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SEPARATION TECHNIQUES - Coggle Diagram
SEPARATION TECHNIQUES
Purity of substances
A pure substance only consists fo one type of substance, either an element or a compound
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For most substance, the presence of an impurity increases its boiling point but decreases its melting point.
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Mixtures
A mixture consists of two or more different substance (elements or compounds) that are mixed, but not chemically combined
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Mixtures of compounds: e.g. seawater is a mixture made up of sodium chloride (compound) and water (compound).
Mixtures of elements and compounds: e.g. air is a mixture made up of water (compound), nitrogen (element), oxygen (element) etc.
SEPARATION OF MIXTURES
SOLID-SOLID
Magnetic attraction is used to separate magnetic substances from non-magnetic materials using a magnet
At the junk yard, electromagnets are used to lift, handle and load scrap steel and iron so that they can be recycled, by being re-mouldered into saleable material
In food processing magnetic separators are used to remove small amounts of iron containments such as ash content, unwanted magnetic stones so that the food is safe to be consumed
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Sublimation is used to separate a solid that sublimes on heating from a solid that does not. A solid that sublimes undergoes a state change form solid gas without going through liquid state.
Physical properties of the components in the mixture: Iodine sublimes but not salt. So during heating, iodine turns into vapour while salt remains on the evaporating dish.
- USE OF A SUITABLE SOLVENT FOLLOWED BY FILTRATION (TWO STEP)
This technique is sued to separate a mixture of two solids, in which one of the solids is soluble in a suitable solvent and the other solid is not.
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SOLID-LIQUID
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In the lab, a piece of filter paper is placed in a filter funnel. The mixture is poured onto the filter paper.
The filter paper has very small pores. Only particles with a size smaller than the pores can pass through. The insoluble solids have larger particle size than the soluble liquid, hence are trapped on the filter paper, which allow the liquid to pass through.
The solid that remains on the filter paper is called the residue. The liquid that passes through is called the filtrate.
Evaporation till dryness is used to separate a soluble dissolved solid from a solution. It is only used for substances that do not decompose on heating.
Evaporation till dryness an only be used for substances that do not decompose on heating, otherwise known as heat-stable substances.
The solid obtained by evaporation till dryness is not always pure. Any soluble impurities will be left together with the solid after the process is completed.
Crystallisation is used to separate a soluble dissolved solid from a solution. It is used for substances that are not heat-stable.
The solution is heated to remove most of the solvent (water). Heating is stopped when a saturated solution is formed. The hot, saturated solution is allowed to cool. The dissolved copper (II) sulfate appears as pure crystals. The cold solution with crystals is filtered. The crystals are washed with a little amount of cold distilled water to remove impurities and filtered again. The filtrate contains soluble impurities. The crystals are then dried by pressing them between pieces of filter paper.
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A saturated solution is a solution that contains as much dissolved solute as it can at a given temperature.
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Paper chromatography is used to separate or identify a small volume fo solutes dissolved in a solvent. It is done based on the different solubility of the components in a particular solvent.
- A solution of the dye is obtained
- A drop of the solution is placed on the pencil line near the bottom of a strip of filter paper.
- The chromatographic paper is dipped into a solvent, e.g. water, with the solvent
- The solvent moves up the paper
- When the solvent reaches the spot of the dye, it dissolves the substances in the dye.
- The difference substances move along with the solvent but at different rates.
- The move soluble substances move faster and further up the paper than those that are less soluble in the solvent used.
- The position of the solvent when the paper is removed is known as the solvent front.
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Thus, the substances in the mixture are seperated.
Locating agents is used int eh chromatography of colourless compounds. The locating agent reacts with the colourless substances to produce coloured products.
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Rf value:
The Rf value is the unique characteristic of the pure component and it can be used to identify the component present int the mixture.
The ratio between the distance travelled by teh substance and the distance travelled by the solvent is a constant. This ratio is called the Rf value of the substance.
The positions of the solvent front (position reached by solvent) and spot on a chromatogram depend on how long the experiment was allowed to run.
What affects Rf value:
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Type of solvent: A solvent which has a stronger affinity for a particular chemical will easily overcome any affinity fo the chemical for the chromatography paper and move that chemical farther in a given period of time.
Type of chromatography paper: Components of he mixture(s) will separate on the chromatography paper according to how strongly they absorb to it versus how much they dissolve into the solvent. The absorbance may be different for different types of chromatography paper.
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The different solubility in a solvent causes them to move at different speeds on the chromatography paper
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Simple distillation is used to separate a liquid from a solution. The solution can be a solid-liquid mixture or a liquid-liquid (miscible) mixture
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LIQUID-LIQUID
Fractional distillation is used to separate 3 or more miscible liquids. Generally, the liquids have boiling points that differ by less than 25 degree celcius from each other
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A fractionating column is attached to the round-bottomed flask and the condenser for fractional distillation.
Glass beads/plates/spiral int eh fractioning column provide a large surface area for vapour to condense on.
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Fractional distillation is able to separate different liquids in a mixture because they have have different boiling points.
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A fractionating column allows a liquid of higher boiling point to condense and fall back during the distillation of a substance of lower boiling point. This ensures separation of the liquids.
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