4.6 - Chemical Analysis

This method is used to separate a liquid from a solution

E.g. separating water from salt water: The salt water is boiled. The water vapour condenses back into a liquid when passed through the condenser. The salt is left behind in the flask.

This method is used to separate a mixture of different liquids that have different boiling points

E.g. separating alcohol from a mixture of alcohol and water: Water boils at 100C and alcohol boils at 78C -> only the alcohol remains as vapour all the way up to the top of the column and passes into the condenser. The alcohol vapours then condense back into a liquid.

This method is used to separate an insoluble solid from a liquid

E.g. separating sand from a mixture of sand and water: The mixture is poured into the filter paper. The sand does not pass through and is left behind (residue) but the water passes through the filter paper and is collected in the conical flask (filtrate).

the solid left behind

the solution collected in conical flask

This method is used to obtain a salt which contains water of crystallisation from a salt solution

For example: hydrated copper sulfate crystals (CuSO4.5H2O(s)) from copper sulfate solution (CuSO4(aq)). • Gently heat the solution in an evaporating basin to evaporate some of the water until crystals form on a glass rod (which shows that a hot saturated solution has formed). • Leave to cool and crystallise. • Filter to remove the crystals. • Dry by leaving in a warm place. • If instead the solution is heated until all the water evaporates, you would produce a powder of anhydrous copper sulfate (CuSO4(s)).

This method can be used to separate the parts of a mixture into their components. E.g. the different dyes in ink can all be separated and identified

The coloured mixture to be separated (e.g. a food dye) is dissolved in a solvent like water or ethanol and carefully spotted onto the chromatography paper on the baseline, which is drawn in pencil so it doesn’t ‘run or smudge’. • The paper is carefully dipped into the solvent • The solvent is absorbed into the paper and rises up it as it soaks into the paper. • As the solvent rises up the paper it will carry the dyes with it. • Each different dye will move up the paper at different rates depending on how strongly they stick to the paper and how soluble they are in the solvent.

it is an organic solvent (e.g. ethanol)

• A baseline is drawn on the paper • The mixture is spotted onto the baseline alongside known or standard reference materials. • The end of the paper is then put into a solvent which runs up the paper and through the spots, taking some or all of the dyes with it. • Different dyes will travel different heights up the paper.

The distance of dye ____________________________________ distance solvent travelled from the baseline

• A pencil line (baseline) is drawn 1cm from the bottom of the paper. Pencil will not dissolve in the solvent, but if ink were used instead it might dissolve and interfere with the results of the chromatography. • A spot of each sample of dye is dropped at different points along the baseline. • Paper is put in a beaker which contains a small amount of solvent. • The bottom of the paper should be touching the solvent, but the baseline with the dyes should be above the level of the solvent. This is important so the dyes don’t simply dissolve into the solvent in the beaker. • When the solvent has travelled to near the top of the paper, the paper is removed from the solvent and a pencil line drawn (and labelled) to show the level the solvent reached up the paper. This is called the solvent front. • The chromatogram is then left to dry so that all the solvent evaporates.

squeaky pop test with lit splint when H2 present

glowing splint relights when O2 present

bubbling the gas through limewater turns cloudy

damp red litmus paper turns blue in presence of NH3

blue damp litmus paper turns white –> bleaches

2H2(g) + O2(g) -> 2H2O(g)

measure boiling point and compare to 100C

Ca(OH)2(aq) + CO2(g) -> CaCO3 + H2O