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?

 Rice or pulses are usually washed before cooking. When you add water to these, the impurities like dust and soil particles get separated. These impurities go into water, which becomes a little muddy. Now, what will sink to the bottom of the vessel — rice or dust? Why? Have you seen that the vessel is tilted to pour out the dirty water?

 When the heavier component in a mixture settles after water is added to it, the process is called sedimentation. When the water (along with the dust) is removed, the process is called decantation. (Fig.1)

Let us find a few other mixtures that can be separated through sedimentation and decantation.

 The same principle is used for separating a mixture of two liquids that do not mix with each other. For example, oil and water from their mixture can be separated by this process. If a mixture of such liquids is allowed to stand for some time, they form two separate layers. The component that forms the top layer can then be separated by decantation.

Let us again consider a mixure of a solid and liquid. After preparing tea, what do you do to remove the tea leaves? Try decantation. It helps a little. But, do you still get a few leaves in your tea? Now, pour the tea through a strainer.                                                                                                                                                                                                                                                  Fig.1                                          

                                                                                                                                                                                
 

Did all the tea leaves remain in the strainer? This process is called filtration (Fig. 5.2). Which method of separating tea leaves from prepared tea is better, decantation or filtration?

Let us now consider the example of water that we use. Do all of us, at all times, get safe water to drink? Sometimes, water supplied through taps may be muddy. The water collected from ponds or rivers may also be muddy, especially after rains. Let us see if we can use some method of separation to remove insoluble impurities like soil from the water.

 

Activity 1

Collect some muddy water from a pond or a river. If it is not available, mix some soil to water in a glass. Let it stand for half an hour. Observe the water carefully and note your observations. 

Does some soil settle at the bottom of water? Why? What will you call this process? Now, slightly tilt the glass without disturbing the water. Let the water from the top flow into another glass (Fig.1). What will you call this process? 

Is the water in the second glass still muddy or brown in colour? Now filter 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 threads. These pores in a cloth can be used as a filter. 

If the water is still muddy, impurities can be separated by a filter that has even smaller pores. A filter paper is one such filter that has very fine pores in it. (Fig.2)  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). The mixture is then poured on the filter paper. Solid particles in the mixture do not pass through it and remain on the filter. Fruit and vegetable juices are usually filtered before drinking to separate the seeds and solid particles of pulp.

                                                  

                                         Fig.2                                                                                                                    Fig.3

 

Evaporation

 

Activity 2
 

Heat a beaker containing some water. Allow the water to boil. If you continue heating, would the water turn into steam and disappear completely? Now, add two spoons of salt to water in another beaker and stir it well. Do you 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.4). Let the water boil away. What is left in the beaker?
 

                                                                                                                                                                                 

               Fig.4                                                                                                                                     Fig.5                                                                                                                                                                                       

           

Sea water contains many salts mixed in it. One of these salts is the common salt. When sea water is allowed to stand inshallow pits, water gets heated by sunlight and slowly turns into water vapour, through evaporation. In a few days, the water evaporates completely leaving behind the solid salts (Fig.5). Common salt is then obtained from this mixture of salts by further purification.

Use of more than one method of separation

We have studied some methods for separation of substances from their mixtures. Often, one method is not sufficient to separate the different substances present in a mixture. In such a situation, we need to use more than one of these methods.

 

Activity 3

Take a mixture of sand and salt. How will we separate these? We already saw that handpicking would not be a practical method for separating these.Keep this mixture in a beaker and add some water to this. Leave the beaker 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 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. 6. 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 condenses and forms liquid water. The water drops that you observed falling from the plate, were due to condensation of steam. The process of conversion of water vapour into its liquid form is called condensation.

 Did you ever see water drops condensed under a plate that has been used to cover a vessel containing milk that has just been boiled?
  

After all the water has evaporated, what is left behind in the kettle?

                                       

                                           Fig.6.

We have thus, separated salt, sand and water using processes of decantation, filtration, evaporation and condensation.

 

Can water dissolve any amount of a substance?

Many substances when dissolve in water forms a solution. We say that these substances are soluble in water. What will happen if we go on adding more and more of these substances to a fixed quantity of water?

 

Activity 4

You will need a beaker or a small pan, a spoon, salt and water. Pour half a cup of water in the beaker. Add one teaspoonful of salt and stir it well, until the salt dissolves completely (Fig 7). Again add a teaspoonful of salt and stir well. Go on adding salt, one teaspoonful at a time, and stir.

After adding a few spoons of salt, do you find that some salt remains undissolved and settles at the bottom of the beaker? If yes, this means that no more salt can be dissolved in the amount of water we have taken. The solution is now said to be saturated.

Here is a hint as to what might have gone wrong when Paheli tried to recover large quantity of salt mixed with sand. Perhaps the quantity of salt was much more than that required to form a saturated solution. The undissolved salt would have remained mixed with the sand and could not be recovered. She could solve her problem by using a larger quantity of water.

Suppose, she did not have sufficient quantity of water to dissolve all the salt in the mixture. Is there some way that water could be made to dissolve more salt before the solution gets saturated? Let us try and help Paheli out.

                     
         Fig.7                 

 

Activity 5

Take two glasses and pour half a cup of water in each of them. Add a teaspoon of salt to one glass and stir till the salt dissolves. Go on adding salt, one teaspoon at a time, till the solution saturates. Record the number of spoons of salt that dissolved in the water, in Table-1. Now, repeat the same activity with sugar. Repeat this with some other substances that are soluble in water.

 

What do you notice from Table-1? Do you find that water dissolves different substances in different amounts?

 

Substance	Number of spoons of substance that dissolved in water
Salt
Sugar

We have discussed a few methods of separating substances. Some of the methods of separation presented in this chapter are also used in a science laboratory. We also learnt that a solution is prepared by dissolving a substance in a liquid. A solution is said to be saturated if it cannot dissolve more of the substance in it.

 

 SUMMARY

 

• 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.