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Experimental Techniques, Criteria of Purity, Measurement, Methods of…
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Criteria of Purity
Paper Chromatography
The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so the samples don´t wash into the solvent container
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The solvent travels up the paper by capillary action, taking some of the coloured substances with it
Different substances have different solubility so will travel at different rates and the substances with higher solubility will travel further than the others
Locating agents are substances which react with colorless samples (like proteins) and produce a coloured product which is then visible
the chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye
Assessing Purity
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mixtures have a range of melting and boiling points as they consist of different substances that melt or boil at different temperatures
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unknown pure substances can be identified by experimentally determining its m.p and b.p and comparing to data tables
Retention Factor
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it is a ratio, therefore it has no units
Importance of Purity
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having pure substances for food and drugs is very important as impurities could be dangerous even in small amounts
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Interpret Chromatograms
If two or more substances are the same, they will produce identical chromatograms
If the substance is a mixture, it will separate on the paper to show all the different components as separate spots
An impure substance will show up with more than one spot, a pure substance should only show up with one spot
Measurement
Mass
measured using a digital balance, gives readings to two decimal places, must be tared (set to zero) before use
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Time
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measured using a stopwatch or stop-clock, usually accurate to one or two decimal places
Temperature
measured using a thermometer - readings to the nearest degree. digital thermometers are more accurate
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Volume
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gas
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if the gas isn’t water-soluble, a graduated cylinder inverted in water may also be used
if the gas is heavier than air and is colored, the cylinder can be used upright
Methods of Purification
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Distillation
Simple Distillation
Used to separate a liquid and soluble solid from a solution or a pure liquid from a mixture of liquids
The solution is heated and pure water evaporates producing a vapour which rises through the neck of the round-bottomed flask
The vapour passes through the condenser, where it cools and condenses, turning into pure liquid H2O which is collected in a beaker
After all the water is evaporated from the solution, only the solid solute or pure liquid will be left behind
Fractional Distillation
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This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker
All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture
Mixtures
Solids
Differences in density, magnetic properties, sublimation and solubility can be used
For a difference in solubility, a suitable solvent must be chosen to ensure the desired substance only dissolves in it and not other substances or impurities
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Crystallisation
Used to separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in cold
The solution is heated, allowing the solvent to evaporate to leave a saturated solution behind
Test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution. If the solution is saturated, crystals will form on the glass rod
The saturated solution is allowed to cool slowly and solids will come out of the solution as the solubility decreases, and crystals will grow
Crystals are collected by filtering the solution and are washed with cold, distilled water to remove impurities and allowed to dry