chemical analysis Flashcards
(59 cards)
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
