chemical analysis Flashcards
define a pure substance in chemistry
a single element or compound not mixed with any other substance
define a pure substance in everyday language
a substance that has had nothing added to it; unadulterated and in its natural state
what do pure elements and compounds do
they melt and boil at specific temperatures - this data can be used to distinguish pure substances from mixtures
easiest method to test if a substance is chemically pure
heat it and measure boiling and melting point
- pure substance always melts and boils at fixed temp
- impure melt and boil over a range of temps
define a formulation
a mixture that has been designed as a useful product - many products are complex mixtures in which each chemical has a particular purpose
how are formulations made
by mixing the components in carefully measured quantities to ensure that the product has the required properties
examples of formulations
- fuels
- cleaning agents
- paints
- medicines
- alloys
- fertilisers
- foods
what is chromatography
a physical separation technique that separates different substances in a mixture based on their solubilities
what does separation in chromatography depend on
the distribution of substance between the stationary and mobile phase
what is an rf value
the ratio of the distance moved by a compound to the distance moved by the solvent
rf value formula
distance moved by substance / distance moved by solvent
what is the stationary phase in chromatography
the paper
what is the mobile phase in chromatography
the solvent
how does paper chromatography separate mixtures
different chemicals have different solubilities so are each attracted to the stationary phase to a different extent
chromatography method
- draw pencil line at bottom of chromatography paper
- place two dots of mixture using capillary tube / draw dot of ink if pen used
- place bottom of paper into solvent; don’t let touch pencil line
- solvent will travel up paper and ink dissolve in it and carried up paper with solvent
what can be determined if one spot is formed from chromatography
the substance is pure in that solvent
what can be determined if multiple spots are formed from chromatography
there are different solvents with different solubilities within it, so an impure mixture
what happens to a more soluble substance in chromatography
it will travel further up the paper because it is more attracted to the mobile phase and less attracted to the stationary phase
vice versa for less soluble substance
if only one spot is formed with a solvent, how can we be sure the substance is pure
by testing it with a range of solvents, because a pure compound will produce one spot in ALL solvents
what do rf values help with
identifying the compounds; look the rf value up in a database and match it to the solvent used to identify the chemical
what happens if an rf value shows several chemicals
repeat the experiment using a different solvent
what happens if a chemical has never been analysed before
there will not be an rf value on the database
test for hydrogen
burning splint held at the open end of a test tube of the gas
positive test for hydrogen
hydrogen burns rapidly with a squeaky pop sound
test for oxygen
glowing splint inserted into a test tube of the gas
positive test for oxygen
splint relights in oxygen
test for carbon dioxide
carbon dioxide shaken with or bubbled through an aqueous solution of calcium hydroxide (limewater)
positive test for carbon dioxide
limewater turns cloudy
test for chlorine
damp litmus paper is put into chlorine gas
positive test for chlorine
litmus paper is bleached and turns white
what are flame tests used for
identifying metal ions
what colour flame do lithium compounds produce
Li+ ions produce a crimson flame
LiCk
what colour flame do sodium compounds produce
Na+ ions produce a yellow flame
SoY
what colour flame do potassium compounds produce
K+ ions produce a lilac flame
PoLl
what colour flame do calcium compounds produce
Ca2+ ions produce an orange-red flame
CalOR
what colour flame do copper compounds produce
Cu2+ ions produce a green flame
CoG
what happens if a sample contains a mixture of ions
some flame colours can be masked
issue with flame tests to identify ions in a compound
colour of flame is difficult to distinguish esp if low conc of metal compound
what can sodium hydroxide solution be used for
identifying positive metal ions (cations)
what forms white precipitates when sodium hydroxide solution is added
aluminium, calcium and magnesium ions
what happens when excess sodium hydroxide solution is added
aluminium hydroxide precipitate dissolves and goes clear again
what forms coloured precipitates when sodium hydroxide solution is added
solutions of copper(II), iron(II) and iron(III)
what precipitate is formed when copper(II) ions react with sodium hydroxide
blue
what precipitate is formed when iron(II) ions react with sodium hydroxide
green
what precipitate is formed when iron(III) ions react with sodium hydroxide
brown
what is a positive ion
a cation
what is a negative ion
an anion
examples of negative ions (anions)
- carbonate ions
- halide ions
- sulfate ions
how to test for carbonate ions
- add dilute acid to sample
- acid reacts with carbonate to make carbon dioxide (as well as salt and water)
- will effervesce
- limewater test
how to test for halide ions
- add dilute nitric acid
- add dilute silver nitrate solution
- halide ions produce a precipitate of silver halide
chloride ions: white precipitate of silver chloride
bromide ions: cream precipitate of silver bromide
iodide ions: yellow precipitate of silver iodide
how to test for sulfate ions
- add dilute hydrochloric acid to the sample
- add barium chloride solution
- white precipitate of barium sulfate forms if sulfate ions present
what are ionic compounds made up of
positive ions (cations) and negative ions (anions)
advantages of instrumental methods
- highly accurate and sensitive
- quicker
- enable very small samples to be analysed
disadvantages of instrumental methods
- usually very expensive
- takes special training to use
- gives results that can often be interpreted only by comparison with data from known substances
what are instrumental methods used for
detecting and identifying elements and compounds
what is flame emission spectroscopy
an example of an instrumental method used to analyse metal ions in solutions
how does flame emission spectroscopy work
sample put into a flame and light given out passed through spectroscope. output is line spectrum that is analysed to identify metal ions in solution and measure their concentrations
how do you use the line spectrum from flame emission spectroscopy
the position of the lines in the spectrum are specific for a given metal ion
as well as identifying a metal ion, what can FES also tell us
the conc of the metal ion because the lines become more intense as you increase conc
