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Chapter 3

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Chapter 3

  1. 1. Chapter 3 - Some substances are pure while others are mixtures. - Difference : Pure substance Mixture Made up of only one substance, not mixed A Substance that contains two or more with any other substance. substances that are not chemically Eg. While diamond – made only of carbon combined. Pure. Not pure. Eg. While diamond – made only of carbon Eg. Air – mixture of gasses : carbon dioxide, oxygen, nitrogen and water vapor - Why important to determine purity?  Medicine if have impurity must be detected or will have undesirable effects after consummation.  Food and beverage industry need to ensure chemical like preservatives and dyes added into food and beverages to make them last longer , taste better or look more attractive is safe for consumption. - How to determine purity?  Modern lab use elaborates equipment but in school lab, can test by checking boiling & melting point, using chromatography. - Pure solids have fixed melting points  a pure solid has an exact and constant or fixed melting point  a pure solid will melt completely at one temperature - Example : 1. Benzoic acid – found naturally in plums, cinnamon and apple – can kill bacteria – Artificially produced as preservative for cosmetics and food like toothpaste and heat- sterilized food (require extremely pure benzoic acid) – Melting point 122°C - Effect of impurities on melting point?  Lower melting point. The greater the amount of impurities, the lower the melting point of the substance.  Cause melting to take place over a range of temperatures. - Pure liquids have fixed boiling points  Pure liquid has an exact and constant or fixed boiling point.  Example pg. 34 - Effect of impurities on boiling point?  If liquid is impure, boiling point will increase?  Liquid will boil over a range of temperatures.  The greater amount of impurities, the higher the boiling point of the liquid. - Effect of pressure on boiling?  If pressure acting on liquid is increased, the boiling point of liquid is raised.  If pressure is decreased, the boiling point is lowered.
  2. 2. - Chromatography  Food coloring used to colour things that we eat and drink.  Food coloring is mixture of colour compounds called dyes.  Government ensures that only dyes that are safe for consumption are used in food coloring.  Technique of using a solvent to separate a mixture into its components – paper chromatography used to separate dyes in ink, pigments in plants, amino acids obtained from proteins, identify poisons or drugs, detect traces of banned substances in food another way used to separate dyes in ink – allow solvent to ascend or travel up the chromatography paper  Mixture through solvent components separated.  Solute dissolves in solvent to become solution.  How does it work? A spot of food coloring applied to chromatography paper. Paper dipped in ethanol (the solvent) and the paper soaks up the solvent. Solvent soaked up by paper dissolves the dyes and continue to travel up the paper carrying the dyes along. A dye not very soluble in solvent will not be carried far along paper but very soluble dyes can be carried far along the paper. Colored spots left in different places on paper at end of experiment.  How to interpret result of chromatography? The chromatography paper with separated components is called chromatogram. Chromatogram shows the dyes and components.  What is Rf values? Rf – the “f” is smaller than R and at bottom right subscript. Solvent front is where solvent run until on paper. It depends on time experiment allowed to run. Length of solvent front from start depends on solvent and temperature. Rf = Distance travelled by dye Distance of solvent front Ration between distance travelled by substance and distance travelled by solvent Is a constant – Rf value Rf value does not change as long as experiment conditions same. Property allows us to identify substance on chromatogram.  Using chromatography to analyze sample Paperchromatography used to separate dyes. Each dye identified by comparing position in chromatogram with a known dye. Each dye identified by comparing Rf value with Rf value of known dye. Chemist checks if dyes are permitted. (Food coloring)  How banned substance in food coloring identified? Example pg. 38  Identical dyes produce spots at same height and colour on paper when same solvent used.
  3. 3.  How to perform chromatography on colorless substance? Used for colorless substance like amino acids. Need apply locating agent on chromatogram. Separate mixture of amino acids by chromatography using suitable solvent. Before solvent reach top, stop the process and dry the paper. Spray locating agent on paper. The locating agent reacts with amino acid to form colored spots on paper. By checking Rf value of each colored spot, can identify different amino acids.  Uses : separate components of sample identify no. of components in sample identify components present in sample determine whether sample is pure- Separation techniques  Remove or reuse components of mixture after separating them  Separate and purify mixtures – methods of purification / separation techniques  Most materials occur naturally are mixtures and not pure.  In order to produce pure water from sea water, need remove impurities like salt.  Several techniques – consider property of substance in mixture Solid from a liquid solids liquid from solution liquids 1. Solid from a liquid  Decanting  simplest way – separate pebble from water by pouring water away  use to separate dense , insoluble solid from liquid  carried out in everyday life during activities like cooking, water poured off cooked foods  Filtration  Separate sand from water  use to separate small solid particles from liquid Eg. sand, clay, dust particles, precipitates (small solid particles produced in a liquid by chemical reactions)  How can we remove precipitate from a reaction mixture? = Decanting not effective as mixture is a suspension where solid particles are found throughout liquid.  Stir mixture, fold filter paper into filter funnel and moisten it with some distilled water, pour mixture into filter funnel lined with filter paper, collect filtrate in conical flask and residue left on filter paper is washed with distilled water and left to dry.  Filter paper acts as sieve. A liquid can pass thru the pores (small holes) but solid cannot.
  4. 4.  After filtration, solid remain on filter paper called residue and liquid or solution that pass thru called filtrate. Evaporation to dryness and crystallization  Solid soluble in water and form solution like salt.  To separate solution, evaporate water to dryness. Eg. Salt solution evaporated to dryness and recover solid salt. Method used to recover salt from seawater.  Sometimes substance decompose when heated strongly (Eg. sugar)  Most crystals give off water to become powders when heated.  After evaporation, any soluble impurities will be left on crystals.  Best method of obtaining pure solid sample -> crystallization  How to purify by crystallization? = Water removed by heating solution and heating stopped at stage where hot saturated solution formed. Resulting solution allowed to cool to room temperature where dissolved solid form pure crystals.  How test for saturated solution? = Clean glass rod dipped into solution and removed. If small amount of solution on the rod form small crystals as solution cool, it is saturated. We say solution at saturation point or crystallization point. Separating solids  separate mixture of two or more solids , make use of their difference in properties  How do we use filtration to separate two solids? = mixture of 2 solid separated if one soluble in solvent, other not.  Pour distilled water into mixture. Stir and warm mixture.  pour warm mixture into filter funnel lined with filter paper and collect filtrate in conical flask.  Wash residue with little distilled water to remove soluble residue and pour filtrate into evaporating dish and evaporate to dryness to get soluble residue.  How use magnet separate solids? = Some metals are magnetic and can separate metals from mixture. Eg. Magnetic – iron,nickel, cobalt, steel  How use sublimation to separate solids? = Separate substance that sublime from one with a high melting point. Separating liquid from a solution  When solid dissolve in solvent, solution formed.  To collect solute from solution, we evaporate solvent.  Pure solvent separated from solution by simple distillation.  Distillation is process of boiling a liquid and condensing the vapor.  Example pg. 48  In distillation flask, solution boils. Water vaporizes rises and enters condenser.  In condenser, water vapor cooled and vapor condenses and changes back to pure water. (distillate)
  5. 5.  Pure water collected as distillate in receiver. (beaker)  Salt solution remains in distillation flask and become more concentrated as distillation proceeds. If distillation continues, solid residue of salt will be left in flask.  Distillation (steps to remember!!) 1. Thermometer should be placed beside the side arm of distillation flask and not dipped in solution so that it measures the boiling point of substance being distilled. 2. Condenser has inner tube and outer water jacket. Cold running water enter water jacket from bottom of condenser and leave from top. 3. Condenser slope downwards so that pure solvent formed can run into receiver. (beaker) 4. If distillate is volatile, receiver can put in large container filled with ice to keep temperature of distillate low and so that it remain in liquid state.  How temperature recorded varies as salt solution distilled? = As salt solution heated, temperature increase and when it finally boils, the thermometer is 100°C (temperature of vapor). Temperature remains unchanged till water boiled off.  Uses of simple distillation = recover solvent from non-volatile solute (have high boiling point) = separate mixture of liquids with different boiling points  Difficult to separate mixture of liquids with boiling points of difference less than 20°C Separating immiscible liquids  Liquids that do not dissolve in each other – immiscible Eg. Oil and water  Separate by using separating funnel 1. Pour mixture of oil and water into separating funnel (make sure tap closed) 2. Support separating funnel with retort stand and place clean beaker below 3. Allow liquid to separate completely and denser liquid will be bottom layer 4. Open tap of funnel to allow bottom layer to drain into beaker and close tap before top layer run out 5. Place another beaker below funnel and open tap to allow a little top layer liquid out. Dispose it and then collect again. Separating miscible liquids  Mix together completely form a solution.  A column called fractionating column attached to round-bottomed flask and condenser. Many glass beads in column to provide large area for vapor to condense on. May also have spiral or plates in column.  During fractional distillation, liquid with lowest boiling point distil over to condenser first and vapor of liquid with high boiling points condense along column and re-enter flask.
  6. 6.  Example pg. 51 How does temperature change as solution undergo fractional distillation? = Temperature of mixture increase as heated and when one liquid distilled over temperature remain constant until all of it out of flask. The temperature increase again and unchanged while second liquid being distilled. Usage : 1. Industry – obtain nitrogen, argon and oxygen from air 2. Separate mixture of liquid like crude oil 3. Ethanol formed when glucose solution undergo fermentation in presence of yeast. Separate ethanol from glucose solution.