Why Do We Separate Substances?

In our daily lives, we encounter many instances where we need to separate substances from a mixture. A mixture is composed of two or more substances that are physically combined. Separation is the process of isolating the individual components of a mixture.

Common examples include:

• Separating tea leaves from liquid tea using a strainer.
• Churning milk or curd to separate butter.
• Picking out stones from a batch of rice or pulses by hand.

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Purpose of Separation

We separate substances for several key reasons. The purpose determines the method of separation we choose and what we do with the separated components. The main purposes are:

1. To remove non-useful or harmful components: This involves removing impurities or undesirable substances. For example, separating small stones from rice before cooking. Here, the rice is the useful component, and the stones are harmful impurities.
2. To separate two different, but useful components: Sometimes, a mixture contains multiple useful substances that we need to use separately. For instance, churning milk to obtain butter. Both butter and the remaining liquid (buttermilk) are useful.
3. To obtain a pure substance: Separation is essential in science and industry to obtain substances in their pure form for various applications.

The substances in a mixture can be particles of different sizes and materials, and they can exist in any of the three states of matter: solid, liquid, or gas. The properties of the components, such as size, weight, and solubility, dictate the method of separation to be used.

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Summary of Separation Purposes

Separation Process	Purpose for which we do the separation	What do we do with the separated components?
1) Separate stones from rice	c) To remove impurities or harmful components.	ii) We throw away the impurities.
2) Churning milk to obtain butter	a) To separate two different, but useful components.	iii) We use both the components.
3) Separate tea leaves	b) To remove non-useful components.	i) We throw away the solid component.

Methods for Separating Solid Mixtures

When a mixture consists of two or more solid components with different properties, we can use simple physical methods to separate them.

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Handpicking

Handpicking is a method of separation that involves simply picking out substances by hand and separating them from others. It is one of the most straightforward separation techniques.

Principle: This method is used when the components of a mixture can be distinguished by sight (based on differences in colour, shape, or size).

When to use:

• When the quantity of the mixture is small.
• When the unwanted particles are slightly larger in size and fewer in number.

Example: Separating slightly larger impurities like pieces of dirt, stones, and husk from wheat, rice, or pulses.

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Threshing

Threshing is the process used by farmers to separate grain seeds from the harvested stalks of crops like wheat or paddy (rice). Since the grain seeds are very small and numerous, plucking them by hand is impossible.

Principle: This method uses force to free the grain seeds from the stalks.

Method:

1. The harvested stalks are dried in the sun.
2. The dried stalks are then beaten on a hard surface to shake the grain seeds free.

This process can be done manually, with the help of bullocks, or by using large machines called threshers for large quantities of grain.

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Winnowing

Winnowing is a method used to separate heavier components of a mixture from lighter components by using wind or blowing air.

Principle: It works on the principle that lighter particles are carried away by the wind, while heavier particles fall down vertically.

Method:

1. The person stands on a raised platform holding the mixture in a plate or a traditional winnowing basket (soop).
2. The mixture is slowly allowed to fall from a height.
3. The wind or blowing air carries the lighter components (like husk) away, which form a separate heap at a distance.
4. The heavier components (like grain) fall down almost vertically, forming a neat heap near the platform.

Example: Farmers commonly use this method to separate lighter husk particles from heavier seeds of grain. The separated husk is often used as fodder for cattle.

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Sieving

Sieving is a method of separating components of a mixture that have different sizes. It is done using a sieve, which is a tool with a mesh or net containing small holes.

Principle: The fine particles, which are smaller than the holes of the sieve, pass through it, while the bigger particles (impurities) remain on the sieve.

Examples:

• Separating bran and impurities from flour in the kitchen.
• In a flour mill, sieving is used to remove impurities like husk and stones from wheat before grinding.
• At construction sites, large sieves are used to separate pebbles and stones from sand.

Limitation: Sieving cannot be used if the components of a mixture have the same size (e.g., separating finely powdered chalk from flour).

Separating Insoluble Solids from Liquids

Sometimes, we need to separate solid impurities that are mixed with a liquid. If the solid is insoluble (does not dissolve) in the liquid, methods like sedimentation, decantation, and filtration can be used.

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Sedimentation and Decantation

These two processes are often used together to separate a mixture of an insoluble solid and a liquid, or two liquids that do not mix (immiscible liquids).

Sedimentation: This is the process in which the heavier, insoluble component in a mixture settles down at the bottom of the container after the mixture is left undisturbed for some time. The settled component is called the sediment.

Decantation: This is the process of gently pouring out the clear liquid from the top into another container, without disturbing the settled sediment. The liquid that is poured out is called the supernatant.

Example:

• Separating dust from rice/pulses: Before cooking, rice or pulses are washed with water. The heavier rice grains settle down (sedimentation), and the dirty water containing lighter dust particles is poured off (decantation).
• Separating oil and water: Since oil is lighter than water and does not mix with it, it forms a separate layer on top. This top layer of oil can be carefully removed by decantation.

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Filtration

Filtration is a method used to separate fine, insoluble solid particles from a liquid by passing the mixture through a filter. The filter has very fine pores that allow the liquid to pass through but block the solid particles.

Principle: The size of the solid particles is larger than the pores of the filter.

Examples of Filters:

• A strainer: Used to separate tea leaves from prepared tea.
• A piece of cloth: Has small pores between the threads and can be used as a filter.
• Filter paper: A special kind of paper with very fine pores, commonly used in science laboratories. It is folded into a cone shape and placed in a funnel.

Applications:

• Filtering muddy water to remove soil.
• Filtering fruit and vegetable juices to separate seeds and pulp.
• Preparing cottage cheese (paneer) by filtering the solid paneer from the liquid through a fine cloth.

Separating Soluble Solids and Combined Methods

When a solid is dissolved in a liquid (forming a solution), methods like sedimentation or filtration cannot be used. In such cases, and for more complex mixtures, we use other methods like evaporation and condensation, often in combination.

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Evaporation

Evaporation is the process of converting a liquid into its vapour form, usually by heating.

Principle: This method is used to separate a solid that has dissolved in a liquid. The liquid evaporates, leaving the solid behind.

Application: Obtaining Salt from Sea Water

1. Sea water, which contains a large amount of dissolved salts, is collected in shallow pits.
2. Sunlight heats the water, causing it to slowly evaporate.
3. After a few days, all the water turns into water vapour, leaving behind the solid salts.
4. This mixture of salts is then collected and purified to obtain common salt.

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Use of More Than One Method of Separation

Often, a single method of separation is not sufficient to separate all the components of a complex mixture. In such situations, a combination of methods is used.

Example: Separating a Mixture of Sand, Salt, and Water

To separate a mixture of sand and salt, we need a sequence of processes:

1. Dissolving: Add water to the sand-salt mixture. The salt will dissolve in the water, but the sand will not.
2. Sedimentation and Decantation/Filtration: Allow the sand to settle at the bottom. Then, either decant the salt water or filter the mixture to separate the sand. The sand is now separated.
3. Evaporation: Heat the decanted/filtered salt water in a kettle. The water will turn into steam and evaporate, leaving the salt behind in the kettle. The salt is now recovered.
4. Condensation: To recover the water, hold a cold metal plate (with ice on it) in the path of the steam coming out of the kettle. The steam will cool down on contact with the plate and turn back into liquid water. This process is called condensation (conversion of vapour into liquid).

By using the processes of decantation/filtration, evaporation, and condensation, we can successfully separate all three components—sand, salt, and water.

Solutions and Saturation

A solution is a homogeneous mixture formed when a substance (solute) dissolves in a liquid (solvent). For example, when salt dissolves in water, it forms a salt solution.

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Saturated Solutions

Can a liquid dissolve an unlimited amount of a substance? No. There is a limit to how much of a substance can be dissolved in a fixed quantity of a liquid at a certain temperature.

A solution in which no more of a substance can be dissolved at a given temperature is called a saturated solution.

If we keep adding a soluble substance like salt to a glass of water, a point will be reached when the salt stops dissolving and starts to settle at the bottom. At this point, the solution has become saturated.

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Effect of Heating on Solubility

We can often dissolve more of a substance in a saturated solution by increasing its temperature. The activity of heating a saturated salt solution shows that a larger quantity of salt can be dissolved in water on heating.

However, when this hot, concentrated solution is allowed to cool, the excess dissolved salt may reappear as solid crystals at the bottom of the beaker.

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Varying Solubility of Different Substances

Water does not dissolve equal amounts of different soluble substances. The amount of a substance that can be dissolved in a given amount of water is a unique property of that substance.

For example, you can dissolve a different number of spoons of sugar in a cup of water compared to the number of spoons of salt before the solution becomes saturated. This demonstrates that different substances have different solubilities.

Substance	Number of spoons of substance that dissolved in water
Salt	(To be recorded based on experiment)
Sugar	(To be recorded based on experiment)

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