Separation of Substances: science class six
Materials and their classification
Materials can be either pure substances, or a mixture of two or more substances. A pure substances has only one kind of particles in it. Most materials which occur on the earth are mixtures of two or more substances. Soil, air, milk and water are mixtures. For examples, air is a mixture of many gases, such as oxygen, nitrogen, and carbon dioxide. Similarly, milk is a mixture of water, fat and other substances.
Mixture
Materials which contain two or more substances in any proportion are called mixture. A mixture may be a solid, a liquid or a gas. Soft drinks, Air, lemonade, sea water are some examples of mixtures.
Types of mixture
Mixture are two types: Homogeneous mixture and Heterogeneous mixture.
(a) Homogeneous mixtures
Mixtures in which the particles of the substances present cannot be seen are called homogeneous mixtures. Some homogeneous mixtures are
Solution of salt and sugar in water.
Air: A mixture of gases like nitrogen, oxygen, carbon dioxide, etc.
Heterogeneous mixtures
Mixtures in which the particles of the substances present can be seen easily are called heterogeneous mixtures. Some heterogeneous mixtures are,
Water +oil, Chalk + water, Dust in air
Pond water: It is a mixture of many salts and fine clay particles in water.
To make the available substance fit for use, these impurities must be separated from the useful substance.
Methods of separation
Separation of the useful substances from unwanted substances is very common in our day to day life.
Hand Picking
This method is used for separation the solid from solids. This is simplest method of separating unwanted materials from the useful ones. This method is used only when,
The unwanted material is present in smaller quantity,
Size, shape or colour of the unwanted materials is different from that of the useful ones.
Most foodgrains such as, wheat, rice, pulses etc. contain very small pieces of stone and some other impurities are present. These small pieces of stones are removed by hand picking. Handpicking is a convenient method of separating substances.
Threshing
This method is generally used by farmers to separate the grains from the stalks after harvesting. The dried stalks are beaten, or threshed, to separate the grains. In large farms, threshing is done by using- machines, called threshers.
Winnowing
This method is used to separate lighter husk from heavier grains like wheat. In this method, the mixture is allowed to fall from a height. The lighter husk get blown away to a distance while the heavier components, or the grains, fall down closer. The two parts, thus, get separated out into two separate heaps. This method is commonly used by farmers to separate lighter husk and heavy grains.
Sieving
This method is used to separate the components that are of different sizes. In this method, a sieve having holes of proper size is used. The bigger particles are retained by the sieve, where as the smaller ones pass through it.
This method is used for the following purpose.
For separating chaff from wheat flour.
For separating fine sand from gravel etc.
For separating cashewnuts of different sizes.
For separating pearls of different sizes.
Sedimentation, Decantation and Filtration
The following methods are used for separating insoluble solids from liquids.
Sedimentation and Decantation
Sedimentation and Decantation is used for separating coarse particles of a solid which are insoluble in the liquid. Sedimentation is a process by which insoluble, heavy solid particles in a mixture settle down on their own, when left undisturbed for sometime. In order to separate out the two, the liquid has to be gently poured into another container without disturbing the settled down solid particles, or sediments. The process of obtaining clear liquid by pouring it into another container without disturbing the sediment is called decantation.
For example, the coarse particles of sand in muddy water can be separated by decantation. Before cooking, rice, pulses etc. are washed with water. The wash-water is removed by decantation.
Filtration
This method is used for separating fine insoluble solid particles from the liquid. In this method, the mixture is allowed to pass through a filter. The insoluble solid particles are retained by the filter and the clear liquid is collected as the filtrate.
Commonly used filters are apiece of muslin cloth and filter paper. The method of filtration is separate out tea leaves from tea. We use a sieve for it. Filtration is also used by the water works Department to purify water. They use layers of gravel and sand to filter very large quantities of water.
Evaporation
Evaporation is used to recover a solid substance from its solution. Evaporation can be made quicker by heating the solution. This method is commonly used to obtain common salt (that we use in our food) from sea water. Sea water is collected in shallow beds dug near the sea. The heat of the sun helps in evaporating water leaving behind the particles of salt.
Condensation
The process of conversation of water vapour into its liquid from is called condensation. This method is used to separated salt, sand and water.
Crystallisation
Crystallisation is used to obtain pure solids from a given solution. For example, the salt, we get from sea water has many impurities in it. To remove these impurities the process of crystallization is used.
Can water dissolve any amount of a substance?
Water dissolves a wide variety of substances in it. The limiting amount of any substance that water can dissolve (at any temperature and pressure). This phenomena is called solubility of that substance in water.
For example, Sugar dissolves to a greater extent than common salt.
Solute
The substance which is dissolve in water or any other solvent is called solute. Different substances dissolve in water to different extents.
Solution
The homogeneous mixture of water or any other solvent and a substance dissolve in it is called a solution.
Saturated solution
A solution in which no more solute can be dissolved at a given temperature is called a saturated solution.
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