C2.1 - Purity & separating mixtures Flashcards
what is a pure substance?
single substance not mixed with anything else - e.g. distilled water
what is a mixture?
contains 2 or more substances - e.g. seawater, milk, alloys
what is an alloy?
a metal made by combining 2 or more metallic elements
what do impurities do to the melting point of a substance?
lower it - disrupts regular arrangement of particles so they cant form as many bonds
what do impurities do the the boiling point of a substance?
increase - impurities decrease water molecules available for vaporisation during boiling
- greater amount of heat needed to make same amount of impure solution vaporise than heat required to make pure solution vaporise
pure + impure mpt + bpt
pure - fixed mpt + bpt - water bpt=100°C, mpt=0°C
impure - no fixed mpt + bpt - melt + boil at range of temps - e.g. starts boil at 70°C, completes boil at 78°C
why is distillation used?
separate a pure liquid from a solute or mixture of liquids
- works when liquids have different boiling points
- sequence of events = heating –> evaporating –> cooling –> condensing
what is a still?
the whole set of apparatus used in distillation
what is a disadvantage of using simple distillation?
can only use it to separate things with very different boiling points
what is different about fractional distillation?
you add a fractionating column on top of the flask
why is fractional distillation useful?
can separate a mixture of different liquids - lowest bpt evapourates first, will reach top of column
what is filtration used for?
separates insoluble solutes in a solution
what is the filtrate?
the solution that passes through the filter paper
when you mix salt and water why can you not see the salt?
water particles can get between the salt particles - breaks salt crystals up so you can no longer see them
what is crystallisation?
process to produce crystals of the solute from the filtrate
how are crystals produced from crystallisation?
when you leave a saturated solution to cool
how is a filtrate turned into a saturated solution? (crystallisation)
by gently evaporating some of the solvent - as the saturated solution cools, the solute becomes less soluble in the solvent and some becomes out of solution as a solid crystal
describe the process of filtration using a mixture of salt and crushed rock
- add distilled water + mix - so salt dissolves (rock is insoluble)
- set up filter paper in a funnel over a conical flask
- pour solution through paper - rock will be left in filter paper (residue) due to its bigger insoluble particles
- heat solution of saltwater in the flask - this creates a saturated solution - allow it to cool, solution now holds less solute so crystals begin to form
what does paper chromatography separate?
coloured substances due to their different solubility in solvent used or how attracted substances are the the filter paper
what is capillary action? (chromatography)
when liquids can flow in a narrow space, even against gravity
describe how paper chromatography works
- draw a pencil line near bottom of paper (baseline)
- place ink sample(s) on pencil line
- add shallow amount of solvent (water or ethanol) to a beaker
- place paper in solvent, making sure solvent doesn’t touch pencil line
- place lid on top to stop solvent evaporating
- wait for solvent to seep up paper
- diff dyes that make up ink will dissolve in the solvent at different points up the paper
- draw pencil line where solvent stops - solvent front
- insoluble substances will stay on the baseline
what is the mobile phase - paper chromatography
solvent - something molecules can move in
what is the stationary phase - paper chromatography
paper - molecules don’t move
how do you calculate the Rf value?
distance travelled by / distance travelled by solvent
substance
paper chromatography - is a single spot a pure or impure substance?
pure
the shorter the distance travelled by the substance….
….the less soluble it is - smaller Rf value
describe thin layer chromatography
- draw baseline near bottom of plate with pencil
- put small amount of solvent in beaker (water / ethanol)
- dip bottom of plate in solvent - not baseline
- put watch glass over top beaker - stops solvent evaporation
- solvent moves up plate - when chems in mix dissolve in solvent, they will move up plate
- diff chems in sample separate out, forms spots
- remove plate when solvent close to top of plate, draw pencil line - solvent front
- if spots can’t be seen use UV or ninhydrin to turn them purple from colourless
advantages of instrumental analysis
- quick
- accurate
- needs v. small samples
- can prepare several samples to run one after another
disadvantages of instrumental analysis
- costly
- needs high skilled operators
- large equipment
- needs setting up
what does mass spectrometry identify?
identifies components by splitting the molecule into different fragments and measure the mass of the fragments
what does infrared spectrometry identify?
identifies components by subjecting the molecule to radiation which makes covalent bonds within ‘wobble’ at specific frequencies
what is the mobile phase in gas chromatography?
unreactive / inert gas - e.g. nitrogen / helium
what is the stationary phase in gas chromatography?
a viscous (thick) liquid - e.g. oil
gas chromatography - if unknown substance isn’t already a gas, then it has to be….
….vaporised
describe the process of gas chromatography
- unknown mixture injected into long tube coated with stationary phase
- mixture moves along tube with mobile phase until it comes out other end -diff substances spend diff amounts of time dissolved in mobile phase + stuck to stationary phase
- a detector monitors compounds in + out if column + a recorder plots signal as a chromatogram
what is retention time?
time it takes a chemical to travel through the tube - gas chromatography
- diff for each chemical - what’s used to identify each chemical
what does the relative areas under the peaks show you? - chromatogram (gas chromatography)
relative amounts of each chemical in sample
