8 Chemical analysis Flashcards
what is a pure substance
- a single element or compound, not mixed with any other
substance - have specific melting and boiling points
what is a formulation
- a mixture that has been designed as a useful product
- made by mixing components in carefully measured quantaties
- eg. fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods
how can you check an unknown substance is pure
- measure its melting or boiling point and comapre it with the melting/boiling point of the pure substance
- the closer the measure value to the actual melting/boiling point the purer the sample
- impurities in a sample will lower the melting point and increase the melting range
- impurities will increase boiling point
what is chromotography
- an anyltical method used to seperate substances in a mixture
- can be used to identify substances
what are the 2 phases in chromatography
- mobile phase - where molecules can move, always a liquid or gas
- stationary phase - where the molecules can’t move, can be solid or really thick liquid
the time spent in each phase depends on what?
- how soluble they are in the solvent
- how attracted they are to the paper
(molecules with a higher solubility in solvent and less attracted will travel further)
chromotography practical
- Use a ruler to draw a horizontal pencil line 2 cm from the bottom short edge of the chromatography paper- origin line
- add a spot of ink to the line and place the sheet in a beaker of solvent eg. water
- add the piece of paper into the water with only the bottom submerdeged eg. 1cm
- clip the top to a wooden spill resting on the top of the beaker
- put a lid on top to stop the solvent evaporating
- the solvent seeps up the paper carrying the ink with it
- each dye in the ink will move up the paper at different rate forming a spot
- if any ink is insoluable it will stay on the baseline
- when the solvent has nearly reached the top of the paper take it out and dry it
- the end result is a chromotogram
analysis(8): - measure the distance traveled by the solvent
- measure the distance in mm between the centre of the spots of dyes and the base line
- calculate the Rf = distance travelled by substance/distance travelled by solvent
- compare the ratios of the different dyes
why are different solvents sometimes used
- depends on solubility
- some compunds dissolve well in water but sometimes other solvents like ethanol are used
what can chromotography be used for
to find if a substance is present in a mixture:
- run a pure sample of the substance alongside the unknown mixture
- if the Rf values to the refrence and one of the spots match the mixture, the substance may be present
Rf=
distance traveled by substance/distance traveled by solvent
gas tests
hydrogen - squeaky pop with a burning splint over gas
oxygen - glowing splint relights
Carbon dioxide - turns lime water cloudy
Chlorine - bleaches damp litmus paper white
how can you detect Carbonates (anion)
- put a sample of unknown solution and add a couple drops of dilute acid
- connect the test tube to a test tube of limewater, if carbonates present, carbon dioxide will be released that will turn the limewater cloudy when it bubbles
test for sulfates
- use a dropping pippette to add a couple of drops of dilute HCl acid
- then add a couple drops of BaCl barium chloride
- if sulfate ions present a white precipiatate of barium sulfate will form
Ba2+ (aq) + SO42-(aq) —> BaSO4 (s)
test for halides
- add a couple drops of dilute nitric acid (HNO3)
- add a drops of silver nitrate solution AgNO3 to the unknown soluution
- Chloride gives a white precipitate of silver chlorise Ag+ + Cl- —> AgCl
- Bromide gives a cream precipitate of silver bromide Ag+ + Br—> AgBr
- An iodide gives yellow precipitate of silver iodide Ag+ + I —> AgI
flame tests for cations results
- Lithium burn a crimson flame - crimson LIpstick , lithium
- Sodium burn a yellow flame Sodium - Sun - yellow
- Pottasium burn a lilac flame - Potassium Purple - lilac
- Calcium burn an orange-red flame - 2c’s so 2 colours
- Copper burns a green flame
how to do a flame test
- clean a platinum wire loop by dipping it in some dilute HCl and hold it in a blue flame from a bunsen burner until it bruns with no colour - tp get rid of ny impurities
- dip the loop into the sample you want to test and put it back in the flame
- record the colour of the flame
- only works if sample contains a single metal ion
how to do a precipitate test
- add a few drops of sodium hydroxide to a solution of unknown compound
- record the colour and find which metal is present
precipitate test for cations NaOH results
calcium - white = Ca2+ + 2OH- –> Ca(OH)2
copper(II) - blue= Cu2+ + 2OH- —> Cu(OH)2
Iron(II) - green = Fe2+ + 2OH- —> Fe(OH)2
Iron(III) - brown = Fe3+ + 3OH- —> Fe(OH)3
Aluminium - white at first the redisolves excess NaOH to produce a colourless solution = Al3+ + 3OH- —> Cu(OH)3
Magnesium - White = Mg2+ + 2OH- —> Mg(OH)2
describe flame emission spectroscopy
- a sample is placed in a flame
- as ions heat up their electrons become exited, when drop back to their orginal energy levels they transfer energy as light
- the light passes through a spectroscope which detects different wavelengths of light to produce a line spectrum
- different ions hit different wavelengths of light so each ion has a different pattern of wavelengths
- intensity of the spectrum indicates the concentration of that ion in solution
- means line spectrums can be used to identify ions in a solution and calculate their concentrations
- can also be used to identify ions in mixtures - compare spectrums for different ions to the spectrum of a mixture
