C2 - purification and seperation techniques Flashcards
what is a pure substance?
a substance that contains of only one element or compound
what is a mixture?
a substance containing more than 1 compound or different elements no part of the same compound, not chemically joined together
what is an element
one type of atom
what is a compound
2 or more types of atom bonded together
melting point of pure substances
sharp melting point
eg. pure water will boil at 100°c exactly
how can the purity of a sample be tested
comparing the melting point of a sample to the expected value
melting point of a mixture/ impure substance
gradually over a range of temperatures
what are the 2 types of distillation
simple and frational
what is simple distillation used for
separating a liquid(solvent) from a solution
how does simple distillation work
- solution is put into a flask
- run cold water through condenser to keep it cool
- flask is gradually heated until the solution evapourates(bp is much lower than solute)
- vapour passes through condenser where it cools and becomes a liquid again
- liquid is collected in a beaker
- solute remains in flask
problem with simple distillation
only used to separate things with very different boiling points
example of substances that can be separated with simple distillation
pure water from saltwater/ sea water
what is frational distillation used for
separating a mixture of liquids
how does frational distillation work
- mixture put in flask attached to fractionating column attached to condenser
- flask is heated, liquid with lowest bp evapourates first
- when temp on thermometer reaches bp of liquid, it reaches the top then returns to liquid in condenser and collected
why won’t other liquids also reach the condenser in fractional distillation
top of fractionating column is cooler than bottom so other liquids even if they begin to evapourate, wont reach the top before condensing and running back down the column
examples of when fractional distillation is used
to separate different hydrocarbons from crude oil
what is filtration used for
separating insoluable solid from liquid
how does filtration work
filter paper has tiny holes allowing small molecules and dissolved ions through but not larger molecules of insoluable solid
steps of filtration
- mixture of solid and liquid poured into filter funnel (funnel with filterpaper in)
- liquid drips through solid is caught in paper
examples of mixtures separated by filtration
- sand from sandy water
- excess reactant from a solution (copper oxide)
what is crystallisation used for
- separating a soluable solid from a solution
steps of crystallisation
- solution is poured into an evapourating dish and heated
- water will begin to evaporate
- once crystals begin to form dish removed from heat
- pour excess liquid away
- allow crystals to dry
problems with crystallisation
liquid is not collected (simple distillation should be used if liquid is to be collected)
what substances can be separated by crystallisation
salt and water
what is chromatography used for
separate a mixture of soluable substances and identify them
what are the 2 phases of chromatography
mobile phase
stationary phase
what is a mobile phase
molecules can move. always liquid or gas
what is a stationary phase
molecules cant move. solid or thick liquid
what is the stationary phase in paper chromatography
filter paper
what is the mobile phase in paper chromatography
solvent/ water
steps of paper chromatography
- line drawn in pencil near bottom of paper
- a spot of mixture is put on line
- bottom of paper is put in a beaker of solvent below the line
- solvent will travel up paper and the mixture will begin to separate
- paper removed before solvent reached top
- solvent front marked in pencil
why should the line be marked in pencil
pencil is insoluable, pen in will dissolve in the solvent
what is a solvent front
how far the solvent travelled
how can the soluability of solute be told in chromatography
more soluable = travel further up the paper
what is a chromatogram
the piece of paper left after chromatography with all separated substances
how can Rf value be calculated
distance travelled by solute/
distance travelled by solvent
how to tell if Rf value is correct
has to be less than 1, solute cant travel further than solvent
how can chromatography be a purity test
pure substance wont separate, will move as one blob
mixture separates into multiple blobs
what is potable water
water that is safe to drink
sources of water that is purified for potable water in UK
surface water: lakes, rivers
groundwater:rockstrapwaterunderground
waste water: contaminatedby humanprocess
3 steps of water treatment
filtration, sedimentation, chlorination
what is filtration
- removes insoluable solids eg. twigs, stones with wire mesh
- sand beds and gravel filter any other solid bits out
what is sedimentation
- iron sulfate/ aluminium sulfate added to water making fine particle settle at bottom
what is chlorination
- chlorine gas bubbled through to kill microorganisms and sterilize water
problems with distillation for potable water from sea water
used lots of energy so it is very expensive to produce large quantities
why must pure water be used in experiments
potable water from taps is safe to drink but still contains ions and minerals which can interfere with reactions and give false result
Ways to get potable water from sea water
Desalination:
- distillation (simple)
- reverse osmosis
