Separating an insoluble solid from a liquid.
E.g.: Separating sand and water from a mixture of sand and water.
Aim: - To obtain sand and water separately from a mixture of sand and water.
Materials needed: - filter funnel, filter paper, beaker and conical flask.
Procedure: - • Place the folded filter paper inside the funnel and set the apparatus as shown below.
• Pour the mixture carefully through the funnel.
• Sand gets trapped in the filter papers as the
• Water passes through the filter paper as the filtrate.
Other examples: When aqueous barium nitrate is reacted with dilute sulphuric acid. Barium sulphate is precipitated out. Filtration can be carried out to separate the barium sulphate from the mixture.
Filtration can be used to separate out the silver chloride precipitates from its reacting mixture.
2. Evaporation
To separate the soluble solid (solute) from a solution.
E.G. – Separating salt from salt water.
Aim: - Obtain salt from sea water.
Procedure: - • Initially heat sea water under direct flame.
• Water will evaporate leaving behind salt.
• Final heating carried out using a water bath avoids loss of salt by spitting.
Other examples: - This method is used t obtain many salts form their solution. The salt obtain using this method would be as a powder. Other example includes sodium sulphate, sodium nitrate and magnesium chloride.
Application: - This method is used to obtain salt from sea water in large scales in many parts of the world.
Disadvantages: - If there are any impurities dissolve in the solution evaporating all the solvents would produce solid with impurities present in their.
3. Crystallization
Crystallization is obtaining the solute from the solution.
E.G.: - Obtaining copper (II) sulphate crystals from copper (II) sulphate solution.
Aim: - To obtain dry crystals of copper (II) sulphate from copper (II) sulphate solution.
Method: - • The solution is evaporated to crystallization point (the point at which crystals of the solute will form on cooling the solution to room temperature).
• Crystallization point is identified by removing drops of solution at intervals to a microscopic slide using a glass rod.
• The formation of crystals in one of these drops tells you that the solution has reached crystallization point.
• The solution is than allowed to cool and crystallize.
• Once crystals form they are filtered with water before drying with filtered paper.
Advantages: - Even if impurities are present in the solution unlike evaporation the solid obtained will be free of impurities in crystallization.
Other examples: - This method is used to obtain dry salt crystals from their solution. Other examples includes zinc nitrate, cobalt (II) chloride and copper (II) chloride.
4. Distillation
Separating pure solvent out of a solution, the process involves changing liquid to gas and gas changing back to liquid.
E.G. Separating pure water from salt water.
Aim: - To obtain pure water from sea water.
Method: - • The apparatus is setup as shown in the diagram.
• The solution is heated in a flask and allowed to boil.
• Steam rises into the condenser.
• When steam reaches the cold condenser it condenses to water and this pure water drips into the beaker as the distillate.
• Salt water is left behind in the flask.
Application: - Distillation is used in desalination plants to obtain pure water out of salt water.
Fractional Distillation: •To separate two or more miscible liquids with different boiling points.
• Miscible liquids are liquids that mix completely with each other.
E.g.: - Separating water (boiling point 100°c) and ethanol (boiling point at 78°)
Aim: - To separate ethanol from water from a mixture of ethanol and water.
Procedure: - • The apparatus is setup as shown in the diagram.
• As the mixture is heated both ethanol and water boils and the vapor rises to the fractionating column and condenses back many times as the temperature rises.
• But the liquid with the lower boiling point will vaporise more (in this case ethanol).
• The glass beads provide a large surface area for condensation.
• When the temperature exceeds the boiling point of ethanol (78°c) all of the ethanol would have change to vapor and rose through the fractionating column to the condenser where the vapor is converted back to liquid and collected as the distillate.
• As the temperature is below the boiling point of water (100°) water vapor only travels as far as the fractionating column where it is condensed back.
• The temperature stays at 78°c until all ethanol is distillate and only then would the temperature rise allow water to be distilled.
POINTS TO REMEMBER ABOUT DISTILLATION & FRACTIONAL DISTILLATION
• The thermometer has to be placed in such a way that the tip of the thermometer is exactly near the entrance of condenser. (in order to measure the boiling point correctly).
• If the solution used is flammable a water bath should be used.
• The apparatus that collects the distillate should not be fixed to prevent the build up of pressure. Pressure may sent back the vapors into the heated mixture instead of it being liquefy and collected.
• Condenser is placed in such away that the water in is at the bottom and at the end farfiest from the reactor to ensure the condensation of any uncondensed gas left near outlet at the distillate collector.
• The use of the condenser hear is used to turn the gas back to a liquid in order to collect it.
Application: -
• To separate oxygen and nitrogen in liquid air.
• Separation of crude oil into patrol, paraffin etc.
• Manufacture of spirit such as gin, whiskey and brandy.
6. Separating funnel
Separating to immiscible liquids.
Immiscible: Do not mix completely but form two layers.
E.g.: - Separating oil and water.
Aim: - To separate oil from water from a mixture of oil and water.
Method: - • The mixture is transferred to the separating funnel with the tap closed and stopper on and the mixture is shaken. This makes the mixture milky due to the formation of an emulsion.
• Then the two liquids are allowed to separate out by allowing the separating funnel to stand.
• The less denser liquid (in this case oil) would be on the top leaving the more denser liquid (water) at the bottom.
• The stopper is removed and tap slowly released to allow the water to carefully leave the separating funnel and close the tap just as the water leaves.
• This would leave the oil in the separating funnel.
• A mixture if paraffin and water.
• A mixture if petrol and water.
7. Sublimation
Separating a mixture of two solids where one solid sublimes when heated.
E.g.: - Separating iodine solid and salt.
Method: - • The apparatus is setup as shown above.
• The mixture is heated, iodine turns straight to gas and rises up where it touches the cold flask and is sublimed back to solid state.
• While the salt is left in the beaker.
8. Using a magnet
Separating a mixture of two solids where one solid is magnetic (can be attracted by a magnet).
E.g.: - Separating iron fillings and sulphur powder.
Method: - • A strong magnet is brought near the mixture.
• All the iron filling will be attracted to the magnet while the sulphur remains.
9. Chromatography
Separating mixture of substances which are usually colored.
Simplest method of chromatography is called paper chromatography as is mainly used to identify substances and to find the purity of a substance.
Method: - • A pencil line is drawn near one of the chromatography paper strip.
• A concentrated drop of ink is placed on the pencil line, allowed to dry and another drop of ink is placed on the same spot to make a concentrated drop.
• The paper is carefully placed in the beaker if solvent taking care not to submerge the drop in the solvent.
• Solvent moves up the paper taking in different components along with it in different rates.
• When solvent reaches almost the top end of the chromatography strip, remove the strip and carefully draw a line in order to identify the ‘solvent front’.
• The paper would shown the different components or colours of substances separated out at different position.
RF value (retention factor)
• In chromatography a value known as RF values are often used for identification purpose.
• The RF value is a measure of the solubility of the substance in the solvent used and it varies from substance to substance.
• It is calculated using the formula:
RF= Distance traveled by a substance/Distance moved by solvent.
Additional Information:
• Starting line should be drawn using pencil.
• At the end of the experiment the solvent front should be near the top end of the paper to ensure complete separation of the compounds.
• The initial spot of the solution (dye) should be small and concentrated, as large drops produce poor separation where spots spread sideway are overlapped.
• In cases where invisible compounds are used to be separated a locating agent is used. The locating agent is sprayed on the finished chromatography paper strip and allowed to dry. The locating agent is known to bind onto the invisible component to produce a coloured compound.
Application:-
Chromatography is used in forensic science and in hospitals usually for identification purpose (usually to find out whether sugar is present in urine).
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