seperation techniques Flashcards
1
Q
displacement
A
- displacement/redox reactions can be used to extract an element from a compound
- elements which are more reactive will displace less reactive elements
- e.g. Mg will displace the Cu in CuSO4 to give solid copper which can be removed by filtration
2
Q
electrolysis
A
- the breakdown of an ionic compound using electricity
- ions must be mobile for electrolysis to work (must be molten/in solution)
- electrolyte: the mixture containing mobile ions
- a direct current is applied through the electrolyte using two electrodes
- +ve ions (cations) are attracted to the -ve electrode (cathode) and -ve ions (anions) are attracted to the +ve electrode (anode)
- reduction occurs at cathode, oxidation occurs at anode
- dc supply is used so that the current passes in one direction only
3
Q
decanting
A
- separates a solid and a liquid/solution
- the solid should settle to the bottom of the beaker and the liquid can be poured away
4
Q
filtration
A
- separates a liquid/solution from an insoluble solid
- solid particles are larger so are trapped by the mesh of the filter paper
- smaller liquid particles are able to pass through the filter paper into the collection vessel
- the separated liquid is called the filtrate and the solid is the residue
5
Q
seperation funnel
A
- used to separate two immiscible liquids
- the liquids form separate layers, the upper layer has a lower density
- when the tap is opened, the lower layer can be poured out
- the narrowing walls of the separating funnel make it easier to close the tap the moment the last drop of the lower layer has passed through
6
Q
distillation
A
- used to separate two substances with different bp
- the mixture is heated to above the bp of the solvent
- the solvent becomes a vapour and starts to spread out through the apparatus
- as the vapour hits the condenser it cools and condenses as droplets which run down the condenser to the collection vessel
- the higher bp solute remains in the heated flask and can also be collected once the solvent has been removed if it is stable to heating.
7
Q
simple distillation
A
-used to separate the solvent from a solution, leaving the solute behind
8
Q
condensor
A
- cold water is passed through the condenser in an outer jacket which maintains a cooling atmosphere for vapours to pass through
- water is always added at the bottom of the jacket to ensure that there is always cold water in the jacket and that condensation occurs all the way along the condenser
9
Q
fractional distillation
A
- used to separate miscible liquids from a mixture
- requires each liquid to have a different bp
- mixture is heated
- when the temp of the mixture reaches the bp of the component with the lowest bp, the temp no longer rises and the first component boils and rises up the fractionating column towards the condenser where it is condensed and collected
- the thermometer at the entrance to the condenser monitors the bp of the component being distilled
- the remaining mixture then rises in temp until the bp of the next lowest component is reached and the process repeats
10
Q
evaporation & crystallisation
A
- used to separate the solvent from a solute when you want the solute rather than the solvent
- the solution is heated gently until crystals just begin to appear (crystallising point)
- the remaining concentrated solution is then left to evaporate
- don’t heat to dryness because some solids may decompose if heated too strongly
11
Q
chromatography
A
-used to separate components of a mixture
-a small sample of the mixture is spotted on to a piece of chromatography paper and it is place in the solvent
-the solvent rises up the paper and the different components each have a different tendency to adsorb to the paper
-if the component adsorbs more strongly to the stationary phase then it travels up more slowly
-if the component adsorbs more strongly to the mobile phase then it travels up more quickly
Rf value=distance travelled by sample/distance travelled by solvent
12
Q
centrifugation
A
- used to separate mixtures based on different densities
- the mixture is spun at high speed in a horizontal circle
- the heavier particles form a pellet at the bottom of the tube
- the remaining liquid (the supernatant) may be decanted into a new tube and spun again at a higher speed to separate smaller particles
- used in medicine for the separation of blood components e.g. RBCs and platelets from plasma
13
Q
choosing the right technique
A
- If the liquid component of a solid-liquid mixture is wanted, distillation is used
- If the single solid component is wanted, decantation, evaporation, filtration or centrifugation can be used
- to separate immiscible liquid-liquid mixtures, the separation funnel is used
- gas-gas mixtures can be separated by centrifugation
14
Q
testing for purity
A
- chromatography: if there are spots present with different Rf values, these would indicate an impurity. (However, an impurity might have the same Rf value as a desired compound by coincidence so can’t be 100% sure of purity)
- distillation: during the boiling of a liquid, the bp can be found. If a liquid starts to boil off at a higher temp than expected at a particular pressure, this indicates the presence of an impurity
- evaporation: if a solution is evaporated to dryness, any solid residue is a dissolved impurity
15
Q
removing unwanted substances
A
- filters can remove impurities such as clay particles/gravel
- odours can be removed by carbon filter
- microbes can be killed by chlorine gas
