure solid is dissolved in the hot solvent till it becomes saturated. Afterwards, to separate the impurities in the impure solid, above solution is filtered while it is still hot. Pure crystals of the solid is obtained by cooling the filtrate. Here, crystallization occurs because the cold solution is saturated with the solute although the hot solution is not. The soluble components present as impurities in minor quantities are not crystallized as the solution is not saturated in them.
Activity 02
Take about 50 g of common salt available in the market. Take about 50 cm^3 of water at a temperature of about 90 0c, into a beaker and dissolve crystals of salt until the solution is saturated. Filter the solution while it is still hot, using filter paper. Take the filtrate into a beaker, place it in a container of ice and shake slowly. Observe the crystals formed.
Solvent extraction
You have learnt that the nature of both solute and solvent affect solubility. Some solutes are soluble in large amounts in one solvent but dissolve in very small quantities in another solvent. For example, when solid iodine is added to water, a very small amount dissolves giving a light colored solution. But a larger amount of iodine dissolves in solvents like carbon tetra-chloride and cyclohexane.
When carbon tetra-chloride is added to an aqueous solution of iodine, they do not mix, and the layers get separated. After some time, it can be seen that the carbon tetra-chloride layer turns violet while the aqueous layer becomes pale. What has happened here is the extraction of iodine from the aqueous layer into the carbon tetra-chloride layer in which it is more soluble. The specialty here is the adequacy of a small volume of carbon tetra-chloride to extract the iodine from a large volume
of aqueous solution.
After this, iodine can be recovered by separating layers and evaporating carbon tetra-chloride.
Hence, solvent extraction is the method of drawing up a substance from a solvent in which it is less soluble, into another solvent in which it is more soluble, where the two solvents are immiscible and are in contact with each other. The medicinal components in some plants are found only in trace amounts. Medicinal solutions of higher concentration are prepared using solvent such as ethanol. Solvents extraction is used in the production of medicinal extracts and potions.
Simple distillation, fractional distillation and steam distillation
The separation of components by boiling a solution or a mixture and condensing the vapour is called distillation.
So, there must be a mechanism to cool the distillate or the vapour that evolves when a given mixture is heated. The Liebig condenser in the school laboratory is an apparatus designed for this. The vapour is allowed to pass through the condenser and cold water is circulated around it, in order to cool down the vapour. This condenser has an inlet and an outlet for water.
The Liebig condenser
Activity 03
Collect a sample of distilled water using the Liebig condenser available in the laboratory. Discuss with your science teacher the special facts that should be taken into consideration when setting up this apparatus.
Simple distillation
Simple distillation is used to separate components in a mixture which contains a volatile component with other non-volatile components. Only the volatile components are vaporized during distillation. The other components are left in the solution. For example, let us assume that a sample of well water is subjected to distillation.
In addition to water, it contains various salts and some gases dissolved in it. When heated slightly, the gases escape without getting condensed. Boiling points of salts are higher than that of water. Therefore, when the sample of well water is heated and vaporized, only the water vaporizes. Salts can be seen deposited at the bottom of the container. For this kind of distillation, special control of conditions is not essential. Hence, this is known as simple distillation. For this use of simple equipment such as the lie big condenser is adequate. The figure illustrates the apartments set up to
obtain distilled water from a sample of well water. Some countries use this method to obtain potable water from sea water.
Simple distillation
Fractional distillation
If the solution or the mixture subjected to separation contains several volatile components, simple distillation or the apparatus used in simple distillation cannot be used to separate them. It has to be perfumed under controlled conditions and for it, a fractionating column should be used. If two liquids are to be separated by fractional distillation, there should be a considerable difference in their boiling
points. That means, their volatilities need to be considerably different. Here, the vapour contains a higher percentage of the more volatile
Component and a lower percentage of the less volatile component.
Let us assume that the boiling point of a component a in A mixture is 80 0C and the boiling point of a component B is 40 0C. Upon heating, the mixture containing A and B begins to boil at a temperature slightly above 40 0C. Therefore, the vapour formed is richer in component B. When this vapour is collected and condensed at a temperature closer to 40 0C, the resulting liquid contains more B. A is present in smaller amount. When more of B gets removed from the mixture like this, its percentage of A increases. Then, the temperature at which the mixture boils also increases. This way, the components can be separated by collecting the vapours at different temperatures and condensing them. Separation of several components by distillation under controlled cooling conditions like this, is known as fractional distillation.
Crude oil is a mixture of many hydrocarbon components. When refining crude oil, a fractionating tower is used to control the cooling conditions. In this tower, the temperature is appropriately controlled at different level and the components are separately withdrawn at the respective positions. Components with lower boiling points are separated from the upper levels of the tower. Components with high boiling points are (bitumen) deposited at the bottom of the tower. This can be further understood by studying below figure.
Fractionating tower
Steam distillation
We know that certain parts of plants contain volatile components. Cinnamon, clove, cymbopogon, nutmeg and cardamom are few such examples. It is difficult to increase the temperature uniformly up to the boiling point of these compounds. Moreover, at the temperatures close to the boiling point, there is a possibility of destruction of these compounds by decomposition or getting converted into other compounds. Therefore, heat is supplied to the mixture by steam.
When water soluble compounds are mixed with water, the boiling point of such mixtures are above the boiling point of water. On the other hand, when the compounds that do not mix well with water are together with water, the boiling point of the mixture drops below the boiling point of water.
Most of the essential oils are immiscible with water and their boiling points are greater than that of water. They occur in living cells, mixed with water. Extraction of essential oils can be demonstrated in the laboratory by using an apparatus such as the one given below.
Steam distillation
When heat is supplied to these mixtures by steam, both water and the essential oil get liberated as a mixture of vapors at a temperature below the boiling point of water (100 0C). When the distillate (vapour) is cooled it separates into two layers because water and the essential oil are immiscible. Therefore, they can be easily separated as pure substances.
Chromatography
Chromatography is used to separate and identify the components present in a mixture (solid or liquid) containing non-volatile components. There are many different types of chromatography. The method carried out using paper (cellulose) is known as paper chromatography.
Add a little amount of water to a petri dish and dip one end of a dry strip of a filter paper in it. It can be observed that a stream of water particles is soaked up the strip of paper from bottom to top. Even when water is replaced by compounds such as acetone, ether and ethyl alcohol, a flow of liquid which flows up from the bottom to top can be seen. The strip of paper is called the stationary phase while the solvent that is soaked into it is called the mobile phase. When a small portion of the mixture whose components need to be separated is placed on this paper, the components in the mixture dissolve in the solvent and move up with the solvent front. This upward movement depends on
the forces of attraction of the components of the mixture, to the stationary phase. For example, if one component in the mixture is strongly attracted to the stationary phase (the paper), its rate of upward movement decreases. If there is another component in the mixture that is relatively less attracted to the mixture, it moves up faster through the stationary phase. Because of this difference in the speed of movement of the components in the mixture, they get separated from one another. Let us do the following activity to separate out the components in a chlorophyll mixture using paper chromatography.
Activity 04
Materials required ;- Chromatography papers or filter papers or A4 papers, spinach leaves, mortar and pestle, a thin piece of silk cloth, a boiling tube, a rubber stopper with a hook
Method ;- Crush a few spinach leaves thoroughly using mortar and the pestle available in the laboratory. Collect the chlorophyll extract onto a watch glass by placing the crushed paste on a piece of thin silk cloth and squeezing it.
• Cut a strip of chromatography/filter/A4 paper
• Take a little bit of the chlorophyll extract to a capillary tube and place it on the strip of the paper a little above the end of it as shown in the diagram. The solvent vaporizes leaving chlorophyll on the paper. Place another drop on same spot.
• Connect a piece of string to the end of the paper strip opposite to that with the chlorophyll drop.
• Add a solvent such as acetone/kerosene/petrol to the boiling tube and stopper it. Leave it to saturate. Connect a hook to the stopper as shown in the diagram and suspend the strip on the hook so that its other end dips in the solvent. Ensure that the sides of the strip do not touch the walls of the boiling tube.
Leave it for some time and then take out the strip of paper and observe It can be seen that the components of different colours are separated. This leads to the conclusion that chlorophyll contains different components.
Therefore, Chromatography technique can be used to separate and identitfy several components when they are mixed together. Chromatography is used to find whether poisonous chemicals are mixed with water. It is also used to check whether harmful substances are associated with food items. Chromatography technique is also useful in identifying active chemical compounds in plants.
Uses of separating techniques
Extraction of salt from sea water
In sri lanka salt is produced by the evaporation of sea water in salterns. The sea water collected in the salt pans are concentrated by evaporation during which salt crystallizes out finally. The separating techniques evaporation and crystallization are used in this method.
The location and structure of a saltern is very important in the production of salt. The geographical and environmental factors that should be taken into consideration when setting up a saltern are as follows.
1. A flat land situated closer to a coastal area to obtain sea water easily
2. Presence of a clayey soil with minimum percolation of water
3. Prevailence of dry and hot climate with bright sunlight and wind throughout the year
4. An area with minimum rainfall.
Regarding the structure of a saltern, three types of tanks can be identified.
- Large, shallow tanks
- Medium tanks
- Small tanks
The main steps of the production of salt in a saltern are as follows.
Step 1 :- The sea water is either made to flood into the big, shallow tanks during high tide or is pumped into them and allowed to evaporate by sunlight. When the concentration is twice as double the initial concentration of sea water, calcium carbonate (CaCO3) begins to crystallize and
precipitate at the bottom of the first tank.
Step 2 :- This water is then transfered into the medium- sized tanks in which the water evaporates further. When the concentration of water is about four times the initial concentration, calcium sulphate (CaSO4) crystallizes and settles down at the bottom.
Step 3 :- Following the precipitation of calcium sulphate, the solution is allowed to flow from the medium tanks into the smaller tanks in which water is evaporated further. When the concentration is nearly ten times the concentration of initial sea water, salt (NaCl) crystallizes and precipitates
at the bottom.
While salt is precipitating, the concentration of the solution increases further. Even before the total precipitation of sodium chloride is complete, magnesium chloride (MgCl2) and magnesium sulphate (MgSO4) begin to precipitate. These give a bitter taste to salt. The solution left after the precipitation of salt is known as mother liquor or bittern.
Salt deposited in the third tanks is taken out, heaped in prismatic piles at another place and stored for a period of six months. Pure sodium chloride is not hygroscopic. But if salt contains magnesium chloride and magnesium sulphate, it becomes bitter and hygroscopic, when exposed to the atmosphere. But, as magnesium chloride and magnesium sulphate absorb moisture in the atmosphere and go into solution, with the elapse of about six months most of them get removed, salt is retained as a solid.
Extraction of essential oils
Volatile compounds obtained from plant materials are referred to as essential oils. The reason for the characteristic aroma of some plant materials is these volatile compounds these they contain. Some main essential oils produced in our country are :
• Cinnamon leaf oil
• Cinnamon bark oil
• Citronella oil
• Pepper oil
• Cardamom oil
• Nutmeg oil
• Clove bud oil
• Eucalyptus oil
Cinnamon bark oil, pepper oil and cardamom oil promote the flavour and the scent of food. Cinnamon leaf oil, pepper oil and cardamom oil have medicinal properties as well and are frequently used in the production of medicinal ointments, toothpaste and the perfumes added to soap. Some plant parts in which essential oils are formed are given below.
The separating techniques such as steam distillation and solvent extraction are used to extract essential oils. From cinnamon leaves, oil is obtained by passing steam through them.
Extraction of essential oils by steam distillation
In this method steam generated by the steam bath is passed through the plant parts. Essential oils, being mixed with water vapour, vapourises at a temperature below 100 0C. Condensation of this mixture of vapours gives essential oil and water. As they are immiscible, they can be obtained separately.
Obtaining essential oils by solvent extraction
Solvent extraction is another method of extracting essential oils. Organic solvents such as ether, chloroform and toluene are used in this method. When plant parts are shaken with the solvent, essential oil dissolves in the solvent. The essential oil is separated by letting the solvent to vaporize.
Volatile oils in some plant parts can also be obtained by compressing them under a suitable pressure.
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