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Chemistry > C2.1 - Purity & separating mixtures > Flashcards

C2.1 - Purity & separating mixtures Flashcards

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1
Q

what is a pure substance?

A

single substance not mixed with anything else - e.g. distilled water

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2
Q

what is a mixture?

A

contains 2 or more substances - e.g. seawater, milk, alloys

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3
Q

what is an alloy?

A

a metal made by combining 2 or more metallic elements

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4
Q

what do impurities do to the melting point of a substance?

A

lower it - disrupts regular arrangement of particles so they cant form as many bonds

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5
Q

what do impurities do the the boiling point of a substance?

A

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

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6
Q

pure + impure mpt + bpt

A

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

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7
Q

why is distillation used?

A

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
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8
Q

what is a still?

A

the whole set of apparatus used in distillation

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9
Q

what is a disadvantage of using simple distillation?

A

can only use it to separate things with very different boiling points

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10
Q

what is different about fractional distillation?

A

you add a fractionating column on top of the flask

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11
Q

why is fractional distillation useful?

A

can separate a mixture of different liquids - lowest bpt evapourates first, will reach top of column

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12
Q

what is filtration used for?

A

separates insoluble solutes in a solution

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13
Q

what is the filtrate?

A

the solution that passes through the filter paper

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14
Q

when you mix salt and water why can you not see the salt?

A

water particles can get between the salt particles - breaks salt crystals up so you can no longer see them

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15
Q

what is crystallisation?

A

process to produce crystals of the solute from the filtrate

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16
Q

how are crystals produced from crystallisation?

A

when you leave a saturated solution to cool

17
Q

how is a filtrate turned into a saturated solution? (crystallisation)

A

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

18
Q

describe the process of filtration using a mixture of salt and crushed rock

A
  • 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
19
Q

what does paper chromatography separate?

A

coloured substances due to their different solubility in solvent used or how attracted substances are the the filter paper

20
Q

what is capillary action? (chromatography)

A

when liquids can flow in a narrow space, even against gravity

21
Q

describe how paper chromatography works

A
  • 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
22
Q

what is the mobile phase - paper chromatography

A

solvent - something molecules can move in

23
Q

what is the stationary phase - paper chromatography

A

paper - molecules don’t move

24
Q

how do you calculate the Rf value?

A

distance travelled by / distance travelled by solvent

substance

25
Q

paper chromatography - is a single spot a pure or impure substance?

A

pure

26
Q

the shorter the distance travelled by the substance….

A

….the less soluble it is - smaller Rf value

27
Q

describe thin layer chromatography

A
  • 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
28
Q

advantages of instrumental analysis

A
  • quick
  • accurate
  • needs v. small samples
  • can prepare several samples to run one after another
29
Q

disadvantages of instrumental analysis

A
  • costly
  • needs high skilled operators
  • large equipment
  • needs setting up
30
Q

what does mass spectrometry identify?

A

identifies components by splitting the molecule into different fragments and measure the mass of the fragments

31
Q

what does infrared spectrometry identify?

A

identifies components by subjecting the molecule to radiation which makes covalent bonds within ‘wobble’ at specific frequencies

32
Q

what is the mobile phase in gas chromatography?

A

unreactive / inert gas - e.g. nitrogen / helium

33
Q

what is the stationary phase in gas chromatography?

A

a viscous (thick) liquid - e.g. oil

34
Q

gas chromatography - if unknown substance isn’t already a gas, then it has to be….

A

….vaporised

35
Q

describe the process of gas chromatography

A
  • 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
36
Q

what is retention time?

A

time it takes a chemical to travel through the tube - gas chromatography
- diff for each chemical - what’s used to identify each chemical

37
Q

what does the relative areas under the peaks show you? - chromatogram (gas chromatography)

A

relative amounts of each chemical in sample

Chemistry (9 decks)

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