Separation techniques Flashcards
What is meant by the term “pure substance”
A pure substance is one that is made up of only one element or compound
What is a mixture?
A mixture consists of 2 or more elements or compounds not chemically combined together - the chemical properties of each substance in the mixture are unchanged
What processes are used to separate an element from a compound?
Chemical processes:
- Displacement
- REDOX reactions
What processes are used to separate elements or compounds from a mixture?
physical processes;
- Simple/ fractional distillation
- Paper chromatography
- Separating funnel
- Centrifugation
- Filtration
- Evaporation and crystallisation
What process is used to separate a liquid from an insoluble solid?
FILTRATION:
- The solid particles are larger and are trapped by the mesh of the filter paper
- The smaller liquid particles are able to pass through the filter paper into the collection vessel
- The separated liquid is known as the filtrate and the solid is the residue
What process is used to separate 2 immiscible liquids (liquids that are incapable of being mixed together e.g, water and oil)
use of a SEPARATING FUNNEL:
- When 2 immiscible liquids are placed into a separating funnel, they form 2 layers
- The upper layer has the lower density
- When the tap is opened, the lower layer is poured out
- The separating funnel has narrower walls at the lower end (near the tap), so it is easier to close the tap the moment the last drop of the lower layer has passed through
What process is used to separate miscible liquids (ones that can mix/ are soluble in each other) but have significantly different boiling points?
DISTILLATION:
(simple distillation)
-The mixture is heated to ABOVE THE BOILING POINT 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 boiling point solute remains in the heated flask
-If the solute is stable to heating, it can also be collected once the solvent has been removed
What process is used to separate miscible liquids (ones that can mix/ are soluble in each other) and have different but VERY CLOSE boiling points?
DISTILLATION:
(fractional distillation)
-The mixture is heated
-When the temperature of the mixture reaches the boiling point of the component with the lowest bp, it will hold steady (measured on the thermometer)
-This means the vapour of the lowest bp liquid has reached the top of the fractionating column; heating the beads in the column up to roughly the same temperature
-When the vapour of the second lowest bp liquid comes into contact with the beads, it will condense to liquid and fall back into the beaker because it has a higher bp than the first liquid
-The lowest bp liquid (the first liquid) will then reach the condenser and condense back into a liquid in the collecting beaker
-The temperature at the thermometer will ten change; we know the first liquid is done evaporating and now can use a container to collect the next liquid
What process is used to separate a solid from a liquid?
EVAPORATION AND CRYSTALLISATION:
- Heat the solution gently until crystals just being to appear- this is the crystallising point
- The remaining concentrated solution is then left to evaporate
- Do not hat to dryness; some solids may decompose if heated too strongly
What process is used to separate heterogeneous mixtures based on their differences in particle mass?
CENTRIFUGATION:
- The sample is put into a test tube
- It is spun at a low speed
- The most dense mixture (solid particles) forms a sediment/ pellet at the bottom of the tube
- The other particles remain suspended in the supernatant (remaining liquid)
- The supernatant is removed and put into a clean test tube and spun at a higher speed
- The second most dense particle forms a pellet at the bottom, etc
What process is used to separate mixtures of different solutes, such as dyes in ink, or to separate mixtures of different liquids?
CHROMATOGRAPHY:
- A small sample of the mixture is spotted on to a piece of chromatography paper [stationary phase]
- The position of the sample is marked with a pencil line- pencil does not dissolve in most solvents; wont move and will give a clear fixed start position
- This paper is then placed in a beaker which contains a solvent [mobile phase] at the bottom and the lid is replaced (the solvent front should not reach the top of the paper & the lid is kept to ensure the solvent rises up the paper rather than evaporating out of the beaker)
- The solvent then rises up the paper
- The different components each have a different tendency to absorb to the paper- if the component absorbs more strongly to the paper (the stationary phase) then it does not travel up easily
How do you make comparisons between different chromatograms?
Calculate the different Rf values:
“the measure of how far the sample, seen as the dot, travels up the paper compared to how far the solvent travels”
Rf = [distance travelled by SAMPLE] / [distance travelled by SOLVENT]
How would establish the purity of a substance using chromatography?
The Rf value of a particular compound is always the same i the chromatography has been carried out the same way;
- If you found the Rf value of a compound to be different than its known/ standard value, this would indicate an impurity
- HOWEVER, an impurity may have the same Rf value as the standard value by coincidence- not possible to be 100% sure
