Separation of Substances

• You all know about mixture namkeen, it is a famous Indian snack. All variety of snacks is mixed in small quantities with fried nuts, puffed rice, dry fruits and fried grams in any quantity.
• So, mixture is a substance that composed of two or more simpler substance in any proportion.

1.0Introduction

Many things around us such as stones, wood, etc. are used as such. However, a large number of substances available in nature are mixed with certain other substances. Thus, in order to use them we separate them by various methods. On the separation of a mixture, some of its components may be useful or all components are useful. Following examples will illustrate the above statement. (i) Milk is churned to separate butter. (ii) We gin cotton to separate seeds from the fibre. (iii) We took out chillies from poha or daliya. (iv) Pick out one kind of fruit from basket containing mangoes and guavas. (v) Tea leaves are separated from the liquid with strainer.

2.0Mixtures

An impure substance, or a mixture, contains two or more substances in any proportion. Also, these substances can be separated from one another. A mixture may contain elements, compounds, or elements and compounds. A mixture should not be confused with a compound. The proportion of the constituent elements in a compound is fixed. But that of the components of a mixture is not. For example, any amount of salt or sugar can be mixed with any amount of water to form a mixture. Most materials that we use or come across are mixtures. Some of them are useful and others need purification (i.e., separation) before use.

Mixture is a substance with variable composition. For example, different mixture can be prepared by adding variable quantity of sugar in water.

• The reason for which we need to separate mixture are following. Different component of a mixture are separated in order to either separated the un-required component from a mixture.
• The solid tea leaves separated out from the tea by the process called filtration, it a common example which we see in our day to day life.

Some examples of naturally occurring and man-made useful mixtures are given below.

• Air Air contains the elements nitrogen, oxygen and argon and the compounds carbon dioxide and moisture (water vapour).
• Natural water Natural water contains dissolved air, which is essential for aquatic animals.
• Sugar solution and soft drinks A solution of sugar or glucose in water is a mixture. Soft drinks (or fizzy drinks) contain carbon dioxide dissolved in water, to which are added some sweetening and flavouring agents. They are highly refreshing. 4.Medicines Most medicines are mixtures. On the label of a medicine bottle, you will find the substances or ingredients it contains.
• Alloys An alloy is a metal mixed with other metal(s) or non-metal(s). The components are so thoroughly mixed that the whole thing appears to be a single substance. They are usually made by melting the components together and allowing the melt to solidify. Alloys are generally stronger than the metals they contain.

• Steel, the most common alloy, contains iron with small amounts of carbon and manganese, Stainless steel is a special steel containing some chromium and nickel also.
• Brass contains copper and zinc, whereas bronze contains copper and tin. Pure gold is not suitable for making jewellery. Pure gold is soft and gets reshaped by small pressures. So, the gold used for making jewellery is made stronger by alloying it with silver and copper.
  Brass is used to make musical instruments because it produces good sound when vibrated.

3.0Need of separation

As we know that many naturally occurring as well as man-made mixtures are desirable. But there are many mixtures which are undesirable and cannot be used as such. The constituents of mixtures need to be separated for the following reasons.

• To remove the undesirable constituents For example, we need to separate impurities from ordinary water to make it potable (safe for drinking). We must remove husk, stones and dirt from food grains before we cook them.
• To obtain desirable substances For example, we obtain common salt from sea water, which contains many substances including salt. Similarly, we separate petrol, kerosene and diesel from petroleum (crude oil).
• To obtain highly pure substances For example, from ordinary water, we obtain highly pure sample of water for medicinal purposes.

4.0Methods of separation

Methods of separation are based on the properties of materials, present in a mixture. The constituents of a mixture retain their properties, so they can be separated easily. Some of them are,

• Hand-picking If a constituent of a solid mixture is big and visibly different, it can be separated by hand-picking. For example, stones can be separated from rice and wheat or the seeds of different varieties of pulse get mixed up, they are separated by hand-picking. Plucking rotten grapes from a bunch is nothing but hand-picking. For picking, forceps can also be used. However, hand-picking is useful only if you need to separate a small amount of constituents from a mixture. This is because, the process is time-consuming.
  Separating rotten grapes by hand
• Threshing The process used to separate grains from the stalks is called threshing. Bundles of paddy (rice) or wheat stalks are kept in the fields after harvesting the crop. These stalks are dried in the sun before the grain is separated
  Threshing from them. These grains cannot be separated from stalks by hand picking. The farmers make small bundles of these stalks and then hit it against the hard wall of mud or hard substances. In doing so the grain separates from the stalks. These days there are mechanical threshers which hammer the stalks and separate grains from them.
• Winnowing Lighter solids are separated from heavier ones by wind is called winnowing. Farmers use this method to separate chaff (the covering of grain) from grain. A mixture of chaff and grain is made to fall from a height. The lighter chaff drifts away and the heavier grain falls vertically on the ground. Wind helps in Winnowing winnowing by blowing away the chaff more easily.
  If there is no wind, the winnower moves slowly through a length of about five metres while letting the mixture fall. The movement of the winnower stirs the air, which helps the chaff drift. If some chaff falls on the grain, it can be blown away. You can mimic winnowing. Take some roasted groundnuts on your palm and peel the nuts. Blow air at them. The nuts will remain on your palm, while the outer covering will be blown away.
• Sieving If a solid mixture is stirred or shaken on a mesh, particles smaller than the holes in the mesh fall and the bigger ones remain on the mesh. Thus, particles are separated on the basis of their size. This process is called sieving and the mesh (usually fitted into a frame) is called a sieve. Sieve is a shallow vessel having small holes at its bottom. The size of the holes of a sieve to be used, depends on the size of the particles of the material, to be seperated from the mixture.
  
  Some common type of sieves material, to be separated from the mixture. Small and fine sieves are used in the kitchen to separate bran and other impurities from flour. Coarse particles of flour are called wheat bran. The impurities remain on the sieve and flour passes through. Bigger sieves are used at construction sites to separate stones from sand.
  
  (a) Cleaning of grain
  
  (b) Sieving of sand
• Magnetic separation Iron is attracted by a magnet. So, if you move a magnet through a mixture of iron filings and sulphur, the iron particles cling to the magnet. Sulphur is not attracted by a magnet and so the particles of sulphur are left behind. This is how a mixture of iron and sulphur is separated. Such a method, in which a magnet is used to separate the constituents of a mixture, is called magnetic separation.
  
  Iron particles stick to the magnet and sand is left behind. Huge magnets, held by cranes, are used to pick up scrap iron in scrapyard.
• Sedimentation and decantation Suppose you have a solid - liquid mixture in which the solid is heavier than the liquid, for example, a mixture of sand and water. If you allow the mixture to stand for some time, the solid will settle at the bottom of the container and the liquid will remain over it. This process is called sedimentation, and the solid layer at the bottom is called the sediment.
  
  Sedimentation The liquid above the sediment is known as the supernatant liquid. We can pour out the supernatant liquid carefully into another vessel, leaving behind the sediment. The process is called decantation. However, if you have a solid-liquid mixture containing a solid lighter than the liquid, For example, a husk-water mixture, sedimentation and decantation will not work.

• Loading: Loading helps sedimentation. You may have seen that water obtained from natural sources like wells and rivers is muddy, especially in the rainy season. Very fine, solid particles do not sink to the bottom they remain suspended in water. If the particles join some other particles, they become heavier and sink. We then say that the particles are loaded.
  
  Suspended solid particles in water are loaded by the addition of alum.

• Filtration By decantation, a liquid is not completely separated from an insoluble solid. Filtration is a better method. The process of removing insoluble solid from a liquid by using a filter paper is called filtration.
• Churning

Fruit salad has cream in it. But do you know how cream is obtained? It is obtained by churning milk. When milk is churned, cream is separated from it. As cream is lighter than milk, it floats over the liquid. Churning is done either manually or by using an electric churner. In a washing machine also, the dirt separated from the cloth by a similar action.

• Evaporation A solid can be recovered from its solution by evaporating the solvent. A solution of salt in water, when heated on a flame for some time, leaves a residue of the salt. By improved, but similar processes, distilled water is prepared for laboratory and medicinal purposes.
  
  Evaporation of water from salt solution
• Dissolution, evaporation, and condensation This method is useful when one constituent of a solid mixture is soluble in a solvent (water) and the other is not.

• Centrifugation is the principle used in process of churning separation technique.
• We can apply evaporation process where the solid component of a mixture is soluble in liquid. It work on a principle that liquid vaporize easily while solid does not.
• We can also apply two or more separation technique to separate out component of a single mixture.

5.0Water (An important solvent)

It is because of its property of dissolving a large number of substances that water is so essential for life. (i) Our body cannot absorb food substances unless they are soluble in water. In the process of digestion, food is reduced to simple substances that are soluble in water. They can then be dissolved in water and absorbed by the body. (ii) Several waste materials produced in the body are dissolved in water and excreted. A number of chemical reactions occur inside our body. They all occur in the presence of water. (iii) Plants can absorb nutrients from the soil only if they are soluble in water. Minerals from the roots and food from the leaves are transported to different parts of the plant in the form of solution in water. (iv) Gases such as oxygen and carbon dioxide which are soluble in water are important for survival of aquatic life. Fish breathe through their gills. Aquatic plants use the carbon dioxide dissolved in water for photosynthesis. Carbon dioxide is dissolved in aerated drinks to give them their tangy taste. The solubility of gases in water decreases with increase in temperature. Thus, dissolved gases escape from water when it is heated. This can easily be seen by gently heating water in a pot. You will see small bubbles coming out much before water starts boiling. These are bubbles of air dissolved in water.

6.0Basic terminology

• Separate - cause to move or be apart
• Churned - shake (milk or cream) in a machine in order to produce butter
• Gin - combing the fibre
• Strainer - a device having holes punched in it for separating solid matter from a liquid
• Retain - continue to have
• Pluck - quickly or suddenly remove something
• Rotten - very bad
• Stalks - the main stem of a plant
• Harvesting - gather (a crop) as a harvest
• Thresher - machine that separates grain from corn or other crops by beating
• Husk - the dry outer covering of some fruits or seeds
• Fodder - food, especially dried hay or straw, for cattle
• Bran - pieces of grain husk separated from flour
• Supernatant liquid-liquid after a mixture of liquid and solid has been left to settle out
• Funnel - a tube or pipe that is wide at the top and narrow at the bottom
• Woven - form (fabric or a fabric item) by interlacing long threads passing in one direction with others at a right angle to them.
• Shallow pits - hole in the ground of little depth
• Seashore - an area of sandy, stony, or rocky land bordering and level with the sea
• Saturated - unable to absorb or dissolve any more of a solute
• Bunch - a number of things, typically of the same kind

7.0Memory Map

Mixture (is composed of two or more substances in any proportion)

• Why do we need to separate different components of mixture? Give two examples. When two or more than two substances are mixed together, we call it a mixture. Sometimes, different components of a mixture are not useful or may be harmful. So, we need to separate different components of the mixture. For example, (a) After preparing tea, the tea leaves are not useful. (b) Small stones present in rice are harmful. So, we need to separate them.
• How will you separate a mixture of kerosene and water? Kerosene and water are two immiscible liquids. The mixture can be separated by using a separating funnel. The mixture of kerosene with water form two distinct layers of liquids. Kerosene being lighter will form a top layer. On opening the stop cock water will be collected in beaker leaving the kerosene in the separating funnel.
  
  Separating funnel
• In villages milk or curd is churned to separate the butter by using a device. Can you explain this technique in a scientific manner? This method of separation is known as centrifugation. It is used for separating particles suspended in milk. When the mixture is rotated (churned) at a high speed, the lighter particles collect at the centre and float at top (butter). The denser particles move outwards and settle at the bottom. These days a mixer grinder is used for this separation method.
• How is common salt obtained from sea water? Sea water is collected in shallow pits dug on the seashore. The heat of the sun makes the water evaporate. Since, sea water contains salt, when enough water has evaporated, the solution becomes concentrated. Then the salt begins to crystallise. The salt is further purified and mixed with a chemical called sodium iodate. This mixture is iodised table salt. Sodium iodate contains iodine. The lack of iodine in our diet might cause the disease goitre.
  
  Obtaining common salt from sea water.
• While dissolving the salt in water, you heated the mixture. Is there a connection between temperature and the amount of solute that can be dissolve in a solution? Yes, there is. Only a certain amount of a solute can be dissolved in a given amount of a solvent at a particular temperature. This amount increases on increasing the temperature.