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chemistry > chemical analysis > Flashcards

chemical analysis Flashcards

1
Q

what is a pure substance

A

a substance that only contains one element or one type of compound

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2
Q

characteristics of pure substances

A
  • melts at a specific fixed temperature
  • has a specific fixed boiling point
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3
Q

what are impure substances

A

mixtures

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4
Q

characteristics of impure substances

A

melt and boil over a range of temperatures

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5
Q

how can we use melting and boiling points to see if a substance is pure

A

we would identify if the substance melts and boils at a specific temperature

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6
Q

what are formulations

A

complex mixtures that has been designed as a useful product

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7
Q

how are formulations formed

A

by carefully measuring the quantity of each component so that the product have the properties we need

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8
Q

examples of formulations

A
  • fuels
  • cleaning products
  • medicines
  • alloys
  • food
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9
Q

what does paper chromatography do

A
  • allows us to separate substances based on their different solubilities
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10
Q

how do we use paper chromatography to test the purity

A

1) draw a pencil line at the bottom of the chromatography paper
2) place a dot of your first substance on the pencil line and next to that we place another dot of the second substance on the pencil line (you can add more substances if there is enough space)
3) place the chromatography paper in a solvent, the solvent will then make its way up the paper and dissolve the substances and these are now carried up as well
4) the paper is called the stationary phase as it does not move
5) the solvent is called the mobile phase as it moves
6) a pure compound will produce a single spot in all solvents
7) compounds in a mixture (impure substances) will separate into different spots depending on the solvent

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11
Q

what is a solvent

A

a liquid that dissolves substances

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12
Q

why does paper chromatography work

A

as different substances have different solubilities and a more soluble substance travels further up the paper than a substance that is les soluble

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13
Q

why do we draw our starting line in pencil in paper chromatography

A

as if we drew the line in pen the pen int would move up the paper with the solvent

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14
Q

how do we use paper chromatography to identify an unknown substance from different food colouring- REQUIRED PRACTICAL

A

1) use a ruler to draw a horizontal pencil line 2cm away from the chromatography paper
2) Mark 5 pencil spots at equal spaces on the line, then use a capillary tube to place a dot of your unknown food colour and your known food colours on the pencil line, its important to keep the dots small to prevent the colours from spreading
3) pour water into a beaker to a depth of 1cm
4) attach the paper to a glass rod using tape and lower the paper into the beaker, the pencil line must be above the surface of the water or the water will wash the ink of the line
5) the sides of the beaker must not touch the sides of the paper, this prevents interference in with the way the water moves, place a lid on the beaker to reduce the evaporation of the solvents
6) the water will then move up the paper with the ink, at this stage we must be careful not to move the beaker, once the water has travelled 3/4 up remove the paper
7) use a pencil to mark the point where the water has reached then hang the paper up to dry
8) if the unknown substance is impure and the colours of the known substances line up with the colours of the unknown substance it tells us that the unknown substance is made up of those colours

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15
Q

how to calculate Rf

A

distance moved by substance/ distance moved by solvent
(Rf has no units)

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16
Q

cons of finding an unknown substance using paper chromatography

A
  • different substances may have the same Rf values so you would have to repeat the experiment using a different solvent
  • if the substance has never been analysed before then there will not be an Rf value on the database so you would have to carry out further analysis to identify it
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17
Q

how do we identify the chemicals in the unknown substance REQUIRED PRACTICAL

A

1) we measure the distance moved the other chemicals its made of, by measuring from the pencil line to the centre of the spot
2) then measure the distance moved by the solvent
3) calculate the Rf value by : distance moved by substance/ distance moved by solvent
4) look the Rf value up on a data base and that will tell us the identity of a substance

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18
Q

how do we test for hydrogen gas

A
  • light the gas with burning splint
  • if its hydrogen the gas will burn rapidly and produce a squeaky pop sound
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19
Q

how do we test for oxygen

A
  • light the gas with a glowing splint
  • if the gas is oxygen the splint will relight
20
Q

how do we test for carbon dioxide

A
  • draw some of the gas into a plastic pipette then bubble the gas into a test tube filled with limewater
  • if the gas is carbon dioxide the limewater will turn cloudy
21
Q

what is limewater

A

an aqueous solution of calcium hydroxide

22
Q

how do we test for chlorine

A
  • insert damp litmus paper into a test tube filled with gas
  • if the gas is chlorine it will bleach the litmus paper and turns it white
23
Q

what do flame tests do

A

identifies metal ions

24
Q

method for a flame test

A

1) place a small amount of our chemical onto wire, mounted on a handle
2) place the wire into the blue Bunsen flame, the colour of the flame can be used to work out the metal ion present

25
Q

what colour does the lithium ion produce

A

crimson flame

26
Q

what colour does the sodium ion produce

A

yellow flame

27
Q

what colour does the potassium ion produce

A

lilac flame

28
Q

what colour does the calcium ion produce

A

orange red flame

29
Q

what colour does the copper ion produce

A

green flame

30
Q

problems of flame test

A
  • the colour of the flame test can be hard to distinguish, especially if there is a low concentration of the metal compound
  • sometimes the sample contains a mixture of metal ions which can mask the colour of the flame
31
Q

what happens in flame emission spectroscopy

A
  • a sample of the metal ion in solution is placed into a flame, the light given out is then passed into a machine called a spectroscope, the spectroscope converts the light into a line spectrum
  • the position of the lines in the spectrum are specific for a given metal ion
32
Q

what do flame emission spectroscopy do

A
  • tells us the concentration of the metal ion as the lines become more intense at a high concentration
  • identifies the metal ion
33
Q

advantages of instrumental methods

A
  • instrumental methods are rapid, we could use the flame emission spectroscopy to analyse sample more rapidly then the flame test
  • instrumental methods are sensitive, flame emission spectroscopy will work even on a tiny sample of metal compound
  • instrumental methods are accurate, flame emission spectroscopy are more likely to identify a metal ion correctly than using a flame test
34
Q

example of instrumental method

A

flame emission spectroscopy

35
Q

how can we test for metal ions

A
  • flame test
  • reacting metals with sodium hydroxide
36
Q

problems when reacting Al, Mg and Ca with sodium hydroxide

A

they all produce a white precipitate which makes it hard to distinguish between them,

37
Q

what is formed when reacting aluminium with sodium hydroxide

A

you form aluminium hydroxide which is a white precipitate

38
Q

what is formed when reacting calcium with sodium hydroxide

A

you form calcium hydroxide which is a white precipitate

39
Q

what is formed when reacting magnesium with sodium hydroxide

A

you form magnesium hydroxide which is a white precipitate

40
Q

how to distinguish between Al, Mg and Ca when reacting with sodium hydroxide

A
  • if we add excess sodium hydroxide solution the aluminium precipitate re-dissolves
  • we would have to use the flame test to work out which one is calcium
41
Q

what is formed when reacting copper (II) with sodium hydroxide

A

copper (II) hydroxide which is a blue precipitate

42
Q

what is formed when reacting iron (II) with sodium hydroxide

A

iron hydroxide which is a green precipitate

43
Q

what is formed when reacting iron (III) with sodium hydroxide

A

iron (II) hydroxide which is a brown precipitate (its iron(III) not iron(II) )

44
Q

what is formed when reacting iron (III) with sodium hydroxide

A

iron (III) hydroxide which is a brown precipitate (its iron(III) not iron(II) )

45
Q

how do we test for carbonate ion (non metal)

A

1) add dilute acid to our sample
2) if carbonate is present the acid will react with the carbonate to produce carbon dioxide gas, we will see effervescence (fizzing), this does not prove for carbon dioxide gas
3) bubble the gas through limewater, if the lime water goes cloudy then this proves carbon dioxide is present and this shows that we started with the carbonate acid

46
Q

how do we test for halide ion (non metal)

A

1) add nitric acid to the sample
2) add dilute silver nitrate solution
3) halide ions produce a precipitate of the silver halide, each halide makes a different coloured precipitate
chloride ions - white precipitate of silver chloride
iodide ions - yellow precipitate of silver iodide
bromide ions - cream precipitate of silver bromide

47
Q

how do we test for sulfate ion (non metal)

A

1) add dilute hydrochloric acid to the sample
2) add barium chloride solution
3) if sulfate ions are present we will see a white precipitate

chemistry (13 decks)

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