Methods For Separating Constituents of Mixtures

Separation of Plants of Class 6

Methods for separating constituents of mixtures

To separate a mixture we have to use such a property that one constituent of the mixture has and the others do not. For example, to separate iron nails from salt, we can use the property than iron is attracted by a magnet but salt does not.

1.         Hand-picking

2.         Threshing and winnowing

3.         Siving

4.         Sedimentation and decantation

5.         Filtration

6.         Magnetic Separation

7.         Evaporation

8.         Distillation / Condensation

9.         Separating Funnel

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 in this fashion. If the grains of rice and wheat or the seeds of different varieties of pulses get mixed up. they picking forceps can also be used.

Hand-picking is possible because the particles of the undesirable substance are different from the food grains in colour, shape and size. This method is convenient when the undesirable components are present in small quantities.

Illustration 5:

Why hand-picking is useful only for separating small amount of mixtures?

Sol.      Because hand-picking process is time-consuming, so its useful only for small amount of mixtures.

Threshing & Winnowing

After the wheat of rice crop is harvested, the grains have to be separated. The first step is to separated the grains from the stalks. This is known as threshing. It is done either by beating the stalks on the ground or a hard surface, by trampling the harvested wheat or rice using animals, or by using a mechanical thresher.

The grains are then separated from the mixture of grain and chaff so obtained, by using the property that grain is heavier than chaff, the mixture is allowed to fall from a height. The breeze blows away the chaff, while the grain falls down almost away the chaff, while the grain falls down almost vertically. The chaff forms a heap at a little distance away from the heap of grain. This process is known as winnowing.

Lighter solids are separated from heavier ones by winnowing. Farmers use this method to separate chaff (the covering of grain) from grain. A wind helps is winnowing by blowing away the chaff more easily.

If there is no wind, the winnower moves slowly through a length of about five metres 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.

Illustration 6:

What is thresher ?

Sol. It is a small machine in which the harvested crop is put and rolled under rollers. These rollers are very heavy and, thus, exert immense pressure on crops. As a result, the grains separate from the crops.

Siving

When the constituents of a mixture have particles of different sizes, a sieve can be used to separate them. The size of the pores in the sieve depends on the size of the particles of the constituent to be separated.

A sieve with fine pores is used to separate undesirable substances in four. A sieve with bigger pores is used at construction sites to separate stones from sand.

A sieve is often used at home to separate in wheat flour (atta). However, it is not considered a healthy practice nowadays, as the bigger particle of roughage are lost in the process.

This method is used on our daily life for the following:

(i)         In the flour mills, wheat grains are separated from the harmful and unwanted substances like stones by slanting sieves.

(ii)        At home, we clean rice grains, wheat flour or  pulses by using sieves which have holes of suitable size.

Sedimentation & Decantation

Food grains which are insoluble in liquids can be cleaned by the method of sedimentation. The process involves the setting down of insoluble solid food components in a liquid. The solid components which settle down at the bottom are known as sediments. The water above the sediments can be easily transferred to another container by pouring it gently. This process is known as decantation and the liquid collected is supernatant liquid.

Decantation is the process of pouring out the liquid without disturbing the sediment. If a solid dissolved in a liquid, decantation cannot be used to separate them.

A common example of cleaning food by sedimentation is that of rice or pulses. We clean these grains by soaking them in water for some time. The dust or other light impurities either dissolve or float in the water. The grains which are insoluble and heavier, settle down at the bottom. The dirty water is thrown away slowly and the grains are cleaned.

Similarly, we cut various vegetables such as potatoes and carrots, in a blow containing water. The impurities flow away in water whereas the small pieces of vegetable settle down at the bottom and get cleaned.

Filtration:

To separate tea leaves from tea, you pour the tea through a strainer which is a wire or a plastic mesh. The tea leaves are left behind in the strainer and the liquid flows out. This process is known as filtration. The strainer acts as a filter.  The same strainer cannot be used to separate mud from water. The particles of mud are too fine and will pass through the holes.

For this we need a fine filter. A filter paper can be used for this purpose.

A filter contains holes. The size of these holes depends upon the size of the particles to be filtered. We can use various substances, such as a piece of cotton, a layer of sand, glass wool filter paper, net, a muslin cloth or a strainer, as filter depending upon the material to be filtered.

The process of filtration cannot be used to separate the soluble components from liquids. But insoluble substances can easily be removed by it. For example, if we want to remove common salt which is soluble in water, we have to use another method. On the other hand, various small insoluble particles which cannot be separated by sedimentation due to light weight or other reasons can be easily separated by filtration.

At homes and in offices or schools, special types of filters are used to remove solid impurities from drinking water. In these filters, a porous pot made of ceramic is used as a filter. As the water passes through the fine pores, the solid impurities are filtered out. In some of these filters, a special type of light called ultraviolet light is used to kill germs in the water.

Illustration 7:

What is alum?

Sol.      Alum is a solid which is soluble in water. It is used to enhance sedimentation. It helps in setting the suspending clay particles in water.

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. A method such as this, in which a magnet is used to separate the constituents of a mixture, is called magnetic separation.

Huge magnets, held by cranes, are used to pick up scrap iron in scrapyards.

Evaporation

You have seen that you can separate insoluble substances from water by various methods. Let us see how we can separate common salt dissolved in a solution.

If we keep a bowl filed with salt solution in the sun, what will we-observe? We will see that water gradually turns into vapour and disappears, leaving behind the common salt in the bowl. This process in which water or any liquid turns into vapour is called evaporation.

Distillation Condensation

Distillation is a method of obtaining pure liquid from a solution. The solution is heated so that the liquid evaporates. The vapours are then cooled. They condense to give the pure liquid. This is known as condensation.

Distilled water used in laboratories is obtained in this way. A special apparatus is used to get distilled water. Water containing dissolved impurities is kept in a flask. An apparatus called the Liebig’s condenser, is fitted to the flask to cool the water vapour. Cold water is made to flow through the condenser.

As the water in the flask is heated, it evaporates. The water vapour cools and condenses in the Liebig’s condenser. The drops of distilled water formed are collected in another flask.

Separating Funnel

You have seen that liquids may be miscible of immiscible with each other. If you vigorously mix oil and water in a breaker and left the breaker stand for some time, the oil and water will separate. Oil will form a separate layer on top of the water. Oil and water are immiscible. Be careful while tilting the beaker it is possible to pour out the top layer of oil. Thus, decantation can be used to separate two immiscible liquids.

A separating funnel is used to separate a mixture of two immiscible liquids. It has a stopper which can be used to drain out the liquid at the bottom.

Solubility

The sugar when added to the water dissolved in the water. When a substance dissolves in a liquid, it breaks up into its individual molecules. These molecules spread out in between the molecules of the liquid. That is why the water tastes sweet. Since individual molecules are too small to be seen, the sugar seems to disappear in the water. The sugar and water together formed a solution of sugar in water.

The water here is called a solvent. Sugar, the substance that dissolves in water is called a solute. A solvent and a solute together form a solution.

Water can dissolve in large number of substances in it. It is, therefore, called the universal solvent.

Some substances such as wood or sand do not dissolve in water. Other, such as sugar or slat, dissolve readily in water.

Substances that dissolve readily in water are said to be soluble in water. Substances such as sand, that do not dissolve in water, are said to be insoluble in water.

During the formation of a solution, the molecules of the soluble or solvent remain, the same. It is easy to get back both in their original forms from the solution.

There are three types of solutions:

1. Unsaturated Solution: The solution in which more amount of solute can be further dissolved is called unsaturated solution.

2. Saturated Solution: The solution in which more amount of solute can be dissolved on raising the temperature of the solution is called saturated solution.

3. Super-saturated Solution: The solution in which no more solute can be dissolved even on raising the temperature is called super-saturated solution.

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