unit 5

science 8th class
of 11
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  5 Separation of Substances Grain is separated from stalks, whileharvesting. Milk or curd is churned toseparate the butter (Fig. 5.2). As welearned in Chapter 3, we gin cotton toseparate its seeds from the fibre.Perhaps you might have eaten salted daliya   or  poha  . If youfound that it hadchillies in it, you may have carefully takenthem out before eating.Suppose you are given a basket containing mangoes and guavas andasked to separate them. What would youdo? Pick out one kind and place themin a separate container, right?Seems easy, but what if the materials we want to separate are much smaller  Fig. 5.1 Separating tea leaves with a strainer  Fig. 5.2 Butter is taken out by churning milk or curds  But, why would we need toseparate substances like thisat all, is what Paheli wantsto know.  Activity 1 In Column 1 of Table 5.1, are given a few processes of separation. The purposeof separation and the way separatedcomponents are used is mentioned inColumn 2 and 3 respectively. However,the information given in Columns 2 and3 is jumbled up. Can you match each  T  here are many instances when wenotice a substance beingseparated from a mixtureof materials. Tea leaves are separated from theliquid with a strainer, while preparingtea (Fig. 5.1).than mango or guava? Imagine you aregiven a glass of sand with salt mixed init. Impossible, even to think of separating salt from this mixture by picking out grains of sand by hand!  36S CIENCE process with its purpose and the way separated components are used? We see that, before we use a substance, we need to separate harmfulor non-useful substances that may bemixed with it. Sometimes, we separateeven useful components if we need touse them separately. The substances to be separated may  be particles of different sizes or materials. These may be solids, liquidsor even gases. So, how do we separatesubstances mixed together if they haveso many different properties? 5.1 M ETHODS   OF   S EPARATION  We will discuss some simple methods of separating substances that are mixedtogether. You may come across some of these methods being used in day to day activities. Hand Picking Activity 2 Bring a packet of grain purchased froma shop to the classroom. Now, spreadthe grain on a sheet of paper. Do youfind only one kind of grain on the sheet of paper? Are there pieces of stone,husks, broken grain and particles of any other grain in it? Now, remove with your hand the pieces of stone, husks andother grains from it. This method of handpicking  can be used for separating slightly larger sized impurities like the pieces of dirt,stone, and husk from wheat, rice or pulses (Fig. 5.3). The quantity of suchimpurities is usually not very large.In such situations, we find that handpicking is a convenient method of separating substances.  Table 5.1 Why do we separate substances? noita ra peS ssecorp ode w hcih w rof esopruP noita ra peseht ehthti w ode w odta h W  ?stnenopmocdeta ra pes et a r a peS)1 ecir mor f senot s ,t ner ef f ido w t et a r a peso T )a  .st nenopmocluf esut u b dlioseht  y a  w a  w or ht e W )i .t nenopmoc k limgninr uhC)2 r et t u bnia t  boot  luf esu-none v omer o T ) b .st nenopmoc eht  y a  w a  w or ht e W )ii .seit ir upmi a et et a r a peS)3 se v a el r oseit ir upmie v omer o T )c .st nenopmocluf mr a h eht ht o besue W )iii .st nenopmoc Fig. 5.3  Handpicking stones from grain   Threshing  You must have seen bundles of wheat or paddy stalks lying in fields after   37S EPARATION   OF   S UBSTANCES harvesting the crop. Stalks are dried inthe sun before the grain is separatedfrom them. Each stalk has many grainseeds attached to it. Imagine thenumber of grain seeds in hundreds of  bundles of stalk lying in the field! How does the farmer separate grain seedsfrom those bundles of stalks?One may pluck mangoes or guavasfrom the trees. But, grain seeds aremuch smaller than mangoes or guavas.So, plucking them from their stalks would be impossible. How does oneseparate grain seeds from their stalks? The process that is used to separategrain from stalks etc. is threshing . Inthis process, the stalks are beaten to freethe grain seeds (Fig. 5.4). Sometimes,this mixture on a plate or a newspaper.Look at this mixture carefully. Can thetwo different components be made out easily? Are the sizes of particles of thetwo components similar? Would it bepossible to separate the components by handpicking?Now, take your mixture to an openground and stand on a raised platform.Put the mixture in a plate or sheet of paper. Hold the plate or the sheet of paper containing the mixture, at your shoulder height. Tilt it slightly, so that the mixture slides out slowly. What happens? Do both thecomponents — sand and sawdust (or powdered leaves) fall at the same place?Is there a component that blows away?Did the wind manage to separate thetwo components? This method of separatingcomponents of a mixture is called  winnowing . Winnowing is used toseparate heavier and lighter componentsof a mixture by wind or by blowing air. Fig. 5.4   Threshing  threshing is done with the help of  bullocks. Machines are also used tothresh large quantities of grain.  Winnowing Activity 3 Make a mixture of dry sand withsawdust or powdered dry leaves. Keep Fig. 5.5 Winnowing   38S CIENCE  This method is commonly used by farmers to separate lighter husk particlesfrom heavier seeds of grain (Fig. 5.5). The husk particles are carried away  by the wind. The seeds of grain get separated and form a heap near theplatform for winnowing. The separatedhusk is used for many purposes suchas fodder for cattles. Sieving Sometimes, we may wish to prepare a dish with flour. We need to removeimpurities and bran that may be present in it. What do we do? We use a sieveand pour the flour into it (Fig. 5.6).Sieving allows the fine flour particlesto pass through the holes of the sieve while the bigger impurities remain onthe sieve.In a flour mill, impurities like husk and stones are removed from wheat  before grinding it. Usually, a bagful of  wheat is poured on a slanting sieve. Thesieving removes pieces of stones, stalk and husk that may still remain with wheat after threshing and separate pebbles and stones fromsand (Fig. 5.7).  Activity 4 Bring a sieve and a small quantity of flour from home, to the class. Sieve theflour to separate any impurities in it.Now, make a fine powder of chalk piecesand mix it with the flour. Can weseparate the flour and the powderedchalk by sieving?Sieving is used when components of a mixture have different sizes. Sedimentation, Decantationand Filtration Sometimes, it may not be possible toseparate components of a mixture by  winnowing and handpicking. For example, there may be lighter impuritieslike dust or soil particles in rice or pulses. How are such impuritiesseparated from rice or pulses beforecooking?Rice or pulses are usually washed before cooking. When you add water tothese, the impurities like dust and soil Fig. 5.6 Sieving   You may have also noticed similar sieves being used at construction sites Fig. 5.7 Pebbles and stones are removed from sand by sieving 
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