A mixture is usually an impure substance and it is necessary to separate mixtures in order to obtain its pure components. The components of a mixture are not chemically combined. Therefore, they can be separated by physical methods; the components of a mixture will still contain their properties. Methods used to separate mixtures depend on the physical differences between the components of the mixture. The main separation techniques include:
Filtration
A method which is used to separate a suspension, a type of mixture produced when a solute does not dissolve in a solvent.
The mixture is poured into a filter funnel containing filter paper. The paper possesses small pores which allow the solvent to pass through but does not allow the solute to pass through. The solid left in the filter paper is known as residue and the liquid which passes through is called the filtrate.
Paper Chromatography
Paper chromatography is a special method used to separate inks and other pigments.
The separation occurs due to different dyes travelling at different speeds up the filter paper. The speed is determined by the dyes ability to dissolve in the solvent. The faster it dissolves in the solvent, the faster and further it moves on the filter paper.
Main Separation Techniques
Simple Distillation
This technique is used to obtain or separate a pure solvent from a solution. Simple distillation separates a solid or liquid solution of substances with widely different boiling points. The method is based on the fact that the solvent vaporizes at a lower temperature than the solute.
The solid which remains the distillation flask is known at the residue and the distilled solvent is known as the distillate.
Solvent Extraction
A method used to separate mixtures, where one component dissolves in a solvent and another does not. These components are usually of different chemical types and solubility.
Main Separation Techniques
Sublimation
Sublimation is used to separate solid mixtures which contain one substance that is able to sublime and other which is not. Sublimation is the ability of a solid to change directly into a gas.
Fractional Distillation
Fractional distillation is used to separate miscible liquids with components of the liquid mixture having similar boiling points. The component with a higher boiling point would condense on the fractionating column and return to the solution. However, the component with the lower boiling point would pass through the column and be collected in the flask.
Main Separation Techniques
Separating Funnel
This equipment is used to separate immiscible liquids. The method is based on the fact that the liquids will not mix but will instead form two distinct layers. The less dense liquid will be on top and the more dense liquid on top.
Evaporation and Crystallization
This method is used to separate a solution of solid and liquid. The mixture is heated using an evaporating dish and the liquid will eventually boil off. The solution will then become extremely concentrated and eventually supersaturated, which will form crystals. If the solution is cooled, crystallization takes place; the crystals are then washed and dried.
Main Separation Techniques
Decantation
This method can be used to separate suspensions. A mixture if left undisturbed until all the solid particles are settled. The liquid layer is later carefully poured or decanted off, leaving the solid residue behind.
Centrifugation
When the mixture takes too long to settle, the process is able to be sped up by centrifugation. This process allows the mixture to be rotated at a high speed and the centrifugal forces cause the particles to settle out.
The Extraction of Sucrose
From Sugar Cane
What is sugar cane?
Sugar cane is a seasonal, tall and usually strong plant, which is native to the warm or tropical regions of the world. The plant is cultivated, grown and harvested in various parts of the world, mainly Africa, South America, Asia, some parts of India and the Caribbean. As the world’s largest crop, it is an economically important plant. A little under two hundred countries are involved in the production of sugar cane due to the wide variety of items it is used to produce.
What is sucrose?
The main product of sugar cane is sucrose, also known as table, white or cane sugar and sometimes as saccharose. Sucrose is an odorless, white and crystalline powder with a sweet taste, which can be in the form of powdered or granulated. Regardless of what it is called, sucrose is used to sweeten foods and give consumers and energy boost.
The Extraction of Sucrose
From Sugar Cane
There are various stages that the sugar cane must go through before sucrose is actually produced. The stages include:
1. Shredding
The sugarcane is placed in a shredder, which contains a cutting tool with round blades known as rotary knives. The rotary knives are used to shred the sugar cane into small pieces.
2. Crushing
A juice extractor is then used to extract the juice from the sugar cane. Water or diluted cane juice is sprayed on the cane, to increase the extraction of sucrose. The juice is then strained to remove sugar cane fibre and the juice is then sent to the juice scale to be weighed. The can fibre, known as ‘bagasse’, is later burnt to supply fuel for the boilers.
3. Precipitation / Clarifying
Cane juice is usually acidic at this point and is sent to the clarifying station to be treated. The slightly acidic juice at this stage is 13% sugar, 85% water and other impurities. It is then neutralized by the addition of calcium hydroxide. Heating allows the impurities to precipitate in the form of insoluble calcium salts.
4. Filtration
Precipitated solid materials, also known as filter press, are removed from the sugar cane juice. The filter press is washed and placed in the fields to be used a fertilizer. The juice, however, is sent to the first of several boilers.
The Extraction of Sucrose
From Sugar Cane
5. Evaporation
The sugar cane juice passes through several boilers. The juices are evaporated under reduced pressure so as to prevent charring; the water is removed and a thick stick syrup is produced.
6. Crystallization
The syrup is boiled at a low pressure to form sugar crystals. The crystals produced are of a uniform size and given great care. At this point a mixture is formed, of molasses and crystals. The mixture is then sent to a centrifugal machine to separate sugar crystals from the molasses.
7. Centrifugation
The centrifugal machine is made up of a perforated drum, lined with a screen and is spun by a motor. The motor is placed at a high speed. This forces the sugar crystals to be trapped by the screen and most of the molasses to be removed. The crystals are then sprayed with water so as to wash off any excess molasses and the machine is then spun to dry the sugar crystals.
8. Collecting
Separated containers are then put in place to collect the sugar crystals, also known as sucrose and the molasses which is used for manufacturing of rum and animal feed.
The Extraction of Sucrose
From Sugar Cane
A flow diagram, showing the stags in the extraction of sucrose.
Bibliography
www.wikipedia.com
www.wisegeek.com
www.benefits-of-chemistry.com
www.answers.com
www.chemistry.about.com
Heinemann Chemistry for CSEC
Chemistry Explained
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