Methods Of Separation Of Substances Under Wet Conditions

Methods Of Separation Of Substances Under Wet Conditions

Some solid particles are insoluble in water. For example chalk powder, dust particles, sand, and tiny pieces of straw are insoluble in water. To separate this type of particles generally we use the following methods

Sedimentation and Decantation

Have you seen pulses being washed in your home? When pulses are kept in a bowl of water, they settle down as they are heavy. However, dirt, insects, tiny pieces of straw, and other lighter impurities float at the top. The water, which contains these impurities, is then poured out and discarded. This process involves two methods: sedimentation and decantation.

The process of separating insoluble solids, suspended in a liquid, by allowing them to settle down is called sedimentation. The solid particles that settle down during sedimentation are called sediments. The process of pouring out the clear upper liquid without disturbing the sediments is called decantation. The liquid above the sediments is called a supernatant.

Methods Of Separation Of Substances Under Wet Conditions 1A mixture of sand and water can also be separated by sedimentation and decantation. The mixture is left undisturbed for some time. Sand, being heavier, settles down and water is poured out into a separate container.

Activity Aim: To observe cleaning of rice by sedimentation and decantation.
Materials needed: A cup of rice and a bowl of water.
Method:

  1. Observe the rice and record your observations. Does the rice look dirty?
  2. Now, mix the rice with water in a bowl and allow it to stand for something.
  3. Without disturbing the layer of rice, decant the water.

Observation: When the mixture is allowed to stand, most impurities came up and floated near the surface, whereas the rice settled down. On decanting the water, we got cleaner rice.
Conclusion: Rice can be cleaned by sedimentation and decantation.

Filtration

The process by which two substances (an insoluble solid and a liquid) are separated by passing the mixture through a filtering device is called filtration.

Filtration is commonly used in our homes. For example, after preparing tea, we filter out the tea leaves using a strainer. Filtration is also done to remove pulp from fresh fruit juice. Water may also contain solid impurities, which can be removed by filtration.

During filtration, the insoluble solid is retained in the filtering device whereas the liquid passes through it. It is important that the particles of the insoluble solid are bigger than the holes in the filtering device for them to be retained in it. A filter paper is a filtering device that has very fine pores in it.

Activity

Aim: To separate a mixture of sand and water by filtration.
Materials needed: Mixture of sand and water, funnel, beaker, flask, and filter paper.
Method:

    1. Fold the piece of filter paper twice to make a cone.
    2. Keep this filter paper cone inside the funnel. Place the funnel into the flask.
    3. Pour the mixture into the flask through the funnel.

Methods Of Separation Of Substances Under Wet Conditions 2

Observation: It is observed that sand is retained in the filter paper but water passes through.
Conclusion: Sand and water can be separated by filtration.

Evaporation

 

The process in which a liquid changes into a gas is called evaporation. In this method, the mixture is heated. The liquid part of the mixture evaporates leaving the solid part behind. For example, a mixture of common salt and water can be separated by evaporation. In fact, evaporating seawater is one of the oldest ways of obtaining salt.

Methods Of Separation Of Substances Under Wet Conditions 3

salt-from-seawater
Fig. Obtaining salt from seawater

Activity

Aim: To separate a mixture of salt and sand.
Materials needed: Mixture of salt and sand, filter paper, burner; test tube, test tube holder and water.
Method:

  1. Take the mixture of salt and sand in a test tube. Add water to it and shake it well.
  2. Salt dissolves in water, whereas sand does not. Sand can be separated using a filter paper.
  3. Now, only salt solution remains in the test tube.
  4. Heat the salt water till all the water changes to vapour, leaving the salt behind.

Observation: Salt is left behind in the test tube.
Conclusion: Two processes, filtration and evaporation, were used to separate a mixture of salt and sand. Thus, sometimes more than one method can be used to separate the components of a mixture.

Condensation

The process in which gas changes into liquid is called condensation. Condensation is the opposite of evaporation. In nature, water vapour in the air condenses to form its liquid form, the dew. Condensation takes place only when water vapour hits a cold surface.

Activity

Aim: To separate both water and salt from saltwater.
Materials needed: Saltwater, kettle, metal plate, a pair of tongs, bowl and burner.
Method:

    1. Take saltwater in the kettle and heat it over the burner.
    2. After some time, steam comes out of the spout of the kettle.
    3. Hold the metal plate (using tongs) above the spout. The steam condenses and changes into water droplets on touching the plate (cold surface).
    4. Collect the falling water drops in the bowl.
    5. Heat until all the water has boiled off.

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Observation: Salt is left behind in the kettle and water is collected in the bowl.
Conclusion: Water and salt could be separated from saltwater by the process of evaporation and condensation.

Methods Of Separation Of Substances Under Dry Conditions

Methods Of Separation Of Substances Under Dry Conditions

Different methods are used for separating different substances that are mixed together such as handpicking, threshing, winnowing and sieving.

Threshing

Grains or seeds of plants like rice and wheat serve as sources of food. The flour (atta) that is used for making chapattis is made from wheat grains. After these crops have been harvested or cut, the grains need to be separated from the stalks (the dried stems). This is done by threshing.
Methods Of Separation Of Substances Under Dry Conditions 1 The process of beating harvested crops to separate the grains from the stalks is called threshing. It is done manually (by hand) or with the help of machines. Manual threshing is done by holding a pile of crop and beating it on a rock or a hard surface. This loosens and separates the grain from the stalk. Sometimes, threshing is also done by crushing the harvested stalks using bullocks.
Methods Of Separation Of Substances Under Dry Conditions 2
Threshing is also done with the help of machines like the combine harvester. Threshed grains may still contain seed coverings and tiny pieces of leaves or stem (collectively called chaff). These are separated by winnowing.

Winnowing

The method used to separate chaff from the grain by wind or blowing air is called winnowing.
Methods Of Separation Of Substances Under Dry Conditions 3 The mixture of chaff and grain is taken in a winnowing basket. The farmer stands at a higher level and lets the mixture fall to the ground. The grain, being heavier, falls almost vertically whereas the lighter chaff is carried away by the wind and forms a separate heap away from the grain. The separated chaff is used as fodder for cattle. The direction of the wind plays an important role in the process of winnowing.

Activity 

Aim: To understand the process of winnowing.
Materials needed: Plate, rice, tiny bits of paper, a chair and old newspapers.
Method:
1. Go to an open ground or garden and spread sheets of newspaper on the ground.
2. Take some rice mixed with bits of paper in a plate. Stand on the chair and let the mixture fall slowly by shaking the plate.
Observation: The rice grains form a separate heap from the paper bits.
Conclusion: The blowing air or wind can be used to separate lighter particles (paper bits) from heavier particles (rice grains).

Hand-picking

Rice, wheat, pulses, etc., that we buy from the market may contain impurities (unwanted or harmful particles) in the form of small stones, unwanted grains, etc. Often, these impurities look very different from the food item and can be spotted easily. The method of separation used in such a case is hand-picking.
Methods Of Separation Of Substances Under Dry Conditions 4This method is preferred when

  • The quantity of the mixture is small,
  • The unwanted substance is present in smaller quantities, and
  • The size, shape, or colour of the unwanted substance is different from that of the useful one.

Sieving

If the components of a mixture are of different sizes, they can be separated by sieving. The smaller component passes through the pores of the sieve whereas the larger component (stones or husk) is left behind in it. This method is used in some homes to separate wheat bran (the bigger particles) from flour.
Methods Of Separation Of Substances Under Dry Conditions 5However, sieving wheat flour is not advisable as wheat bran, which is removed during sieving, is very rich in nutrients and is also a rich is better to remove visible impurities by hand picking. The process of sieving is also used to separate pebbles and stones from sand at construction sites. The stones and pebbles present in the mixture remain in the sieve and the fine sand particles pass through the holes of the sieve.

Activity

Aim: To understand sieving
Materials needed: Flour, powdered chalk, old newspaper, and a sieve
Method:
1. Sieve the flour on the newspaper to separate husk or any impurities.
2. Take a small amount of flour and mix it with fine chalk powder.
3. Sieve the mixture. Can chalk powder be separated from flour?
Observation: Husk and other impurities could be separated from the flour, but chalk powder could not be separated.
Conclusion: Husk particles are bigger than flour particles; so they are retained in the sieve. Particles of chalk powder and flour do not differ much in size; so chalk powder could not be retained in the sieve. Thus, sieving can be used to separate particles of different sizes.