Separation of Substances PDF

3 Separation of Substances I n our daily life, there are many instances when we notice a substance being separated from a mixture of materials. Tea leaves are separated from the liquid with a strainer, while preparing tea (Fig. 3.1)....

3 Separation of Substances I n our daily life, there are many instances when we notice a substance being separated from a mixture of materials. Tea leaves are separated from the liquid with a strainer, while preparing tea (Fig. 3.1). Fig. 3.2 Butter is taken out by churning milk or curd than mango or guava? Imagine you are given a glass of sand with salt mixed in Fig. 3.1 Separating tea leaves with a strainer it. Impossible, even to think of Grain is separated from stalks, while separating salt from this mixture by harvesting. Milk or curd is churned to picking out grains of sand by hand! separate the butter (Fig. 3.2). We gin cotton to separate its seeds from the fibre. But, why would we need to Perhaps you might separate substances like this have eaten salted at all, is what Paheli wants daliya or poha. If you to know. found that it had chillies in it, you may have carefully taken them out before eating. Activity 1 Suppose you are given a basket In Column 1 of Table 3.1, are given a containing mangoes and guavas and few processes of separation. The purpose asked to separate them. What would you of separation and the way separated do? Pick out one kind and place them components are used is mentioned in in a separate container, right? Column 2 and 3 respectively. However, Seems easy, but what if the materials the information given in Columns 2 and we want to separate are much smaller 3 is jumbled up. Can you match each 20 SCIENCE Rationalised 2023-24  Table 3.1 Why do we separate substances? Separation Purpose for which we do What do we do with the process the separation separated components? 1) Separate a) To separate two different, i) We throw away the soild stones from rice but useful components. component. 2) Chur ning milk b) To remove non-useful ii) We throw away the to obtain butter components. impurities. 3) Separate tea c) To remove impurities or iii) We use both the leaves har mful components. components. process with its purpose and the way the sheet of paper? Are there pieces of separated components are used? stone, husks, broken grain and particles We see that, before we use a of any other grain in it? Now, remove substance, we need to separate harmful with your hand the pieces of stone, or non-useful substances that may be husks and other grains from it. mixed with it. Sometimes, we separate This method of handpicking can even useful components if we need to be used for separating slightly larger use them separately. sized impurities like the pieces of dirt, The substances to be separated may stone, and husk from wheat, rice or be particles of different sizes or pulses (Fig. 3.3). The quantity of such materials. These may be in any three impurities is usually not very large. states of matter i.e., solid, liquid or gas. In such situations, we find that So, how do we separate substances handpicking is a convenient method of mixed together if they have so many separating substances. different properties? 3.1 METHODS OF SEPARATION We will discuss some simple methods of separating substances that are mixed together. You may come across some of these methods being used in day to day activities. Handpicking Activity 2 Fig. 3.3 Handpicking stones from grain Bring a packet of food grain purchased from a shop to the classroom. Now, Threshing spread the grains on a sheet of paper. You must have seen bundles of wheat Do you find only one kind of grain on or paddy stalks lying in fields after SEPARATION OF SUBSTANCES 21 Rationalised 2023-24 harvesting the crop. Stalks are dried in this mixture on a plate or a newspaper. the sun before the grain is separated Look at this mixture carefully. Can the from them. Each stalk has many grain two different components be made out seeds attached to it. Imagine the easily? Are the sizes of particles of the number of grain seeds in hundreds of two components similar? Would it be bundles of stalk lying in the field! How possible to separate the components by does the farmer separate grain seeds handpicking? from those bundles of stalks? Now, take your mixture to an open One may pluck mangoes or guavas ground and stand on a raised platform. from the trees. But, grain seeds are Put the mixture in a plate or sheet of much smaller than mangoes or guavas. paper. Hold the plate or the sheet of So, plucking them from their stalks paper containing the mixture, at your would be impossible. How does one shoulder height. Tilt it slightly, so that separate grain seeds from their stalks? the mixture slides out slowly. The process that is used to separate What happens? Do both the grain from stalks etc. is threshing. In components — sand and sawdust (or this process, the stalks are beaten to free powdered leaves) fall at the same place? the grain seeds (Fig. 3.4). Sometimes, Is there a component that blows away? Did the wind manage to separate the two components? This method of separating components of a mixture is called winnowing. Winnowing is used to separate heavier and lighter components of a mixture by wind or by blowing air. direction of the air soop Fig. 3.4 Threshing husk threshing is done with the help of bullocks. Machines are also used to thresh large quantities of grain. Winnowing grain Activity 3 Make a mixture of dry sand with sawdust or powdered dry leaves. Keep Fig. 3.5 Winnowing 22 SCIENCE Rationalised 2023-24 This method is commonly used by farmers to separate lighter husk particles from heavier seeds of grain (Fig. 3.5). The husk particles are carried away by the wind. The seeds of grain get separated and form a heap near the platform for winnowing. The separated husk is used for many purposes such as fodder for cattles. Sieving Fig. 3.7 Pebbles and stones are removed from Sometimes, we may wish to prepare a sand by sieving dish with flour. We need to remove to separate pebbles and stones from impurities and bran that may be present sand (Fig. 3.7). in it. What do we do? We use a sieve and pour the flour into it (Fig. 3.6). Activity 4 Sieving allows the fine flour particles Bring a sieve and a small quantity of to pass through the holes of the sieve flour from home, to the class. Sieve the while the bigger impurities remain on flour to separate any impurities in it. the sieve. Now, make a fine powder of chalk pieces In a flour mill, impurities like husk and mix it with the flour. Can we and stones are removed from wheat separate the flour and the powdered before grinding it. Usually, a bagful of chalk by sieving? wheat is poured on a slanting sieve. The Sieving is used when components of sieving removes pieces of stones, stalk a mixture have different sizes. and husk that may still remain with wheat after threshing and winnowing. Sedimentation, Decantation and Filtration Sometimes, it may not be possible to separate components of a mixture by winnowing and handpicking. For example, there may be lighter impurities like dust or soil particles in rice or pulses. How are such impurities separated from rice or pulses before cooking? Fig. 3.6 Sieving Rice or pulses are usually washed You may have also noticed similar before cooking. When you add water to sieves being used at construction sites these, the impurities like dust particles SEPARATION OF SUBSTANCES 23 Rationalised 2023-24 get separated. These impurities go into strainer. Did all the tea leaves remain water. Now, what will sink to the bottom in the strainer? This process is called of the vessel — rice or dust? Why? Have filtration (Fig. 3.1). Which method of you seen that the vessel is tilted to pour separating tea leaves from prepared tea out the dirty water? is better, decantation or filtration? When the heavier component in a Let us now consider the example mixture settles after water is added to of water that we use. Do all of us, at it, the process is called sedimentation. all times, get safe water to drink? When the water (along with the dust) is Sometimes, water supplied through taps removed, the process is called may be muddy. The water collected from decantation (Fig. 3.8). Let us find a ponds or rivers may also be muddy, few other mixtures that can be separated especially after rains. Let us see if we through sedimentation and can use some method of separation to decantation. remove insoluble impurities like soil The same principle is used for from the water. separating a mixture of two liquids that Activity 5 do not mix with each other. For example, oil and water from their mixture can be Collect some muddy water from a pond separated by this process. If a mixture of or a river. If it is not available, mix some such liquids is allowed to stand for some soil to water in a glass. Let it stand for time, they form two separate layers. The half an hour. Observe the water component that forms the top layer can carefully and note your observations. then be separated by decantation. Does some soil settle at the bottom Let us again consider a mixure of a of water? Why? What will you call this solid and liquid. After preparing tea, process? what do you do to remove the tea leaves? Now, slightly tilt the glass without Usually, we use stainer to remove tea disturbing the water. Let the water from leaves. Try decantation. It helps a little. the top flow into another glass (Fig. 3.8). But, do you still get a few leaves in your What will you call this process? tea? Now, pour the tea through a Is the water in the second glass still muddy or brown in colour? Now filter mixture (soil + water) it. Did the tea strainer work? Let us try filtering the water through a piece of cloth. In a piece of cloth, small holes or pores remain in between the woven undissolved material threads. These pores in a cloth can be (soil) used as a filter. Fig. 3.8 Separating two components of a mixture If the water is still muddy, impurities by sedimentation and decantation can be separated by a filter that has even 24 SCIENCE Rationalised 2023-24 smaller pores. A filter paper is one such filter that has very fine pores in it. Fig. 3.9 shows the steps involved in using a filter paper. A filter paper folded in the form of a cone is fixed onto a funnel (Fig. 3.10). The mixture is then poured on the filter paper. Solid particles in the mixture do not pass through it and remain on the filter. Fig. 3.11 Heating a beaker containing salt water see any change in the colour of water? Can you see any salt in the beaker, after stirring? Heat the beaker containing the salt water (Fig. 3.11). Let the water boil Fig. 3.9 Folding a filter Fig. 3.10 Filtration away. What is left in the beaker? paper to make a cone using a filter paper In this activity, we used the process of evaporation, to separate a mixture of Fruit and vegetable juices are usually water and salt. filtered before drinking to separate the The process of conversion of water seeds and solid particles of pulp. The into its vapour is called evaporation. method of filtration is also used in the The process of evaporation takes place process of preparing cottage cheese continuously wherever water is present. (paneer) in our homes. You might have Where do you think, salt comes seen that for making paneer, a few drops from? Sea water contains many salts of lemon juice are added to milk as it mixed in it. One of these salts is the boils. This gives a mixture of particles common salt. When sea water is allowed of solid paneer and a liquid. The paneer to stand in shallow pits, water gets is then separated by filtering the mixture heated by sunlight and slowly turns into through a fine cloth or a strainer. water vapour, through evaporation. In Evaporation a few days, the water evaporates completely leaving behind the solid salts Activity 6 (Fig. 3.12). Common salt is then Add two spoons of salt to water in obtained from this mixture of salts by another beaker and stir it well. Do you further purification. SEPARATION OF SUBSTANCES 25 Rationalised 2023-24  notice steam coming out from the spout of the kettle? Take a metal plate with some ice on it. Hold the plate just above the spout of the kettle as shown in Fig. 3.13. What do you observe? Let all the water in the kettle boil off. When the steam comes in contact with the metal plate cooled with ice, it Fig. 3.12 Obtaining salt from sea water condenses and forms liquid water. The water drops that you observed falling Use of more than one method from the plate, were due to condensation of separation of steam. The process of conversion of We have studied some methods for water vapour into its liquid form is separation of substances from their called condensation. mixtures. Often, one method is not Did you ever see water drops sufficient to separate the different condensed under a plate that has been substances present in a mixture. In such used to cover a vessel containing milk a situation, we need to use more than that has just been boiled? one of these methods. After all the water has evaporated, Activity 7 what is left behind in the kettle? We have thus, separated salt, sand Take a mixture of sand and salt. How and water using processes of will we separate these? We already saw decantation, filtration, evaporation and that handpicking would not be a condensation. practical method for separating these. Paheli faced a problem while Keep this mixture in a beaker and recovering salt mixed with sand. She add some water to it. Leave the beaker has mixed a packet of salt in a small aside for some time. Do you see the sand settling down at the bottom? The sand can be separated by decantation or filtration. What does the decanted liquid contain? Do you think this water contains the salt which was there in the mixture at the beginning? Now, we need to separate salt and water from the decanted liquid. Transfer this liquid to a kettle and close its lid. Heat the kettle for some time. Do you Fig. 3.13 Evaporation and condensation 26 SCIENCE Rationalised 2023-24 amount of sand. She then tried the method suggested in Activity 7, to recover the salt. She found, however, that she could recover only a small part of the salt that she had taken. What could have gone wrong? Can water dissolve any amount of a substance? In chapter 2, we found that many Fig 3.14 Dissolving salt in water substances dissolve in water and form would have remained mixed with the a solution. We say that these substances sand and could not be recovered. She are soluble in water. What will happen could solve her problem by using a if we go on adding more and more of larger quantity of water. these substances to a fixed quantity of Suppose, she did not have sufficient water? quantity of water to dissolve all the salt in the mixture. Is there some way that Activity 8 water could be made to dissolve more You will need a beaker or a small pan, a salt before the solution gets saturated? spoon, salt and water. Pour half a cup Let us try and help Paheli out. of water in the beaker. Add one teaspoonful of salt and stir it well, until Activity 9 the salt dissolves completely (Fig 3.14). Take some water in a beaker and mix Again add a teaspoonful of salt and stir salt in it until it cannot dissolve any well. Go on adding salt, one teaspoonful more salt. This will give you a saturated at a time, and stir. solution of salt in water. After adding a few spoons of salt, do Now, add a small quantity of salt to you find that some salt remains this saturated solution and heat it. What undissolved and settles at the bottom do you find? What happens to the of the beaker? If yes, this means that undissolved salt in the bottom of the no more salt can be dissolved in the beaker? Does it dissolve, now? If yes, amount of water we have taken. The can some more salt be dissolved in this solution is now said to be saturated. solution by heating it? Here is a hint as to what might have Let this hot solution cool. Does the gone wrong when Paheli tried to recover salt appear to settle at the bottom of the large quantity of salt mixed with sand. beaker again? Perhaps the quantity of salt was much The activity suggests that larger more than that required to form a quantity of salt can be dissolved in water saturated solution. The undissolved salt on heating. SEPARATION OF SUBSTANCES 27 Rationalised 2023-24  Does water dissolve equal amounts Table 3.2 of different soluble substances? Let us Number of spoons of find out. Substance substance that Activity 10 dissolved in water Take two glasses and pour half a cup Salt of water in each of them. Add a teaspoon Sugar of salt to one glass and stir till the salt dissolves. Go on adding salt, one teaspoon at a time, till the solution We have discussed a few methods of saturates. Record the number of spoons separating substances. Some of the of salt that dissolved in the water, in methods of separation presented in this Table 3.2. Now, repeat the same chapter are also used in a science activity with sugar. Repeat this with laboratory. some other substances that are soluble We also learnt that a solution is in water. prepared by dissolving a substance in a What do you notice from Table 3.2? liquid. A solution is said to be saturated Do you find that water dissolves different if it cannot dissolve more of the substances in different amounts? substance in it. Chur ning Saturated solution Condensation Sedimentation Decantation Sieving Evaporation Solution Filtration Threshing Handpicking Winnowing Handpicking, winnowing, sieving, sedimentation, decantation and filtration are some of the methods of separating substances from their mixtures. 28 SCIENCE Rationalised 2023-24  Husk and stones could be separated from grains by handpicking. iii Husk is separated from heavier seeds of grain by winnowing. Difference in the size of particles in a mixture is utilised to separate them by the process of sieving and filtration. In a mixture of sand and water, the heavier sand particles settle down at the bottom and the water can be separated by decantation. Filtration can be used to separate components of a mixture of an insoluble solid and a liquid. Evaporation is the process in which a liquid gets converted into its vapour. Evaporation can be used to separate a solid dissolved in a liquid. A saturated solution is one in which no more of that substance can be dissolved. More of a substance can be dissolved in a solution by heating it. Water dissolves different amount of soluble substances in it. 1. Why do we need to separate different components of a mixture? Give two examples. 2. What is winnowing? Where is it used? 3. How will you separate husk or dirt particles from a given sample of pulses before cooking. 4. What is sieving? Where is it used? 5. How will you separate sand and water from their mixture? 6. Is it possible to separate sugar mixed with wheat flour? If yes, how will you do it? 7. How would you obtain clear water from a sample of muddy water? 8. Fill up the blanks (a) The method of separating seeds of paddy from its stalks is called ___________. (b) When milk, cooled after boiling, is poured onto a piece of cloth the cream (malai) is left behind on it. This process of separating cream from milk is an example of ___________. (c) Salt is obtained from seawater by the process of ___________. (d) Impurities settled at the bottom when muddy water was kept overnight in a bucket. The clear water was then poured off from the top. The process of separation used in this example is called ___________. 9. True or false? (a) A mixture of milk and water can be separated by filtration. (b) A mixture of powdered salt and sugar can be separated by the process of winnowing. SEPARATION OF SUBSTANCES 29 Rationalised 2023-24  (c) Separation of sugar from tea can be done with filtration. (d) Grain and husk can be separated with the process of decantation. 10. Lemonade is prepared by mixing lemon juice and sugar in water. You wish to add ice to cool it. Should you add ice to the lemonade before or after dissolving sugar? In which case would it be possible to dissolve more sugar? SUGGESTED PROJECTS AND ACTIVITIES 1. Visit a nearby dairy and report about the processes used to separate cream from milk. 2. You have tried a number of methods to separate impurities like mud from water. Sometimes, the water obtained after employing all these processes could still be a little muddy. Let us see if we can remove even this impurity completely. Take this filtered water in a glass. Tie a thread to a small piece of alum. Suspend the piece of alum in the water and swirl. Did the water become clear? What happened to the mud? This process is called loading. Talk to some elders in your family to find out whether they have seen or used this process. THINGS TO SEE “The winnowers”, painted by Gustav Courbet in 1853 Reproduced with permission from Museè de Beaus Arts, Nantes, France 30 SCIENCE Rationalised 2023-24