- 1 Separating Mixtures Techniques & Filtration
- 1.1 1. Threshing
- 1.2 2. Winnowing
- 1.3 3. Hand-Picking
- 1.4 4. Sieving
- 1.5 5. Magnetic Separation
- 1.6 To Separate a Mixture of iron Filings and Sulphur Powder
- 1.7 Separation of Scrap Iron
- 1.8 6. Decantation
- 1.9 7. Loading
- 1.10 8. Filtration
- 1.11 9. Evaporation
- 1.12 10. Distillation
- 1.13 Separation of Mixtures by Using More Than One Method
Inorganic chemistry, another important branch of Chemistry Topics, deals with the properties and behavior of inorganic compounds and elements.
Separating Mixtures Techniques & Filtration
Many of the materials around us are mixtures. These mixtures have two or more than two substances (or components) 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 components of a mixture for our use. So, we have to separate the various mixtures into their individual components to make them useful in our daily life.
The various components of a mixture have different physical properties such as hardness, density, solubility, size of particles, behaviour towards magnet, volatility, boiling points, etc. This difference in the physical properties of components is used to separate them from a mixture. So, when we want to separate the components of a mixture, we should first find out some properties which would be different for different components. Some of the methods which are commonly used for separating the components of mixtures are : Threshing, Winnowing, Hand-picking, Sieving, Magnetic separation, Decantation, Loading, Filtration, Evaporation and Distillation. The basis for choosing method of separating the components of a mixture is the difference in the physical properties of the components. We will now discuss all these methods of separating mixtures in detail, one by one. Let us start with threshing.
When a foodgrains crop like wheat or paddy (rice) matures, it is harvested from the fields. In harvesting, the crop plants are cut close to the ground. The harvested crop is then dried in the sun. In this way we get bundles of stalks (or stems) of dried crop plants which have grains (wheat grains or rice grains) attached at their top. The grains attached to the stalks are covered in a thin layer called ‘chaff’. Each stalk has a large number of chaff covered grains. The stalks with chaff covered grains can be considered to be a mixture of stalk, chaff and grains. We will now describe how grains are separated from the stalks and chaff.
The grains are separated from stalks (on which they grow) by the process of threshing. Threshing is the process in which stalks (of wheat, paddy, etc.) are beaten to separate grains from the stalks, and from the chaff that covers the grains. The method of separation is based on the fact that the ‘stalks’ (or stems) of the crop plants and the ‘chaff’ (covering of grains) are soft materials whereas the grains themselves are very hard. Being soft, stalks and chaff can be broken into pieces on beating but the grains remain unaffected.
Threshing is done by holding the bundles of stalks in hands and hitting them on a hard surface (see Figure). In this way, the grains separate from the stalks. Threshing is also done with the help of cattle such as bullocks, etc. In this method, the harvested and dried crop plants (or stalks) are spread on the ground in a small area, and various cattle such as bullocks, buffaloes and camels, etc., are made to walk over them in circles, again and again, for a considerable time. The cattle’s feet crush (or beat) the stalks due to which the grains get separated from stalks. This crushing also breaks the chaff around the grains and hence the grains get separated from chaff. During threshing, the stalks are converted into very small pieces called ‘hay’ which is used as a dry fodder for cattle. The broken chaff forms the ‘husk’. In larger farms, a motorised machine called ‘thresher’ is used for the threshing process. Before we go further and discuss winnowing, please note that ‘blowing air is called wind’.
When a farmer threshes wheat crop in his fields, he gets a mixture of wheat grains and husk. Now, before wheat grains can be used, husk has to be removed from them. The husk is separated from wheat grains by the method of winnowing. Winnowing is the method of separating husk from grains with the help of wind. This method is based on the fact that husk is very light whereas wheat grains are comparatively heavy. Winnowing is done by using a winnowing basket. Winnowing basket is called ‘chhaaj’ in Hindi. The process of winnowing is described below.
The mixture of wheat grains and husk is taken in a winnowing basket. The farmer stands at a higher platform from the ground and allows the mixture of wheat grains and husk to fall down from a height by shaking his winnowing basket continuously (see Figure). The wheat grains, being heavy, fall down vertically to the ground and form a heap of wheat grains (see Figure). The husk particles, being lighter, are carried away by wind to a greater distance. So, the husk forms a separate heap at a small distance away from the heap of wheat grains (see Figure). In this way, husk gets separated from wheat grains. Thus, the method of winnowing is used to separate husk from various types of grains like wheat, rice, etc.
‘Hand-picking’ means ‘to take out by hand’. The method of hand-picking is used to separate those mixtures where one of the components is in small quantity (and it can be picked up easily by hand). The method of hand-picking is usually used to separate undesirable substances such as small pieces of stones from wheat, rice and pulses.
The foodgrains (pulses, rice or wheat) containing small pieces of stones are taken in a flat container (like thali). The pieces of stone are picked up from the grains by hand, one by one, and thrown away (see Figure). Ultimately, all the stone pieces are removed and only foodgrains are left behind. Please note that the quantity of undesirable substances (such as small pieces of stones, etc.) in the foodgrains like pulses, rice or wheat, etc., is usually small which can be separated easily by the method of hand-picking.
A sieve is a shallow vessel having small holes at its bottom. An iron mesh can also be used as a sieve in some cases. The method of separating a mixture by using a sieve is called sieving. Sieving is used to separate those solid mixtures which have components of different sizes. The mixture having components of different sizes is put in a sieve and the sieve is moved back and forth continuously. The smaller particles of the mixture pass through the holes of the sieve and collect in a vessel kept below. The bigger particles of the mixture cannot pass through the small holes of the sieve and remain behind in the sieve. In this way, the mixture gets separated into two components : one having small particles and the other having larger particles. It should be noted that the size of the holes of a sieve to be used depends on the size of the particles of the material to be separated from the mixture. Sieves of different sizes of holes are used for separating different substances (see Figure). We will now take some examples to understand the method of sieving more clearly.
(i) The wheat flour which we bring from a flour mill is sieved by using a sieve having very small holes. The sieving of flour is done to remove the coarse particles of flour (called wheat bran) from it so as to obtain fine flour for making chapattis. When wheat flour is put in a sieve and the sieve is moved back and forth repeatedly, the fine particles of flour pass through the small holes of the sieve and collect in the vessel kept below (see Figure). The bigger particles of wheat bran do not pass through the very small holes of the sieve and remain behind in the sieve. In this way, wheat flour is separated into ‘wheat bran’ and ‘fine flour’. Please note that sieving of wheat flour is not considered good for health because during sieving, the useful roughage is lost from the flour.
(ii) The wheat brought from the fields (after threshing and winnowing) still contains some impurities like pieces of stone, stalk and husk, etc. In a flour mill, the impurities are removed
from wheat before grinding, by the process of sieving. A bagful of wheat is poured on a slanting sieve made of iron mesh. The wheat grains pass through the sieve whereas pieces of stones, stalk and husk are left behind.
(iii) We need fine sand for mixing in cement for plastering walls of a house. Fine sand is obtained by the method of sieving. The coarse sand having larger particles and pebbles in it is put on a sieve consisting of a big iron mesh held in slanting position (see Figure). The fine sand particles pass through the iron
mesh and collect on the ground. The bigger particles of sand and pebbles, etc., do not pass through the iron mesh and remain behind. Thus, fine sand can he separated from larger particles by sieving.
(iv) The method of sieving is also used to separate similar objects of different sizes. For example, some cashewnuts are small in size whereas other cashewnuts are bigger in size. So, the cashewnuts of different sizes are separated in cashewnut factories by the process of sieving. Similarly, jewellers separate the pearls of different sizes by the method of sieving.
5. Magnetic Separation
Magnetic separation means ‘separation of a mixture by using a magnet’. We know that iron is attracted by a magnet. This property of iron is used to separate it from a mixture. For example, a mixture of iron filings and sulphur powder can be separated by using a magnet. This is because iron filings are attracted by a magnet whereas sulphur is not attracted by a magnet.
To Separate a Mixture of iron Filings and Sulphur Powder
In order to separate iron filings from sulphur, a horse-shoe type magnet is moved on the surface of the mixture. The iron filings are attracted by the magnet, they cling to the poles of the magnet and get separated (see Figure). This process has to be repeated a number of times to achieve complete separation of iron filings. Sulphur powder is not attracted by a magnet so it remains behind. A mixture of iron filings and sand can also be separated by using a magnet in a similar way. In industries, the impurity of iron present in several substances is removed by the use of magnets. A carpenter can also separate iron nails from saw-dust by the method of magnetic separation. This is because iron nails stick to the magnet but saw-dust does not.
Separation of Scrap Iron
Waste materials supplied to factories contain a number of scrap metals including iron. So, in factories, scrap iron is separated from the heap of waste materials by using big electromagnets fitted to a crane (A magnet which works with an electric current is called an electromagnet). When a crane fitted with a powerful electromagnet is lowered on to the heap of waste materials, then the scrap iron objects present in the heap cling to the electromagnet (see Figure). The crane is then moved up and away to drop these scrap iron objects at a separate place. In this way, scrap iron is separated from the heap of waste materials.
Before we go further, we should know the meaning of the terms : sediment, sedimentation and decantation. This is discussed below. Suppose we have a liquid which contains some solid particles suspended in it [see Figure]. Now, if this liquid is allowed to stand undisturbed for some time, then the heavier particles of the solid will sink to the bottom of the beaker [Figure (b)]. The solid matter which settles down at the bottom of a liquid is called a sediment. And the deposition of solid matter at the bottom of a liquid is called sedimentation.
After sedimentation, we get two layers. The upper layer is of clear liquid and the lower layer is of sediment (solid matter). We can now pour out the clear liquid into another beaker and separate it from the sediment. The pouring out of a liquid from a vessel without disturbing the sediment (settled matter) is called decantation. After knowing all these things, it will now be easier for us to understand the separation of a mixture by the method of decantation.
There are some mixtures which contain insoluble solid particles suspended in a liquid. The solid particles which are insoluble in a liquid can be separated by decantation. For example, a mixture of sand and water contains sand particles suspended in water. So, sand can be separated from sand-water mixture by the method of decantation. This is done as follows.
The sand and water mixture is taken in a beaker. It contains sand particles scattered throughout water [see Figure (a)]. Allow the mixture of sand and water to stand undisturbed for some time. On keeping, the heavier sand particles will settle down at the bottom of the beaker. This layer of sand is called sediment. The deposition of sand at the bottom of beaker is called sedimentation. When the sand settles down, clear water is left above the layer of sand [see Figure (b)].
We pour the clear water into another beaker gently with the help of a glass rod, without disturbing the sediment of sand [Figure (c)]. This process is called decantation. When all the clear water has been poured out, only sand is left at the bottom of the beaker. In this way, a mixture of sand and water has been separated into ‘sand’ and ‘clear water’ by decantation.
The process of decantation is used in our everyday life. This will become more clear from the following example. Rice and pulses bought from the market usually have dust or soil particles as impurities in them. So, rice and pulses are always washed before cooking. When we add water to rice or pulses taken in a vessel, then the impurities such as dust and soil particles get separated and go into water. Due to this the water becomes a little dirty. The rice or pulses, being heavy, remain at the bottom of the vessel and dirty water is separated by decantation by tilting the vessel to one side.
Thus, when rice and pulses are washed with water before cooking, then the water is separated from them by decantation. Decantation is also used to separate water from some vegetables after boiling. The process of decantation can be used for separation only when the solid does not dissolve in the liquid. Decantation cannot be used for separation if the solid
dissolves in the liquid. For example, sugar dissolves in water, so a mixture of sugar and water cannot be separated by the method of decantation.
The method of decantation can also be used for separating two immiscible liquids (which do not mix together). For example, water and oil are two immiscible liquids which can be separated by decantation. A mixture of water and oil is allowed to stand in a beaker. Since water and oil are immiscible, they form two separate layers in the beaker. Water, being heavier, forms the lower layer and oil, being lighter, forms the upper layer in the beaker. We can decant off the upper layer of oil into another beaker carefully. Water will be left behind. It should be noted that the separation of two immiscible liquids by decantation is not very satisfactory. A better separation of two immiscible liquids can be done by using a separating funnel. We will learn this in higher classes.
Please note that the process of decantation cannot be used for separating two miscible liquids (which mix with each other). For example, alcohol and water are miscible with each other (which mix to form a single layer). So, a mixture of alcohol and water cannot be separated by decantation. Similarly, a mixture of kerosene and petrol cannot be separated by decantation. This is because they are miscible liquids (which mix with each other to form a single layer).
Sometimes the water fetched from a river, lake or pond is muddy due to the presence of fine clay particles suspended in it. These suspended clay particles are very small and take a long time to settle down on their own. So, if we want to obtain clear water (from this muddy water) by decantation, then it will take a lot of time (because the suspended clay particles will settle down very, very slowly). The settling down of suspended clay particles of muddy water can be made faster by the process of loading. Loading is the process in which alum particles are deposited on suspended clay particles of muddy water to make them heavy and settle down rapidly. Loading is done by using a piece of alum (or phitkari). Loading helps the suspended clay particles to settle down rapidly. We will now describe how muddy water is made clear by the method of loading.
Muddy water is a mixture of water and suspended clay particles. The muddy water is taken in a beaker [Figure (A)]. A big piece of alum is taken and a thread is tied to it. The piece of alum is then put in the beaker of muddy water and rotated in it three or four times by holding from thread [Figure (F)]. After this the piece of alum is taken out from the beaker.
When the piece of alum is rotated in muddy water, some of the alum dissolves in water. The particles of dissolved alum deposit on the suspended clay particles of muddy water. In this way the suspended clay particles become loaded with heavy alum particles. Due to this the clay particles become quite heavy and settle down at the bottom of the beaker quite rapidly. When all the suspended clay particles settle down, the water becomes clear [Figure (c)]. The clear water is then decanted into another beaker. The loaded clay particles remain behind in the first beaker.
Before we go further and describe the process of filtration for separating mixtures, we should know something about a filter paper and how it is folded for carrying out filtration.
Filter Paper. A filter paper is a round piece of special paper which has millions of tiny holes in it. The holes of a filter paper are so small that they cannot be seen with naked eyes. They can be seen only through a microscope. The liquids (like water, salt solution, oil, etc.) can pass through the tiny holes of a filter paper but solid particles (like chalk particles or sand particles) being large, cannot pass through the tiny holes of the filter paper. The solid which remains on the filter paper is called residue. The liquid which passes through the filter paper is called filtrate. A filter paper is shown in Figure (a).
Folding of Filter Paper. Before a filter paper can be used for the process of filtration, it is folded properly to make a hollow cone (which is then kept in the funnel).
The filter paper is folded in the following steps :
- The round piece of filter paper is taken and folded in half [see Figure (b)].
- The half folded filter paper is folded again [see Figure (c)].
- The twice folded filter paper is opened to form a hollow cone (by keeping three layers of filter paper on one side and one layer on the other side) [see Figure (d)].
- This cone of filter paper is placed inside a funnel [as shown in Figure (e)]. It is then used for the process of filtration in a science laboratory.
The process of removing insoluble solids from a liquid by using a filter paper is known as filtration. Filtration is used for separating insoluble substances from a liquid.
The mixture of insoluble solid and the liquid is poured into a filter paper cone fixed in a funnel by using a glass rod as a guide (see Figure). The liquid passes through the filter paper and collects in the beaker kept below the funnel. The solid particles (being bigger in size) do not pass through the filter paper and remain behind on the filter paper. The solid substance left behind on the filter paper is called residue. The clear liquid obtained is called filtrate. In this way, a mixture of insoluble solid in a liquid is separated into the ‘solid’ and ‘clear liquid’.
A mixture of chalk and water is separated by filtration. When the mixture of chalk and water is poured on the filter paper fixed in a funnel, then clear water passes through the filter paper and collects as filtrate. The chalk particles remain behind on filter paper as residue. A mixture of sand and water can also be separated by filtration. When the mixture of sand and water is poured on a filter paper fixed in a funnel, then clear water passes through filter paper and collects as filtrate. The sand remains behind on the filter paper in the form of a residue.
In science experiments we use a filter paper as ‘filter’ for carrying out the process of filtration. But in everyday life, different kinds of ‘filters’ are used. For example, a wire-mesh, a piece of cotton, a piece of muslin cloth (fine cloth), strainer (chhalni), or even a layer of sand, can be used as ‘filters’ for various purposes. The size of holes of the filter to be used depends on the size of the solid particles to be separated from the liquid.
When we make tea, we add tea-leaves. Now, to separate used tea-leaves we pour the prepared tea over a tea-strainer (see Figure ). The tea-strainer has a wire mesh in it which acts as a filter. The liquid tea passes through the small holes of a tea-strainer and collects in the cup below. The tea-leaves (being bigger in size) do not pass through the tea-strainer.
The tea-leaves remain behind on the tea-strainer. In this way, the used tea-leaves are separated from prepared tea by the method of filtration. Fresh fruit juice is also filtered by using a strainer as filter to remove the seeds and solid particles of pulp. When milk is cooled after boiling, a layer of cream (malai) is formed on its surface. This cream can be removed from the milk by filtering it through a piece of cloth. The milk passes through the cloth whereas cream is left on the cloth. Flere, a piece of cloth has been used as a filter.
In many homes, drinking water is filtered by using special ‘water filters’ made of ‘porcelain’ which remove bacteria from it. At water treatment works (which supply drinking water to the whole city), the river water or lake water is filtered by using ‘sand filters’. In cities, gutter water (dirty drain water) is filtered through big metallic filters to separate solid materials present in it so as to avoid choking of underground drains (called sewers). Please note that a mixture of two liquids cannot be separated by filtration. For example, a mixture of milk and water cannot be separated by filtration. Also, filtration cannot remove any solid substances which are dissolved in a liquid. For example, sugar dissolved in tea cannot be separated by filtration.
The changing of a liquid into vapours (or gas) is called evaporation. Evaporation is used to obtain a solid substance that has dissolved in water (or any other liquid). The dissolved substance is left as a solid residue when all the water (or liquid) has evaporated. The use of process of evaporation for separating a mixture is based on the fact that liquids vaporize easily whereas solids do not vaporize easily. Though evaporation of a liquid can take place even at room temperature but it is very slow at room temperature. Evaporation can be made quicker by heating the solution.
If we have a mixture of common salt and water, then we cannot separate common salt from water by filtration. This is because common salt is completely dissolved in water and not insoluble in it. We can recover common salt from salt-water mixture (or salt solution) by the process of evaporation.
The common salt dissolved in water can be separated by the process of evaporation. This is done as follows : The solution of common salt and water is taken in a porcelain dish and heated gently by using
a burner [see Figure (a)]. The water present in salt solution will form water vapours and escape into atmosphere. When all the water present in the solution of common salt and water gets evaporated, then common salt is left behind in the porcelain dish as a white solid [see Figure (b)],
The process of evaporation is used on a large scale to obtain common salt from sea-water. Sea-water is trapped in shallow lakes (called lagoons) and allowed to stand there. The heat of sun gradually evaporates water in the shallow lakes and common salt is left behind as a solid. Actually, sea-water contains many salts dissolved in it. The major salt in sea-water is, however, common salt (It is about 80 per cent of all the salts in sea-water).
So, when sea-water is evaporated, we get a mixture of salts. Common salt is obtained from this mixture of salts by further purification. We will study this in detail in higher classes. Please note that the chemical name of common salt is sodium chloride.
When a sugar solution is evaporated, then water is eliminated and solid sugar is left behind. We can also separate copper sulphate (blue vitriol) from its solution by the process of evaporation. The substances like potash alum and potassium nitrate, etc., are also separated from their water solutions by the process of evaporation. It should be noted that during the evaporation of a ‘water solution’ we get the ‘dissolved solid substance’ but water cannot be recovered in this method. Water evaporates into the atmosphere.
We can obtain salt from salt-water mixture (or salt solution) by evaporation but water cannot be recovered during evaporation, it is lost into the air. In order to recover both, salt as well as water, from a salt-water mixture (or salt solution), the process of distillation is used. Distillation is the process of heating water to form water vapour (or steam), and then cooling the hot water vapour (or steam) to get back liquid water. Distillation can be represented as :
The changing of water into water vapour (or steam) on heating, is called evaporation (or boiling). And the changing back of hot water vapour (or steam) into liquid water on cooling, is called condensation. Thus, distillation involves two processes : evaporation (or boiling) followed by condensation.
The separation of a salt-water mixture into salt and water by distillation depends on the fact that water is a volatile liquid which forms vapours (or gas) on heating but salt is a non-volatile solid which does not form vapour on heating. We will now describe a very simple distillation experiment to separate the salt-water mixture into salt and water.
Put the salt-water mixture in a metal kettle and close its lid. Heat the kettle on a burner. After some time, salt-water mixture starts boiling and steam starts coming out from the spout of the kettle (see Figure). Take a frying pan with a wooden (or plastic) handle and put some ice in it (to cool it). Hold the frying
pan in slanting position just above the spout of the kettle in such a way that steam coming out of the spout comes in contact with the bottom of frying pan. When the hot steam comes in contact with the ice cold bottom of frying pan, it gets cooled and condenses to form drops of liquid water. This pure water is collected in a beaker kept below the frying pan (see Figure 18). After all the water of salt-water mixture taken in the kettle boils off, salt is left behind in the kettle. This salt can be taken out from the kettle. In this way, salt-water mixture has been separated into salt and water by the process of distillation.
Separation of Mixtures by Using More Than One Method
We have just studied some of the methods for separating mixtures into their components. Some of the mixtures can be separated by a single method. On the other hand, many mixtures can only be separated into individual components by a combination of methods of separation. These methods are applied turn by turn. The separation of mixtures by the use of more than one method of separation will become clear from the following examples.
Example Problem 1.
How will you separate a mixture of sand and salt ?
This mixture contains two components : sand and salt. Now, salt is soluble in water whereas sand is insoluble in water. This difference in the solubility of salt and sand is used to separate them. This is done as follows :
- Some water is added to the mixture of sand and salt in a beaker and stirred. Salt dissolves in water to form salt solution whereas sand remains undissolved. It is then filtered.
- On filtering, sand is obtained as a residue on the filter paper and salt solution is obtained as filtrate.
- The salt solution (filtrate) is evaporated when pure salt is left behind.
Please note that the mixture of sand and salt has been separated into its components (sand and salt) by using two methods of separation : filtration and evaporation.
Example Problem 2.
Is it possible to separate sugar mixed with wheat flour ? If yes, how will you do it?
Sugar is soluble in water whereas wheat flour is insoluble in water. This difference in the solubilities of sugar and wheat flour can be used to separate their mixture as follows :
- Some water is added to the mixture of sugar and flour in a beaker and stirred. Sugar dissolves in water to form sugar solution whereas flour remains undissolved. It is then filtered.
- On filtering, wheat flour is obtained as a residue on the filter paper and sugar solution is obtained as filtrate.
- The sugar solution (filtrate) is evaporated slowly when pure sugar is left behind.
Example Problem 3.
How will you separate a mixture of iron filings, chalk powder and common salt ?
- Bring a magnet near this mixture several times. All the iron filings will stick to the magnet and get separated. We are then left with a mixture of chalk powder and common salt.
- Some water is added to the mixture of common salt and chalk powder and stirred. Common salt dissolves in water to form salt solution, whereas chalk powder remains undissolved.
- On filtering, chalk powder is obtained as a residue on the filter paper and salt solution is obtained as filtrate.
- The salt solution (filtrate) is evaporated when pure common salt is left behind.
This mixture has been separated by using three methods of separation : magnetic separation, filtration and evaporation.