Purity & Separation Techniques Year 8 Science PDF

Purity & Separation Techniques Year 8 Science  Content Common Separation methods 1. Decanting 2. Filtration 3. Chromatography 4. Centrifugation 5. Distillation 6. Crystallization Is This Pure?  Is This Pure? In Everyday life, the idea of purity is used loosely and i...

Purity & Separation Techniques Year 8 Science  Content Common Separation methods 1. Decanting 2. Filtration 3. Chromatography 4. Centrifugation 5. Distillation 6. Crystallization Is This Pure?  Is This Pure? In Everyday life, the idea of purity is used loosely and in an inexact way. In Chemistry… Pure: There is only one substance present. Would you reconsider what you consider pure now? Why is Purity Important? Take 5 minutes to discuss with your group and come up with 3 different scenarios when purity may be of importance.  When Purity Matters Production of Ice Cream Medical Drugs Food Additives Computer Chips (Silicon) Analytic Investigations Etc…  The Need for Pure Substance Unwanted substances are called impurities. Impurities may greatly affect the properties of a substance  Why is Purity Important? Salt reduces the freezing temperature of water. This means it takes a much lower temperature than 0°C for it to freeze. Can you explain why people add salt on roads and paths during icy weather?  Suggest how to separate the following mixtures Pebbles and sand Gold and iron Salt and sand Mud and water (a mixture of solid and liquid) Oil and water (a mixture of immiscible liquids)  Decanting The process of simply pouring off the solution. Suitable for solids that have very heavy particles Q: Can you think of an example?  Filtration Insoluble solids can be separated from a solution by passing through a piece of filter paper in a filter funnel.  Can you name the different components of filtration? 描 :h 述 ttp://t1.gstatic.com/images?q=tbn:ANd9GcSdrIBSohl2qTR1Stsv24o3NCnB9bflqqWklmtCkHTPljT8JLDPdQ Beaker Mixture (2) (1) Conical flask (3)  Filtration https://www.youtube.com/watch?v=tLmh_rMQu7M  Paper chromatography  Paper Chromatography A technique engaged extensively in researches, experiments, and investigations due to its sensitivity and ability to pick up minute traces of impurities and drugs. Detect as little as 0.000 0001 g of a substance.  Paper Chromatography A technique for the separation of a mixture into its components. or A technique which is used to separate low-molecular-mass compounds based on their distribution between stationary phase and mobile phase. ?? ?? solvent  Paper Chromatography Steps of separating dyes in a sample of black ink Step 1: Use a pencil to draw a baseline across the bottom (about 1~2 cm from the edge) of a filter paper. (Why not pen?) Step 2: Place a drop of testing material (e.g. black ink) on the baseline. Step 3: Dry the spot. Paper Chromatography filter paper strip black ink baseline spot 1 cm  Paper Chromatography Step 4: Suspend paper in an alcohol- water mixture (a suitable solvent), with the ink spot above the liquid level. (Why not at the same level?) Note: Solvent rises up the filter paper, various dyes in the black ink also move upwards at different speeds Different colour patches appear at different distances from baseline. Paper Chromatography Step 5: Remove and dry the paper when the solvent is near the top of the paper.  Paper Chromatography plastic wrap glass rod clip filter paper strip solvent and dyes beaker move up the paper spot of black ink solvent (alcohol- water mixture)  Paper Chromatography Separation depends on two factors: the ability of a solute to dissolve in a solvent at a certain temp. 1. Solubility of component substances in the solvent (a.k.a. mobile phase) Higher solubility -> travel more quickly  Paper Chromatography 2. Degree of adsorption onto the paper (stationary phase) greater adsorption -> travel more slowly  Paper Chromatography Identify substances based on distance traveled, given constant condition Find the Rf values (under same conditions) Rf = baseline  Paper Chromatography If a component substance travels 5.2 cm and the distance travelled by the solvent is 7.5 cm. Rf = solvent front a component substance 7.5 cm 5.2 cm origin baseline 1 cm Paper Chromatography Chromatogram solvent front A B Are A and B the same component?  Paper Chromatography solvent front C D Likely that C and D are the same component.  Question O Chromatography is used by the ‘Horse Racing Forensic Laboratory’ to test for the presence of illegal drugs in racehorses. O A concentrated sample of Meh? urine is spotted onto chromatography paper on the start line. Alongside this, known drugs are spotted. The chromatogram is run using methanol as the solvent.  Question 1. Calculate the Rf value for caffeine. 2. State two factors which determine the distance a substance travels up the paper. 3. From the results the sample from one horse contains an illegal substance a. State the horse, and the drug present. b. Give a reason for the use of this drug. Centrifugation  Centrifugation Used to separate small amounts of solid and liquid.  What is Centrifuge A machine which spins test tubes round and round at very high speeds. The spinning pulls the denser solid to the bottom of the tube. Next, decant the liquid from the solid. https://www.youtube.com/watch?v=KEXWd3_fM94 Before and After Centrifuging 1) Before centrifuging, the solid is mixed all through the liquid 2) After centrifuging, all the solid has collected at the bottom supernatant 3) The layer of solution above the pellet (sediment or precipitate) is called supernatant. pellet  Some examples of centrifugation Separating water from clothes by spin-drying in washing machines Removing water from wet lettuce in a salad spinner  Some examples of centrifugation Removing fat from milk to produce skimmed milk Separating solid components of blood and urine centrifugation of blood https://www.youtube.com/watch?v=UWwM71SNpJ8 Distillation  Separating miscible liquids Liquids that do mix together are described as miscible. An example of this is water and alcohol – these two liquids mix together easily. Can you think of any more examples of miscible liquids? + How could you separate a mixture of miscible liquids?  Distillation A technique used to separate liquids from a mixture  Simple Distillation (3) thermometer (1) (2) boiling tube clamp (4) delivery tube (7) sodium chloride (5) solution test tube (as a receiver) heat (8) (6) anti-bumping beaker granule (9) (10) cold water distilled water  Simple Distillation thermometer Prevents bumping boiling If bumping occurs, some solution (Not yet vapourized) clamp tube may spurt out into the receiver delivery tube sodium chloride solution test tube (as a receiver) heat anti-bumping beaker granule cold water distilled water  Simple Distillation To obtain a solvent from a solution To separate a mixture of 2 or more liquids with a great difference in boiling points (ie. NaCl: 1,413°C and H2O: 100°C ) The liquid collected is called the distillate. “Quickfit” Apparatus  thermometer screw-cap adaptor water out (to sink) thermometer bulb rubber tubing pear-shaped flask solution receiver adaptor Liebig anti-bumping condenser granule heat cold water in (from tap) test tube (as a receiver) cold water distillate  Simple Distillation Position of the thermometer bulb allows for: Measuring the boiling point of the solvent Measurement of vapour actually distilled over can be recorded (e.g. the liquid collected will not be counted when the temperature is not high enough) A Liebig condenser is used to cool and condense the vapour effectively. Crystallization Evaporation to Dryness =/= Crystallization How could you separate a soluble solid, like salt from seawater solution? The solution is heated so that the water evaporates and leaves behind soluble solids from its solution But Powder =/= Crystal https://www.youtube.com/watch?v=nztV4w0DtOo&t=207s Crystallization Method to obtain purer substances from solution Product in form of a crystal, not powder (If a solution is evaporated to dryness by heating, what is left is powder, not crystals.) crystallization of copper sulphate https://www.youtube.com/watch?v=QdwKhbtzsug  The Set Up To obtain soluble solid from its solution → (1) Evaporation (2) Crystallization The Set Up (2) Crystallization glass rod beaker glass rod solution wire gauze tripod The formation of small crystals on heat glass rod upon cooling, shows that the solution has become concentrated enough (saturated solution) Crystallization Crystallization thermometer solution cools crystals of solute cold solution (still saturated, but hot concentrated solution with less solute dissolved) The solubility of most solid decreases as temperature drops. Therefore, as hot concentrated solution cools down, the solvent can no longer dissolve all of the solute. The ‘extra’ solute comes out as crystals. stand Wash bottle Cold distilled water clamp crystals as residue filter funnel with filter paper Wash with cold distilled water to beaker remove any soluble impurities on the solution as surface of the filtrate crystals Dry the crystal with filter paper Crystallization allows a purer crystal to be obtained. Impurities are excluded from the growing crystal. Why? - when the hot saturated solution is cooled slowly, only solute particles of same shape and size come together and form crystals. solute impurity Where does the salt around the Dead Sea come from? The Dead Sea is a salty lake, located between Jordan and Israel. The Dead Sea contains some of the saltiest water in the world. The Dead Sea is almost six times more salty than the ocean, so nothing is able to live in it and that’s why it is called ‘dead’. The heat of the Sun evaporates the water from the Dead Sea, and the salt that is left behind is collected in salt beds.  Scenario 1 What will we obtain when all the water evaporates? What’s the problem? (Hint: Remember we want purity)  Scenario 2 What’s the problem? (Hint: Sugar decomposes at 100°C)  Scenario 3 Goal: To obtain hydrated copper sulphate Crystals https://www.youtube.com/watch?v=M6IIbZx7xvQ Finally… Another use of crystallization – sugar sticks for coffee.  Now that we know various Separation Techniques… How do we know that the substances obtained are pure?  To Determine Purity Solid by melting point Liquid by boiling point Sharp boiling/melting point Check against data booklet  Determination of Melting Point Method 1: An oil-bath thermometer to determine the melting clamp point of a low-melting solid. stirrer boiling tube rubber band melting point tube test sample paraffin oil heat  Determination of Melting Point Step 1: Place a little of dry powdered thermometer solid in a thin-wall glass melting point tube (or sealed clamp capillary tube). stirrer boiling tube Step 2: rubber band melting point Attach the tube to a tube thermometer. test sample paraffin oil heat  Determination of Melting Point Step 3: thermometer Heat the paraffin oil gently. clamp Step 4: Record two temperature stirrer readings. boiling tube t1 at which the solid rubber band starts to melt melting point tube t2 at which the solid test sample melts completely paraffin oil heat Melting Point of an Organic Compound - MeitY OLabs https://www.youtube.com/watch?v=xaQUTlruvFU  Determination of Melting Point Method 2 : An electrical melting point apparatus To determine the melting point of a low-melting solid. Sample is inserted into and heated thermometer by an electrically heated block Observation made through magnified port Safer and more convenient to use than oil bath Carrying out a melting point determination https://www.youtube.com/watch?v=sh-96_KfqgY  Determination of Melting Point Pure solid has a sharp melting point. Sharp melting point if (t2 − t1) is smaller than 0.5°C. An impure solid does not have a sharp melting point. It melts gradually over a wide temperature range. Presence of impurities lowers the melting point. e.g. The melting point is lowered by 1.85 degrees Celsius if 29.2 grams of salt are dissolved in each kilogram of water.  Determination of Boiling Point thermometer water out hot receiver water flammable adaptor bath liquid anti- ice/water bumping cold water in mixture (as granule cooling agent) heat distillate (flammable liquid condensed)  Determination of Boiling Point A flammable liquid Use distillation set-up. Use water-bath for heating, not a naked flame. The thermometer bulb should be immersed in the liquid. The temperature at which the liquid boils steadily is its boiling point.  Determination of Boiling Point A non-flammable liquid Use simple distillation apparatus.  Determination of Boiling Point Boiling point of a pure liquid is quite sharp. Presence of non-volatile solutes (e.g. common salt) will raise the boiling point of a liquid. e.g. The boiling point is raised by 0.5 degrees Celsius for water with 29.2 grams of salt dissolved in each kilogram of water.