This chapter is about different methods used to separate substances from mixtures in our daily lives. We often need to separate things for various reasons, such as removing unwanted impurities or obtaining useful components.

A famous saying by Kabir compares this process to winnowing, where the useful grain is kept while the husk is blown away. This illustrates the core idea of separation: keeping the valuable part and discarding the unwanted one.

A mixture is formed when two or more substances are mixed together.

Separation Of Solids From Solids

Several methods are used to separate solid components from other solids in a mixture, usually based on properties like size, shape, or weight.

Handpicking

This is a simple method where unwanted substances are picked out by hand from a mixture. It is effective when:

• The particles to be separated are large enough to be seen clearly.
• The quantity of the unwanted substance is relatively small.
• There are clear differences in size, shape, or color between the components.

Examples:

• Separating small stones and husk from rice or wheat grains.
• Removing whole black peppers from cooked food like pulao.
• Separating green chillies from a dish.
• Taking out seeds from watermelon.
• Picking out withered flowers from a heap of fresh ones.

Threshing

Threshing is the process used to separate grains from the stalks of harvested crops like wheat or rice.

Traditionally, this involves beating the harvested stalks against a hard surface (like a wooden log) to loosen and detach the grains.

Nowadays, machines called threshers are often used. These machines efficiently separate the grains from the stalks, and sometimes also the husk.

Winnowing

Winnowing is a method used to separate lighter components from heavier ones in a solid mixture, using wind or by blowing air.

In this method, the mixture is dropped from a height (often from a raised platform) using a winnowing tray (like a 'soop' made of bamboo), while wind or air is blown across the falling mixture.

The lighter particles (like husk) are carried away by the wind and form a separate heap at a distance, while the heavier particles (like grains) fall almost vertically and form a heap closer to the person winnowing.

This method relies on the difference in weight between the components of the mixture.

Threshing machines often perform both threshing and winnowing simultaneously.

Separation Based On Particle Size

When the components of a solid mixture have different sizes, methods based on filtering or allowing particles to pass through openings of specific sizes can be used.

Sieving

Sieving is a method used to separate components of a solid-solid mixture that have different particle sizes, using a sieve.

A sieve is a mesh or screen with holes of a specific size. When the mixture is placed on the sieve and shaken, the smaller particles pass through the holes, while the larger particles are retained on the sieve.

This method is effective when the holes of the sieve are smaller than the larger particles but larger than the smaller particles.

Examples:

• Separating bran or small stones from flour using a kitchen sieve.
• Separating pebbles and stones from sand at construction sites using a large sieve.
• Separating corn from potatoes if the sizes are significantly different.
• Separating coconut pieces from rice flour.

Filtration

Filtration is a process used to separate insoluble solid particles from a liquid or gas using a filter medium that allows the fluid to pass through but retains the solid particles.

The solid material that remains on the filter is called the residue, and the liquid that passes through is called the filtrate.

Examples:

• Separating tea leaves from brewed tea using a strainer (a type of sieve/filter).
• Separating muddy water to get clear water. A piece of cloth can act as a filter (it has small pores between woven threads). For finer separation, a filter paper with very fine pores can be used.
• Fishing nets are a form of filtration, allowing water to pass while trapping fish.

Other materials like cotton, charcoal, and sand can also be used as filter media, depending on the size of particles to be removed.

Extra Information: Tea bags, historically made of cloth like silk or muslin, are now commonly made of filter paper, allowing hot water to extract flavour while keeping the tea leaves contained.

Separation Of Dissolved Solids From Liquids

When a solid substance is dissolved in a liquid, special methods are needed to separate them, as the solid particles are no longer visible or retained by simple filters.

Evaporation

Evaporation is the process where a liquid turns into its gaseous state (vapour), usually by heating or exposure to air/sunlight.

This property can be used to separate a solid that is dissolved in a liquid.

When a solution (a dissolved solid in a liquid) is heated or left to evaporate, the liquid turns into vapour and escapes into the air, leaving the solid substance behind.

Example: Obtaining salt from saltwater.

Obtaining Salt From Solution

Salt (common salt) is obtained from seawater. Seawater is a mixture of many salts (including common salt) dissolved in water.

Large shallow pits are made near the coast where seawater is collected. The sun's heat and wind cause the water to gradually evaporate over a few days, leaving behind the solid salts.

This solid mixture of salts is then purified to obtain common salt.

Similarly, if you dissolve salt in water at home and spread a few drops of the salt solution on a dark paper, as the water evaporates, you will see white patches of salt left behind.

Heating a salt solution strongly will cause the water to evaporate faster, leaving the salt in the container (e.g., in a china dish). The process of evaporation recovers the solid, but the liquid (water) is lost to the atmosphere.

Extra Information: White patches on dark clothes in summer are due to dried sweat, which contains salts that remain on the fabric after the water evaporates. Drying medicinal plants in the shade (as in Ayurveda) helps evaporate water while preserving the medicinal components.

Separation Of Insoluble Solids From Liquids

When solid particles do not dissolve in a liquid (are insoluble), methods like sedimentation and decantation can be used, often as preliminary steps before filtration.

Sedimentation

Sedimentation is the process where heavier, insoluble solid particles settle down at the bottom of a liquid when the mixture is left undisturbed for some time.

Example: Mud settling down in muddy water after standing, or tea leaves settling at the bottom of a cup of tea.

Decantation

Decantation is the process of gently pouring off the liquid after the solid particles have settled down during sedimentation, without disturbing the settled solid.

Example: Carefully pouring the tea from a teapot into a cup after allowing the tea leaves to settle. Also used in washing rice or pulses, where the water containing lighter impurities is decanted after the heavier grains settle.

Decantation alone may not remove all the solid particles, especially very fine ones, which is why it is often followed by filtration.

Extra Information: To separate a mixture of immiscible liquids like oil and water, decantation can be used after allowing the layers to separate. The lighter liquid (oil) floats on top of the heavier liquid (water), and the top layer can be carefully poured off.

Separation Of Liquids From Liquids

Methods exist to separate liquids from other liquids or to separate components within a liquid mixture.

Churning

Churning is a process of vigorously mixing or agitating a liquid, often to separate components based on density or other properties.

A common example is the churning of curd (or cream) to extract butter. By rapidly stirring the curd, the lighter butterfat particles collect and separate from the liquid buttermilk.

Electric mixers or blenders are modern kitchen appliances that perform a similar churning action.

Separation Of Magnetic Substances

This method uses the property of magnetism to separate substances that are attracted to a magnet from those that are not.

Magnetic Separation

Magnetic separation is used to separate components of a mixture where one component is magnetic (attracted to a magnet) and the other components are non-magnetic.

By bringing a magnet close to the mixture, the magnetic substance is attracted to the magnet, separating it from the non-magnetic substances.

Example: Separating iron nails from sawdust. Iron is magnetic, while sawdust is not. A magnet will attract and collect the iron nails, leaving the sawdust behind.

Extra Information: Magnetic separation is used on a larger scale in industries and recycling plants to separate magnetic metals (like iron and steel scrap) from waste materials using strong magnets often attached to cranes.

In summary, various methods of separation are used in our daily lives and in industries, chosen based on the properties of the substances in the mixture and the desired outcome of the separation.

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