9thChemistry Lesson No 2
Is Matter Around Us Pure(part 2nd)
Separating Components of a Mixture
The separation of a mixture depends upon the nature, physical state and difference in properties of various constituents of the mixture.
Evaporation
This method is used for separating a solid dissolved in a liquid. The solution is heated on a water bath or sand bath. The liquid evaporates leaving the solid behind. eg, to obtain dye from blue or black ink or to obtain salt crystals from sea water evaporation method is used
Centrifugation
This method is based on the principle that when a mixture containing very small particles is rotated at a high speed the lighter particles come on the surface and heavier particles settle down. eg, separating milk from cream is done through Centrifugation
Applications of centrifugation :
1:-The technique of centrifugation is used in diagnostic laboratories for blood and urine tests.
2:- It is used in dairies and homes to separate butter from cream.
3:-Centrifugation technique is also used in washing machines to squeeze out water from wet clothes.
Separation Using a Separating Funnel
A mixture of two immiscible liquids can be separated by using a separating funnel. Two liquids which do not mix with each other are called immiscible liquids. These liquids form heterogeneous mixture. On standing they separate out into two separate layers depending upon their densities.eg To separate a mixture of kerosene and water or edible vegetable oil and water we use this technique in both the cases liquids vary in their densities.
Sublimation
This process is used to separate those solids from their mixtures which directly pass to the vapour state on heating without passing through the liquid state . eg, we use sublimation to separate the mixture of ammonium chloride and common salt
Heat, sublimation
Solid ------------------------->Vapours
<------------------------
Cool, de sublimation
Magnetic Separation
This method is based on the property of iron to get attracted by magnet. The mixture in which one component is magnetic in nature can be separated out by this method. Eg, To separate a mixture of iron and sulphur we use this method
Chromatography
This is a modern method used for separation of mixture into their components and to test the purity of components. The name chromatography is based on Greek word "kroma" means "colour" and "graphy" means "writing" because it was first used to separate coloured substances found in plants. Separation of different components of a mixture based upon their different solubilities in the same solvent is known as chromatography eg, we use this technique to separate the components of ink.
Distillation
If liquids in a mixture are miscible and have different boiling points, they can be separated by distillation. Distillation involves conversion of a liquid into vapours and then condensing the vapours back into liquid. Distillation is used only if the liquids have a difference in boiling point of more than 25 K. eg, acetone is separated from water by distillation
Distillation can be represented as,
Heating
Liquid ------------------------->Vapours
<----------------------------
Cooling
The liquid obtained by condensing the vapour is called distillate.
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| Distillation procedure |
Fractional Distillation
If the boiling point of the two miscible liquids of the mixture are very close to one another, i.e., less than 25 K or so, the separation cannot be achieved by the simple distillation method. This is due to the reason that at the boiling point of the more volatile liquid of the mixture there will be sufficient vapours of the less volatile liquid as well. As a result, both the liquids of the mixture will distil together and the separation cannot be achieved.
The separation of such a liquid mixture into individual components can, however, be achieved by fractional distillation, which involves repeated distillations and condensations. Fractional distillation is carried out using a fractionating column. It usually consists of a long glass tube with a wide bore packed with glass beads or small stones or porcelain pieces. The actual purpose of the fractionating column is to increase the cooling surface area and to provide hurdles or obstructions to the ascending vapours and descending liquid. , this method is used to separate a mixture of alcohol and water .
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| Fractional distillation |
Fractional Distillation of Air
Air is a homogenous mixture of many gases which can be separated by fractional distillation. Air is cooled and compressed by applying pressure and decreasing temperature. Air is compressed to form a Liquid air. Liquid air is subjected to fractional distillation and different gases are Separated according to their boiling points.
The flow diagram is as under.
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| Separation of components of air |
Crystallisation
The process by which a pure soluble substance is separated in the form of crystals from its hot and saturated solution on cooling is called crystallisation. Crystallisation is better than simple evaporation as it is a method to get pure solids from liquids as
(i) some solids may decompose or get charred when heated to dryness
(ii) on evaporation some dissolved impurities may also get deposited with the solid while in crystallisation, pure solid are crystallised leaving the impurities in the solution.
The process of inducing crystallisation by adding a crystal of the pure substance into the saturated solution is called seeding and the crystal of the pure substance is called the seed crystal which acts as nuclei for crystallisation. To obtain pure crystals of copper sulphate and sugar we use this method
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| Process of crystallization |
Purification of Drinking Water
A number of dissolved and suspended impurities are to be removed from the river or lake water before making it fit for drinking purposes.
A typical water works involved in the process includes the following processes :
- Removal of suspended impurities (Sedimentation tank) : Water is allowed to stand for sometime so that suspended impurities are settled down.
- Removal of colloidal particles (Loading tank) : To remove the small particles present in colloidal state some alum is added to water. Particles like clay get neutralised and are coagulated at the bottom of the tank.
- Filtration of impure water (Filtration tank): The water from which insoluble impurities have been removed is passed through filtration tank which has three layers. Coarse gravel at the bottom, fine gravel in the centre and fine sand at the top acts as filters. Impure water is introduced from the bottom. The impurities are retained in the three layers of gravel and pure water goes to the top and sent to chlorination tank,
- Chlorination tank : Filtered water is treated with bleaching powder or ozone or any other germicide to kill bacteria. The pure water is now supplied to homes for drinking purpose.
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| Purification of Drinking Water |
Colloid
Colloids are contrasting compounds in which the particle size is Between 10-7cm (actual solution size = 1nm) and 10-5Cm (size of course suspension = 100nm). The size of the solution particles in colloids is larger than the actual solution but smaller than that Of a suspension. Colloids particles may look similar to us, but they are actually found Varied Or different on observation through a high power microscope. Some examples of colloids are , soap solution, milk, ink, blood etc.
Types of Colloids
Depending on the states of dispersed phase and dispersion medium, eight different types of colloids are possible. Gases mix to form homogeneous mixtures. As shown in below chart
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| Different types of Colloids |
- Oil-in-Water Emulsions :-When oil is the dispersed phase and water is dispersion medium eg, milk (in which liquid fat is dispersed in water) and vanishing cream
- Water-in-Oil Emulsions:-When oil acts as dispersion phase and water is dispersed in it eg, butter and cold cream
- The process of making an emulsion is called emulsification. The substances which are added to stabilise emulsions are called emulsifiers or emulsifying agents. These stabilises the sols by reducing the surface tension of water.
- For stabilising an emulsion, an emulsifying agent is required along with dispersed phase and dispersion medium. Some common emulsifying agents are soaps, detergents, proteins, gum, agar, etc.
- The separation of an emulsion into its constituents is called demulsification.
- The various techniques applied for demulsification are boiling, freezing, filtration, centrifugation, etc.
Properties of Colloids
Since the particle size of colloids lies between true solution and suspension, they show some specific properties. Which are as under
Brownian Movement
Colloidal particles are in a state of constant and zig-zag motion when viewed under an ultra microscope. This motion arises due to the collision of the colloidal particles with the particles of dispersion medium
Tyndall Effect
The scattering of a beam of light by colloidal particles is called Tyndall effect. Due to scattering of light, the path of the light becomes visible. When a beam of sunlight enters a dark room through a small hole from the window, the path of light becomes visible and dust particles present in air can be seen in the rays of light.Sky looks blue due to scattering of light by dust particles
Electrophoresis
All the colloidal particles of a particular colloid carry same charge which may be either positive or negative. When an electric field is applied to a colloidal sol, the charged particles move towards the oppositely charged electrode depending on the charge present on them. This movement of charged colloidal particles under the influence of an electric field is called electrophoresis.
Coagulation
The process by which the colloidal particles are separated by addition of small amount of electrolyte is called coagulation. It is generally carried out by addition of electrolytes like sodium chloride, barium chloride alum, ete. For example, bleeding from a cut can be immediately stopped by applying alum or ferric chloride..
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https://jkgovtschoolnotes.blogspot.com/2022/01/9th-chemistry.html
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