CHEMISTRY              HOMOGENEOUS MIXTURES: Homogeneous mixtures are those which have a uniform composition and similar properties at all points. They are uniform in appearance and have only one phase i.e. they are either all solid or liquid or gas. However, there are differences while studying them very minutely. But most of the pure substances are considered homogeneous for practical purposes.     HETEROGENEOUS MIXTURES: These are mixtures which are non uniform in composition and appearance i.e. different portions/samples from the same mixture show different composition. They are composed of two or more substances which retain their own identifying properties. Also, they may have mixed phases.     METHODS OF SEPARATION OF THE CONSTITUENTS OF MIXTURES: The separation of the constituents of a mixture depends upon the properties of its constituents. The following are a few methods employed for this purpose:     SEDIMENTATION AND DECANTATION This is a simple procedure to separate insoluble, heavy particles from a liquid which is often used. It involves keeping the liquid aside for some time and waiting for the heavy particles to settle down at the bottom of the container. This is called Sedimentation. After this the liquid may be poured into another container without disturbing the settled particles. This transfer of liquid is called Decantation.             FILTRATION This method is employed to separate the solid from a liquid. Normally we use a strainer to filtrate things in a kitchen but in a laboratory, a filter paper is used for the purpose. As an example, take the mixture of water and chalk powder. Take a filter paper and fix it in a funnel. Then, pour the mixture slowly into a container; through the funnel. You will be able to see the particles of chalk powder on the filter paper. The solid particles of this type are called the ‘residue’ and the clear liquid like the water in the container is known as the ‘filtrate’.         EVAPORATION This method is employed to separate the soluble solids from the solutions. For example take the solution of salt and water (or sea water) where salt is the solute and the water is the solvent. Heat the solution in an evaporating dish till the solvent evaporates. The remaining residue is the solute i.e. salt. NOTE: usually in evaporation only the solute is obtained, not the solvent.         DISTILLATION This method is adopted to obtain both the solute and the solvent from a solution. Here the solute is a soluble solid. Distillation actually involves both evaporation as well as condensation. Let us take the example of a salt solution. The solution is taken in a distillation flask and heated so that the solvent slowly starts evaporating. The distillation flask is attached to a Leibig’s condenser which has a lower tube for the inlet of cool water and an upper tube for the outlet of water. The circulation of cold water in the condenser helps to cool the vapors from the solution and they form the distillate which is collected in a separate container attached to the condenser. The complete solution is not allowed to evaporate in this case but when a little is left, it is heated separately in an open container leaving it to evaporate completely. The remaining residue is the solute i.e. salt and the distillate collected is the solvent i.e. water.         FRACTIONAL DISTILLATION This method is adopted to separate two or more miscible liquids whose boiling points are quite different. The liquid with the lower boiling point is heated first so that it evaporates and after the vapors pass from through the condenser, they fall into a container as liquid. After this the liquid with the next higher boiling point evaporates and again is collected in a separate container. Gradually, all the solutions are separately collected in the same manner. NOTE: the temperature remains constant for each boiling liquid till it evaporates completely more or less.         LIQUID-LIQUID EXTRACTION This method is used while separating two or more immiscible liquids with different densities. The mixture is taken in a separating funnel and allowed to stand for some time. This separates the liquids into layers. The liquid with the lowest density floats on the top and the one with higher density lies below it. After this the funnel tap is opened and the liquid at the bottom of the funnel is transferred into a container till a little of it is left along with the next liquid. That little part is not collected as it may contain a little of the other liquid too. After this the next layer is collected in another container, giving us the separate liquids.         SUBLIMATION This method of separation is adopted when one of the components of a mixture is sublime. Let us take the example of a mixture of iodine and sand. The mixture is heated and iodine being sublime transforms into vapors. These vapors cool down to a solid state on the cooler part of the container being heated. The sand remains in the vessel and thus the two get separated.