Class 6 Science Chapter 9 Notes Methods of Separation in Everyday Life
Pure Substances and Impure Substances or Mixtures
Pure Substances:
Substances that contain only one type of particle are called pure substances.
For example, sugar, salt, etc.
Mixtures or Impure Substances:
Substances made up of two or more pure substances called components are mixtures or impure substances. All the components of the mixture should retain their original properties in the mixture. Examples of mixtures are air, seawater, soil, tea, milk, coffee, petroleum, etc. Mixtures are classified into two types based on their composition.
- Homogeneous mixtures have a uniform or the same composition throughout the mixture (it looks the same everywhere).
Examples include seawater, sugar solution, etc. - Heterogeneous mixtures: They do not have a uniform or same composition throughout the mixture (do not look the same everywhere).
For example, soil, tea, etc.
Why do we need to separate different components from a mixture?
The constituents of a mixture are segregated from each other by the process called separation. For example, grain is separated from stalks, tea leaves are separated from the liquid with a strainer while preparing tea, milk, or curd is churned to separate the butter. Most substances that occur in nature may contain one or more useful substances or one useful substance and one or more harmful or unwanted (undesirable) substances. To make the available substance fit for use, these impurities must be separated from the useful substance. The substances to be separated may be particles of different sizes or materials. These may be solids, liquids or even gases. The separation of substances is carried out for the following purposes.
(i) To remove impurities or harmful compbnents:
Separation helps us to remove harmful substances from the mixture.
For example, small pieces of stones and undesirable seed grains are removed from rice and pulses before cooking. The useless pieces of stones and seed grains are thrown away.
(ii) To remove non-useful components:
Mixtures often contain non-useful substances that need to be separated from the mixture.
For example, tea leaves are separated from the liquid tea with a strainer after preparing tea or before taking it.
(iii) To separate different useful and pure components from mixture:
A mixture may contain one or more useful constituents. These pure components should be separated for their proper use.
For example, when we churn milk, we separate the cream from, the milk to obtain butter, which is used in cooking and baking. Also, we get pure salt from seawater by evaporating the water, leaving only the salt behind.
Methods of Separation of Mixtures
Separation of useful substances from unwanted substances is very common in our day-to-day lives and laboratories/industries. In day-to-day life, we use simple methods to carry out separation. Some most commonly used methods for separating the constituents from a mixture are discussed below.
Handpicking
The process of separating solid particles from a mixture by hand is called handpicking. This method is one of the simplest ways to separate substances from a mixture based on their physical properties, particularly when the components have noticeable differences in appearance (size, shape, or color). This method is used when highly larger-sized particles are present in small quantities (impurities) like stone, husk, pieces of dirt, etc. from the grains of wheat, etc. In our everyday lives, we use the method of handpicking to separate stones, husks, which are not useful, from food grains like wheat, rice, and pulses.
Threshing
The process of separating grains from stalks of harvested crops is called threshing. Fully matured crops like paddy (rice) or wheat is harvested from the field. During this, the crop plants are cut close to the ground. Stalks are dried in the sun before the grains are separated. Each stalk has many grains attached to it. The grains attached to the stalks are covered in a thin layer called chaff. The grains are then separated from stalks by the process of threshing. In this process, the stalks are beaten to free the grain seeds. Threshing is essential in agriculture to separate grains efficiently from harvested crops like wheat, rice, barley, and oats. Threshing can be done in the following ways.
- By holding the bundles of stalks in hand and beating them on hard surfaces.
- By allowing bullocks to trample the stalk.
- By threshing machines, called threshers, in case of large quantities of grains.
Winnowing
The method used to separate heavier and lighter components of a mixture by wind or by blowing air is called winnowing. This method is commonly used by farmers to separate lighter husk particles (thin, dry outer covering of various seeds) from heavier seeds of the grains. The steps involved in the separation of husk from grains with the help of wind are as follows.
- The mixture of wheat grains and husk is taken in a bamboo winnowing basket (also called chhaaj in Hindi) or plate or newspaper.
- Farmers take this mixture in an open ground and stand on a risen or elevated platform.
- The winnowing basket containing the mixture is held at shoulder height then the mixture is allowed to fall by shaking the basket continuously.
- As a result, lighter husk particles are carried away by the blowing wind. Heavy wheat grains fall straight to the ground and form a heap near the platform for winnowing. The separated husk is used for many purposes such as fodder for cattle.
With technological developments, a modern machine thresher has been developed which can be used to perform both the tasks of threshing and winnowing simultaneously.
Sieving
A method used to separate solid particles of different sizes by passing a mixture through a sieve, which is a shallow vessel having holes of the same size at the bottom, is called sieving. The sieves can be of different sizes which depends on the size of the substances to be separated. Some examples of separation of mixtures by sieving are as follows.
- Sieving allows the fine flour particles to pass through the sieve holes while the bigger impurities (Bran-outer coarse part of the grain) remain on the sieve.
- On a large scale, such as in a flour mill, impurities like stones and husk are removed from grains before grinding them. Usually, a bag full of wheat grains is poured on a slanting sieve.
- The sieve removes pieces of stones, stalks, and husk that may still remain with wheat grains after threshing and winnowing.
- Similar slanting sieves are used at construction sites to separate pebbles and stones from coarse sand.
Sieving of wheat flour is not considered a healthy practice because wheat bran which is removed during sieving is a rich source of fibre.
Evaporation
The process of conversion of water into its vapour is called evaporation. Heat and wind help in the evaporation of water. The process of evaporation takes place continuously, wherever water is present. This process is used to obtain a solid material that has dissolved in water or any other liquid. The dissolved substance is left as a solid residue when all the liquid gets evaporated. For example, obtaining salt from seawater. Sea water contains large amounts of salt and some other substances dissolved in it. One of these salts is the common salt. Now, let us understand the way by which it can be obtained.
When seawater is allowed to stand in shallow 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. Common salt is then obtained from these solid salts by further purification. In Ayurveda, the traditional Indian system of holistic health and medicine, herbs or parts of plants are prescribed as remedies. This practice facilitates the evaporation of excess water by drying the ingredients like roots, leaves, flowers, or seeds of various medicinal plants in shade.
The process of evaporation takes place at room temperature, but the rate of evaporation increases with the rise in temperature, thus, the evaporation process is faster when the mixture is heated.
Condensation
The process of conversion of water vapour into its liquid form is called condensation. The process of condensation is the opposite of evaporation. In this process, water vapor condenses into a liquid after making contact with the cooled surface. This method is used when the solid and liquid both need to be recovered, e.g. separation of pure water from salt solution.
Sedimentation, Decantation and Filtration
We observed that sometimes, separation of the mixture is not possible with the help of winnowing and handpicking as there may be lighter impurities present such as dust or soil particles. So,, we use other methods for the separation of such a mixture. These methods are as follows.
Sedimentation
The process of settling down of insoluble heavy solid particles in a mixture at the bottom of the vessel is known as sedimentation. For example, when we mix up sand and water in a glass and stir it with a spoon or any other tool and wait for some time, we see that the sand settles down at the bottom and clear water stands on it.
Decantation
The process of transfer or separation of clear liquid from one vessel to another vessel without disturbing the sedimented particles is known as decantation. For example, transfer of water to another vessel from a mixture of water and sand.
The same principle is used for separating a mixture of two liquids, that do not mix with each other, e.g. oil and water can be separated from their mixture by this process. If a mixture of such liquids, which do not mix with each other, is allowed to stand for some time, they form two separate layers. The component that forms the top layer (i.e. oil) can then be separated by decantation.
Filtration
The process of removing insoluble solid particles from a liquid by using a filter is known as filtration. Filter paper is a commonly used filter, other than this, cotton, charcoal, and sand can also be used as filters. The choice of filter depends upon the size of the particles of the materials to be removed. Some examples of the separation of mixtures by filtration are as follows.
- Separation of tea leaves from prepared tea by using a strainer.
- Making tea by using tea bags made up of filter paper. Initially, tea bags were made of soft cloth like silk. It is because it could hold the tea leaves and let hot water pass through. Later, people started using gauze or muslin. Eventually, they began using filter paper.
- Fruit and vegetable juices are usually filtered before drinking to separate the seeds and solid particles of pulp.
- In everyday applications, such as purifying drinking water, mud from water, etc.
Our kidneys act as a filter to clean the blood. Waste chemicals dissolved in the blood pass through a membrane in the kidneys and are excreted as urine acts or gets filtered.
The filtration method can not be used for the separation of a mixture of two liquids, like milk and water or oil and water. Also, this method only works if the solid component of the mixture is insoluble in the liquid component, thus a mixture of salt in water can not be separated by this method.
Churning
A mechanical process used to separate butter from buttermilk is called churning. It is traditionally significant in households and dairy farms. Churning is done by pouring the buttermilk or curd into a churner (mathni) and rotating the buttermilk or curd vigorously by using a churner. This causes the separation of butter (solid) from the buttermilk or curd. Butter being lighter, floats on the top of liquid buttermilk.
Magnetic Separation
A process where magnetic materials are separated from non-magnetic materials, by using a magnet is known as magnetic separation. The magnetic material, like, iron nails is attracted to the magnet and sticks to it, while, the non-magnetic material, like sulphur powder, is not attracted to the magnet.
This method is used in various industries and everyday situations. It is employed in recycling processes to separate magnetic materials (like iron) from the waste materials through a magnet fitted to a crane. This iron is called scrap iron and it can be recycled and reused.
→ Pure Substance: A substance that contains only one type of particle is called a pure substance.
→ Mixture: It is made up of two or more substances, which are called the components of the mixture.
→ Handpicking: It is the method of picking particles from mixtures by hand when the impurities are of slightly large size and in small quantity in the mixture.
→ Chaff: It is the thin, scaly layer that covers the grains attached to the stalks.
→ Threshing: The process of separating grains from stalks of harvested crops.
→ Winnowing: It is the method used to separate heavier and lighter components of a mixture by wind or by blowing air.
→ Sieving: It is a method used to separate solid particles of different sizes by passing a mixture through a sieve.
→ Evaporation: It is a process where a liquid substance is converted into its gaseous state.
→ Condensation: It is the process of conversion of water vapour into its liquid form.
→ Sedimentation: The process of settling down of insoluble heavier solid particles at the bottom of the vessel.
→ Decantation: It is the process of separating the top layer of liquid after sedimentation.
→ Filtration: It is a method used to separate a mixture of an insoluble solid and a liquid by passing it through a filter.
→ Magnetic Separation: It is a process where magnetic materials are separated from non-magnetic materials by using a magnet.
→ Churning: It is a mechanical process used to separate butter from buttermilk.