In our daily life, we often need to separate substances from mixtures. We strain tea with a strainer to remove tea leaves, we use a sieve to remove stones from grains, and farmers separate grains from chaff by winnowing. Separation is a very common and useful process. In science, we study the different methods of separating the components of a mixture.
A mixture is a substance made by mixing two or more different substances together. Most substances around us are mixtures. Air is a mixture of gases, sea water is a mixture of water and salts, and milk is a mixture of many substances. Sometimes we need to separate the components of a mixture to get a pure substance.
In this chapter, we learn why separation is needed, what mixtures and pure substances are, and the various methods of separation like hand-picking, threshing, winnowing, sieving, sedimentation, decantation, filtration, evaporation, and condensation. Each method works because of a difference in the properties of the components, such as size, weight, solubility, or state.
Separation is needed for many reasons: - To remove impurities: Farmers remove stones, husks, and dirt from grains before storing them. - To get useful substances: We extract sugar from sugarcane, salt from sea water, and spices from plants. - To make food safe and tasty: We remove tea leaves from tea, and separate stones from rice before cooking. - For health and medicine: Pure substances are needed to make medicines. - For industrial processes: Many industries separate components of mixtures to get pure materials.
Separation is also important in environmental science. We separate garbage into biodegradable and non-biodegradable waste. We separate dirty water from clean water in water treatment plants. Without separation, our food, water, and medicines would not be safe.
Before learning separation, we must understand what a pure substance and a mixture are.
The properties of a mixture are different from the properties of its components. For example, when salt is dissolved in water, the mixture (salt solution) is different from both salt and water. The components of a mixture do not lose their individual properties, and they can be separated back.
Hand-picking is the simplest method of separation. It is used to separate substances that are different in size, colour, or shape, and are present in small amounts.
In hand-picking, the unwanted substances are picked out by hand. For example, stones and pieces of dirt are removed from rice and dal by hand-picking. We also pick out small stones and damaged grains from wheat before storing it.
Hand-picking is useful when: - The unwanted substances are present in small amounts. - The unwanted substances are different in size, colour, or shape. - The unwanted substances are not too many.
This method is simple and requires no special tools, but it is time-consuming when the mixture is large or when the unwanted substances are many.
Threshing is the process of separating grains from the harvested crop by beating. Farmers beat the harvested stalks on the ground or use machines to separate the grains from the stalks. This process breaks the stalks and releases the grains. Threshing is done for crops like wheat, rice, and maize.
Winnowing is the method of separating grains from the chaff (the outer covering of grains and light pieces of straw). In winnowing, the mixture of grains and chaff is dropped from a height. The wind blows away the lighter chaff, while the heavier grains fall down and collect near the farmer.
Winnowing works because of the difference in weight between the grains and the chaff. The grains are heavy and fall straight down, while the chaff is light and is carried away by the wind. Today, winnowing machines are also used on large farms.
Sieving is a method of separating solid materials of different sizes using a sieve. A sieve is a net made of metal wires or cloth with small holes of a specific size.
In sieving, the mixture is poured onto the sieve and shaken. The smaller particles pass through the holes, while the larger particles remain on the sieve. For example, in a flour mill, bran and husk are removed from flour by sieving. We also use sieving to separate stones and dust from sand, and to remove impurities from grains before cooking.
Sieving is also used to separate grains from husk when the grains are larger than the holes of the sieve. Sieving is more efficient than winnowing in separating fine impurities from food grains.
Sedimentation is the process in which insoluble, heavy solid particles settle down at the bottom of a liquid when the mixture is left undisturbed for some time. For example, when a mixture of sand and water is kept still, the sand settles at the bottom.
Decantation is the process of pouring out the clear liquid from the top, leaving the settled solids behind. After the sand settles at the bottom, the clear water above it is poured out carefully into another vessel. This is decantation.
Sedimentation and decantation are used to separate insoluble solids from liquids. This method is used in villages and towns to clean muddy water. When muddy water is allowed to stand, the mud settles down, and the cleaner water above can be decanted.
Filtration is a method of separating insoluble solid particles from a liquid using a filter. A filter has tiny holes that allow the liquid to pass through but stop the solid particles. Common filters are filter paper, cloth, sand, and charcoal.
In filtration, the mixture is poured over the filter. The liquid that passes through the filter is called the filtrate, and the solid that remains on the filter is called the residue. For example, when we filter tea through a strainer, the tea liquid passes through, and the tea leaves remain on the strainer.
Filtration is used in many places: - Water filters in homes clean water by filtration. - Water treatment plants use layers of sand and gravel to filter water. - In laboratories, filter paper is used to separate solids from liquids.
Filtration is different from decantation because it uses a filter, while decantation just pours off the clear liquid.
Evaporation is the process of converting a liquid into vapour by heating. When we heat a salt solution, the water evaporates and turns into water vapour, leaving the salt behind. Evaporation is used to recover a dissolved solid from a solution.
Condensation is the process of converting vapour back into liquid on cooling. When water vapour is cooled, it turns back into liquid water. We see condensation when water droplets form on the outside of a cold glass of water. The water vapour in the air touches the cold glass and condenses into droplets.
Evaporation and condensation together are used to separate salt from sea water. Sea water is collected in shallow pits and allowed to dry in the sun. The water evaporates, and the salt is left behind. When the vapour cools, it condenses back into water. The same principle is used to obtain drinking water from impure water.
Different mixtures need different methods. The table below summarises which method is used for which mixture:
| Mixture | Method of Separation |
|---|---|
| Stones from rice | Hand-picking |
| Grains from stalks | Threshing |
| Grains from chaff | Winnowing |
| Bran from flour | Sieving |
| Sand from water | Sedimentation and decantation or filtration |
| Tea leaves from tea | Filtration |
| Salt from salt water | Evaporation |
| Water from a solution | Evaporation followed by condensation |
The choice of method depends on the properties of the components, such as their size, weight, state, and solubility.
| Method | Based On | Example |
|---|---|---|
| Hand-picking | Difference in size, colour, shape | Removing stones from rice |
| Threshing | Beating to separate grains | Separating grains from stalks |
| Winnowing | Difference in weight | Separating grain from chaff |
| Sieving | Difference in size | Separating bran from flour |
| Sedimentation | Settling of heavy solids | Sand settling in water |
| Decantation | Pouring off clear liquid | Pouring water from settled sand |
| Filtration | Passing through a filter | Straining tea leaves |
| Evaporation | Heating a liquid to vapour | Obtaining salt from salt water |
| Term | Meaning |
|---|---|
| Mixture | Two or more substances mixed together |
| Pure substance | Only one kind of material |
| Filtrate | The liquid that passes through a filter |
| Residue | The solid that remains on the filter |
| Saturated solution | A solution that cannot dissolve more solute |
In this chapter, we discovered that separation is an everyday skill and an important scientific process. We learned what pure substances and mixtures are, and why we need to separate the components of mixtures. We studied simple methods like hand-picking, threshing, winnowing, and sieving, and liquid-based methods like sedimentation, decantation, filtration, evaporation, and condensation. Each method works because of a difference in some property of the components, such as size, weight, solubility, or state. By choosing the right method, we can obtain clean water, pure salt, safe grains, and useful substances from nature. These simple ideas of separation are used in homes, farms, laboratories, and industries all over the world.