Distillation involves 2 stages and both are physical state changes.
(1) The liquid or solution mixture is boiled to vaporize the most volatile component in the mixture (liquid ==> gas). The ant-bumping granules give a smoother boiling action.
(2) The vapour is cooled by cold water in the condenser to condense (gas ==> liquid) it back to a liquid (the distillate) which is collected.
This can be used to purify water because the dissolved solids have a much higher boiling point and will not evaporate with the steam, BUT it is too simple a method to separate a mixture of liquids especially if the boiling points are relatively close.
Fractional distillation involves 2 main stages and both are physical state changes. It can only work with liquids with different boiling points. However, this method only works if all the liquids in the mixture are miscible (e.g. alcohol/water, crude oil etc.) and do NOT separate out into layers like oil/water.
(1) The liquid or solution mixture is boiled to vaporise the most volatile component in the mixture (liquid ==> gas). The ant-bumping granules give a smoother boiling action.
(2) The vapour passes up through a fractionating column, where the separation takes place (theory at the end). This column is not used in the simple distillation described above.
(3) The vapour is cooled by cold water in the condenser to condense (gas ==> liquid) it back to a liquid (the distillate) which is collected.
This can be used to separate alcohol from a fermented sugar solution.
It is used on a large scale to separate the components of crude oil, because the different hydrocarbons have different boiling and condensation points (see oil).
FRACTIONAL DISTILLATION THEORY:
Imagine green liquid is a mixture of a blue liquid (boiling point 80oC) and a yellow liquid (boiling point 100oC), so we have a coloured diagram simulation of a colourless alcohol and water mixture! As the vapour from the boiling mixture enters the fractionating column it begins to cool and condense. The highest boiling or least volatile liquid tends to condense more i.e. the yellow liquid (water). The lower boiling more volatile blue liquid gets further up the column. Gradually up the column the blue and yellow separate from each other so that yellow condenses back into the flask and pure blue distils over to be collected. The 1st liquid, the lowest boiling point, is called the 1st fraction and each liquid distils over when the top of the column reaches its particular boiling point to give the 2nd, 3rd fraction etc.
To increase the separation efficiency of the tall fractionating column, it is usually packed with glass beads, short glass tubes or glass rings etc. which greatly increase the surface area for evaporation and condensation.
In the distillation of crude oil the different fractions are condensed out at different points in a huge fractionating column. At the top are the very low boiling fuel gases like butane and at the bottom are the high boiling big molecules of waxes and tar.
Paper or Thin Layer Chromatography
This method of separation is used to see what coloured materials make up e.g. a food dye analysis.
The material to be separated e.g. a food dye (6) is dissolved in a solvent and carefully spotted onto chromatography paper or a thin layer of a white mineral material on a glass sheet. Alongside it are spotted known colours on a 'start line' (1-5).
The paper is carefully dipped into a solvent, which is absorbed into the paper and rises up it. The solvent may be water or an organic liquid like an alcohol (e.g. ethanol) or a hydrocarbon, so-called non-aqueous solvents. For accurate work the distance moved by the solent is marked on carefully with a pencil and the distances moved by each 'centre' of the coloured spots is also measured. These can be compared with known substances BUT if so, the identical paper and solvent must be used (See Rf values below).
Due to different solubilities and different molecular 'adhesion' some colours move more than others up the paper, so effecting the separation of the different coloured molecules.
Any colour which horizontally matches another is likely to be the same molecule i.e. red (1 and 6), brown (3 and 6) and blue (4 and 6) match, showing these three are all in the food dye (6).
The distance a substance moves, compared to the distance the solvent front moves (top of grey area on 2nd diagram) is called the reference or Rf value and has a value of 0.0 (not moved - no good), to 1.0 (too soluble - no good either), but Rf ratio values between 0.1 and 0.9 can be useful for analysis and identification.
Rf = distance moved by dissolved substance (solute) / distance moved by solvent.
Some technical terms: The substances (solutes) to be analysed must dissolve in the solvent, which is called the mobile phase because it moves. The paper or thin layer of material on which the separation takes place is called the stationary or immobile phase because it doesn't move.
It is possible to analyse colourless mixture if the components can be made coloured e.g. protein can be broken down into amino acids and coloured purple by a chemical reagent called Ninhydrin and many colourless organic molecules fluoresce when ultra-violet light is shone on them. These are called locating agents.
Thin layer chromatograpy (t.l.c) is where a layer of paste is thinly and evenly spread on e.g. a glass plate. The paste consists of the solid immobile phase like aluminium oxide dispersesd in a liquid such as water. The plate is allowed to dry and then used in the same way as paper chromatography.
Gas-liquid chromatography is described below
FILTRATION, EVAPORATION AND CRYSTALLIZATION
Filtration use a filter paper or fine porous ceramic to separate a solid from a liquid. It works because the tiny dissolved particles are too small to be filtered BUT any insoluble 'non-dissolved' solid particles are too big to go through!
Evaporation means a liquid changing to a gas or vapour. In separation, its removing the liquid from a solution, usually to leave a solid. It can be done quickly with gentle heating or left out to 'dry up' slowly. The solid will almost certainly be less volatile than the solvent and will remain as a crystalline residue.
Crystallisation can mean a liquid substance changing to its solid form. However, the term usually means what happens when the liquid from a solution has evaporated to a point beyond the solubility limit. Then solid crystals will 'grow' out of the solution because the solution is too concentrated for all the solid to remain dissolved at that temperature. Crystallisation is often done from a hot concentrated solution, because most substance are more soluble the hotter the liquid. Consequently on cooling a hot concentrated solution, crystals form as the solubility gets less and less.
These separation methods are involved in e.g. (1) separation of sand and salt mixture or (2) salt preparations(a) from dissolving an insoluble base in an acid to form a soluble salt or (b) preparing an insoluble salt.
(1) The sand/salt mixture is stirred with water to dissolve the salt. The sand is filtered off and washed with pure water to remove remaining traces of salt solution. The salt solution (filtrate) is carefully heated in a dish to evaporate the water and eventually the salt crystals form. Here the solvent is water, but other mixtures can be separated using the same sequence of procedures using a different solvent. e.g. copper and sulphur can be separated using an organic solvent like tetrachloromethane which will dissolve the sulphur (hazardous chemical solvent).
(2a) When the water insoluble base (e.g. a metal oxide) is dissolved in an acid, the excess solid base is filtered off and the filtrate solution heated to evaporate the water to produce the salt crystals.
(2b) Two solutions of soluble substances are mixed and react to form an insoluble salt. The insoluble salt is filtered off to separate it from the solution, washed with pure water to remove any residual salt solution. The solid is then removed from the filter paper and dried to give the pure dry insoluble salt.
Some important words-phrases to do with the above procedures.
A solvent is a liquid that dissolves things.
The solute is the solid that dissolves in a solvent.
A solution is a mixture of a liquid with something dissolved in it.
The technique of solvent extraction involves using a liquid to dissolve a solid to separate it from a mixture (e.g. in purifying salt in the experiment described above.
A saturated solution is one in which no more substance will dissolve in the liquid.
Soluble means the substance (gas, liquid or solid) dissolves in a liquid to form a solution.
Insoluble a substance won't dissolve in a particular liquid. Remember, a solid may dissolve in one liquid but not in another.
Distillation, described is used to separate miscible liquids that dissolve in each other. If two liquids do NOT mix, they form two separate layers and are known as immiscible liquids (e.g. oil/water). This is illustrated in the diagram on the left, where the lower grey liquid will be more dense than the upper layer of the yellow liquid and shows how you can separate these two liquids using a separating funnel. (Particle picture on gas-liquid-solid page)
1. The mixture is put in the separating funnel with the stopper on and the tap closed and the layers left to settle out. 2. The stopper is removed, and the tap is opened so that you can carefully run the lower grey layer off first into a beaker. 3. This leaves behind the upper yellow layer liquid, so separating the two immiscible liquids.
MAGNET
This can be used to separate iron from a mixture with sulphur (see below). It is used in recycling to recover iron and steel from domestic waster i.e. the 'rubbish' is on a conveyer belt that passes a powerful magnet which pluck's out magnetic materials.
GASES
Methods of collecting gasesare on a separate web page. Includes the preparation of ammonia, carbon dioxide, sulphur dioxide, hydrogen and a cracking experiment.
Use of U tube to collect things in e.g. condensing out water in a combustion investigation
Their use is included inSalt Preparationsand titrations with the Acids, Bases, Salts, pH page. A burette is used to measure volumes accurately and a pipette is used to separate out an accurate volume of a solution from a bulk container of the solution.
Decantingis the simplest possible way of separating a liquid (pure or a solution) from an insoluble solid which has a density greater than water (i.e. > 1.0 g/cm3). The solid-liquid mixture is allowed to stand e.g. in a beaker, until all the solid settles out to the bottom of the container. Then the liquid is carefully poured off to leave the insoluble solid behind. However it is inefficient e.g. a small amount of liquid is always left in the solid residue and very fine solid particles take some time to settle out and any disturbance of the liquid can mix them in with the liquid being poured off. Wine may be served in a decanter to leave the undesirable solids behind - no good for bits of cork though, they float!
In its simplest form these techniques involveusing a liquid to dissolve a solid to separate it from a mixture. Theextraction of pure salt from a sand-salt mixtureis a simple example of the technique.
For more complex examples see the advanced level chemistry page.ASA2 solute distribution between two immiscible liquids, partition coefficient , calculations and uses
Centrifuges and centrifuging
Centrifuges are devices or apparatus that can be usedto separate insoluble materials(usually a solid)from a liquid, where normal filtration does not work welle.g. a suspension of very fine (tiny) solid particles.The centrifugeconsists of carriage or glass tube holder, mounted on an electrically motor driven vertical axle. The carriage holds the balanced glass tubes ofequal amountsof the solid-liquid mixture in each tube, all tubes initially in a horizontal position before the motor is switched on. The tube carriage is rotated at high speed safely in afully enclosed container. Unbalanced tubes can break with the extra vibration and this situation has a 'knock on' effect, quite literally, as other tubes are likely to shatter with the erratic high speed unbalanced motion. High velocity glass fragments are not good for you! Onrapid rotationof the carriage the tubes whirl round horizontally and the centrifugal force causes themore denseinsoluble material particles to move outwards, separating from the liquid. When rotation ceases the solid particles end up at the 'bottom' of the glass tubes with the liquid above. After thecentrifuging operationthe liquid can be decanted off and the solid is left at the bottom of the glass tube. You might be interested in the solid, liquid or both products depending on the context. Centrifuges come in all sizes and centrifuge technology has many applications in the separation of mixtures and the purification of materials.
If [ ] represents the glass tubes, the horizontal rotation situation is shown below ..
Uses-applications: In biology cells can be separated from fluids. A waste 'sludge' can be treated e.g. removing toxic solids from contaminated water from an industrial process. Milk can be separated from whey. Edible oils, wines and spirits can be cleaned or 'clarified' of solid impurities. Expensive oils and other fluids used as lubricants in machining metal parts in industry become contaminated with tiny metal fragments. The larger pieces of metal are easily removed by filtration or sedimentation (allowing to settle out) but the very fine metal particles can only be removed by using a centrifuge. This is likely to be a cheaper option than buying more machine fluid AND reducing pollution since the fluid is recycled leaving less waste to dispose of.
Why Instrumental methods of detection and separation are are useful?
Instead of testing for chemicals using standard laboratory equipment such as test tubes etc. Special instruments have been developed to carry out such testing. These are quick, accurate and can be used on very small samples.
Elements and compounds can also be detected and identified using a variety of instrumental methods. Some instrumental methods are suited to identify elements while other instrumental methods are suited to the identification of compounds.
Instrumental methods are accurate, sensitive and rapid and are particularly useful when the amount of a sample is very small.
Mass spectroscopycan be used toidentify elementsand their relative ratio of isotopes and for molecules it can help to determine a molecular structure (it’s expensive, and nmr is much better for molecular structure analysis - especially organic molecules, see below).
oThe atoms or molecules are vaporized and converted to positive ions (based on a single atom or molecular fragment) by bombardment with high energy electrons. The gaseous ions (e.g. Na+or CH3+etc.) are analyzed according to their mass in a powerful magnetic field.
Atomic emission spectroscopycan be used toidentify elements and analyze element mixtures.
oBasically atomic spectroscopy is about 'exciting atoms' with heat or electrical energy until theyemitthe absorbed energy as visible light. You see this effect when fireworks go off, most of the color comes from the 'excited' metal atoms in the salts added to the explosive powder mixture.
oIn a simple way flame color tests in the school laboratory are used to identify elements e.g. sodium is yellow, barium green etc. BUT these colors are formed from many specific frequencies of visible light added together, so how do you sort out e.g. two shades of greens from copper or barium?
oThe answer is that detailed analysis of thedifferent emitted frequenciesof visible light (e.g. using aprism) gives a 'finger print pattern' by which to identify elements.
oAND thegreater the relative intensity of lightfrequencythe more there isof that element.
oSo atomic spectroscopy is used to identify elements and analyze a mixture of elements or detect traces of elements in a solid or solution.
oThis analytical method has many applicationse.g.
§It’s used in the steel industry to monitor the composition of steel as the molten mixtures are being made
§Astrophysicists can identify elements in distant stars from the light emitted.
§Tiny traces of metal ions can be detected in water e.g. for pollution monitoring.
Nuclear magnetic resonance spectroscopy(nmr) is one of the most powerful analytical tools fordetermining the molecular structure of an organic compound.
oIt’s very expensive for routine analysis but is invaluable in designing and analyzing new molecules or finding the structure of natural molecules that the drug industry might find useful in developing new pharmaceutical products.
Infra-red spectroscopycan help todetermine molecular structure and identify an organic compound.
oEach molecule has a 'fingerprint' pattern of absorption of different infrared frequencies. Can be used to determine alcohol concentrations in breath!
Ultra-violet spectroscopycan be used to thedetermine purity or concentration of solutionof a substance that absorbs uv light.
Gas-liquid chromatography(gc/glc) can be used toanalyze liquid mixtureswhich can be vaporized (e.g. petrol, blood for alcohol content). The instrument is called agas chromatograph.
Asample of the substance under investigation isinjected and vaporized into a tube containing a carrier gas(called themobile phase, it moves). The gas carries the vaporized substance through a long 'separating' tube orcolumn wound around inside a thermostated oven.
·Thesubstances in the mixture are partially absorbed by an absorbent materialheld in orthe column(called the immobile phase, doesn't move), but only temporarily. However different substances are held back, or 'retained', for different times so that the mixture separates out in the carrier gas stream.
oThe column is filled with a porous solid so gas can get through but passes over a large surface area OR it is coated in a very high boiling organic liquid which can also provide a large absorbing surface but still allows gas flow.
·The gases emerge from the oven into adetector systemwhich electronically records the different signal as each substance comes through. A printout or computer display of the results from the gas chromatograph, called thegas chromatogram, shows aseries of peaksin the graph line imposed on a steady baseline when only the carrier gas is passing through the detector.
·The time it takes for a substance to come through is called theretention timeand isunique for each substancefor a particular set of conditions (flow rate, length of separating column, nature of separating column material, temperature etc.).Generally speaking, the greater the molecular mass of the mixture molecule, the longer the retention time. This is because the component molecule - immobile phase intermolecular force of attraction increases with the size of the component molecule, so it is absorbed/retained temporarily a bit more strongly (see right of diagram).
·Theheight of the peak, or more strictly speaking, the area under the peak, isproportional to the amount of that particular substance in the mixture.
·Therefore it is possible to identify components in a mixture and calculate their relative proportions in the mixture.
·The chromatogram shown above (right of diagram) illustrates the separation of some alkane hydrocarbons in petrol (in reality it is far more complicated with dozens of hydrocarbon molecule peaks on the chromatogram). The different peak heights give the relative proportions i.e. hexane >pentane > heptane. The retention time order follows the trend of increasing molecular mass gives increasing retention time i.e. in time heptane C7H16> C6H14> C5H12
·The gas chromatographic instrument can becalibratedwith known amounts of known substances.
Industry requires rapid and accurate methods for the analysis of its products. There have also been increasing demands from society for safe and reliable monitoring of our health and environment. The development of modem instrumental methods has been aided by the rapid progress in technologies such as electronics and computing.
Various factors have influenced the development of instrumental methods. With modern methods you get...
ogreater sensitivityi.e. smaller amounts of material can be used OR much smaller amounts of a trace element or compound can be detected in a bulk mixture (drug testing of athletes)
omore accurate data(perhaps analyzed by computer)
oautomation of analysis, multi-samples efficiently analyzed
oagreater range of analytical techniques, today's laboratory is far more versatile these days
oGreater reliability and consistencyonce the instrument is set up and procedures in place and checked.