Class 7 Separation of Substances

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The process of separating grains from sun-dried stalks is called threshing. In this process, the stalks are beaten against any hard surface such as large rock or a stone slab etc. to free the grains. Threshing can also be done with the help of a mechanical device called a thresher.


A farmer, after harvesting the wheat from the field, threshes it to loosen the chaff from the grain inside. The chaff is lighter and the grain is heavier. Therefore, on a windy day, the farmer allows the mixture of chaff and grain
to fall from a height. The chaff, being lighter, blows with the wind to a distance while the grain being heavier falls down. The farmer gets two separate heaps of chaff and grain. This process of separating the wheat grain
from chaff is called winnowing.

This method is used to separate small pebbles which are mixed with seeds in a large quantity. In this process, a sieve is used for sieving grains. A sieve has small holes which allow pebbles to pass through it. The grains are retained in the sieve and are thus separated. Depending upon the size of particles to be separated.

This method is used for the following purposes:
• for separating pearls of different sizes.
• for separating the chaff from wheat flour.
• for separating cashew nuts of different sizes.

Magnetic Separation

Iron filings can be separated from a mixture of iron filings and sand by using a magnet. This process is known as magnetic separation.

Separation of Insoluble Solids from liquids
The following methods are used for separating insoluble solids from liquids.
This process is based on the densities of different components of a mixture. The process of setting down of heavy insoluble particles in a mixture of water and insoluble substances is called sedimentation.
The process of pouring out a clear liquid from a vessel (after sedimentation) without disturbing the sediments (heavy insoluble settled particles) is called decantation.
The above method has two disadvantages.
(a) It cannot be used for the miscible liquids that dissolve in one another. For example, petrol mixed in kerosene oil or salt or sugar solution in water.
(b) During the process of decantation, a small quantity still remains unseparated thus it gets wasted.

Coagulation or Loading
Coagulation is the process of improving the settling property of solids by addition of specific chemicals. When solid particles present in a mixture are not heavy enough to settle easily, some chemicals can be added to the mixture to enable the solids to settle. For example, when alum is added to dirty water, it attaches itself to the dirt particles and makes several dirt particles stick to each other. This makes them heavier and helps them to settle down. Alum is said to be a coagulating agent.
Filtration is the process of separating suspended solid matter from a liquid, by causing the latter to pass through the pores of some substance, called a filter. The liquid which has passed through the filter is called filtrate. The filter may be paper, cloth, cotton-wool, asbestos, slag or glass wool, unglazed earthenware, sand, or any other porous material.

Condensation is the change of water vapour into liquid water.

Distillation is a process of separating the component of substances from a liquid mixture by selecting evaporation and condensation.

Separation of Substances Using More Than One Method.
A mixture of sand and salt can be separated by a combination of methods. The first method is sedimentation and decantation. This mixture is put in water and left for the sand to settle for some time. Then, we will decant the salty water, which will separate the sand from the mixture. Now the salt can be separated from the water by evaporation. The water will boil away, leaving the salt behind. So, the mixture of the sand, salt and water has been separated successfully using a combination of sedimentation, decantation, evaporation and condensation.

Water the Universal Solvent
Water is capable of dissolving different substances, which is why it is a good solvent. In fact, water is called the “universal solvent” because it can dissolve with more substances than any other liquid.

• Liquids that mix in water completely in all proportions are called miscible liquids. For example, Alcohol is miscible with water.
• Liquids that do not mix in and form a separate layer are called immiscible liquid. For example, Oils are immiscible with water.

Saturated Solutions

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

Characteristics of a Solution
• A solution is a homogeneous mixture of two or more substances.
• The particles of solute in a solution cannot be seen by naked eye.
• A solution is stable.
• The solute from a solution cannot be separated by filtration (or mechanically).
Solute: A substance dissolved in another substance, water (or any other solvent) is called solute.

Solubility is the ability of a substance to dissolve in a given liquid.

The maximum amount of a substance which can be dissolved in 100 grammes of water at a given temperature is known as the solubility of that substance in water (at that temperature). The solubility of a solid solute generally increases with the rise in temperature.

Solubility of gases in water
• Aquatic plants use dissolved carbon dioxide to make their food by photosynthesis.
• All aerated drinks contain dissolved carbon dioxide.
• Gases are usually more soluble at cooler temperatures.

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