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AQA Chemistry GCSE > Chemical Analysis > Flashcards

Chemical Analysis Flashcards

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1
Q

why do we draw our starting line in pencil

A

because if we drew the line in pen, the pen ink would move up the paper with the solvent

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2
Q

how to find out an unknown substance using paper chromotography

A

After seeing how far the substance has moved and if it is pure :

  • measure the distance from the pencil line to the centre of the spot to see how far the substance moved
  • measure the distance that the solvent moved
  • find the Rf value by dividing the distance moved by the substance over the distance moved by the solvent
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3
Q

Rf equation

A

distance moved by substance / distance moved by solvent

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4
Q

how to increase the reliability of a paper chromatography ecperiment

A

repeating the experiment using different solvents

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5
Q

problems with paper chromotography

A
  • several different substances may have the same Rf values
  • if the substance has never been analysed, then there will not be an Rf value on the database
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6
Q

what is limewater

A

the aqueous solution of calcium hydroxide

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7
Q

what is a pure substance

A

a substance that consists of only one element or compound

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8
Q

what is a mixture

A

a substance that contains two or more different elements or compounds that are not chemically joined together

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9
Q

what is an element

A

a substance that contains one type of atom

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10
Q

what is a compound

A

a substance that contains two or more types of atom chemically joined together

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11
Q

what are the differences between pure and impure substances

A
  • a pure substance melts at a specific fixed temperature
  • a pure substance has a specific boiling point
  • impure substances melt and boil over a range of temperatures
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12
Q

what is a formulation

A

a mixture which has been designed as a useful product, In a formulation, every chemical has been added in specific amountst. Each chemical has a specific purpose in the formulation.

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13
Q

examples of formulations

A
  • fuels
  • cleaning products
  • paints
  • medicines
  • alloys
  • fertilisers
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14
Q

why is paper chromatography used

A

to separate mixtures of soluble substances and identify substances present in the mixture, often coloured substances such as food colourings, inks, dyes

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15
Q

What are the steps of paper chronatography

A
  1. Take a piece of chromatography paper and draw a pencil line near the bottom
  2. Put a dot of your first colour onto the pencil line, and next to that, put a dot of your second colour
  3. Place the piece of paper into a solvent
  4. The solvent moved up the paper and dissolves the ink and the two coloured dots
  5. The colour with only one spot is pure, the colour with more than one is impure
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16
Q

what is a solvent

A

a liquid that’ll dissolve substances

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17
Q

what is the stationary phase

A

the paper since it does not move

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18
Q

what is the mobile phase

A

the solvent as it moves up the paper

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19
Q

what is a chromotogram

A

the results of separating mixtures by chromatography

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20
Q

how to test for hydrogen

A

place a lighted splint into a test tube, if hydrogen is present, you’ll hear a squeaky pop sound

21
Q

how to test for oxygen

A

put a glowing splint into the mouth of a test tube, if it relights, oxygen is present

22
Q

how to test for carbon dioxide

A

if carbon dioxide is bubbled through limewater, it turns milky or cloudy white

23
Q

how to test for chlorine

A

hold damp litmus paper into the mouth of the test tube, chlorine bleaches litmus paper and turns it white

24
Q

what are some problems with doing flame tests

A
  • the colour of the flame could be hard to distinguish, especially if there is a low concentration of the metal compound
  • sometimes a metal compound contains a mixture of metal ions, which can mask the colour of the flame
25
Q

what does it mean if a method is instrumental

A

it is carried out by a machine

26
Q

advantages of instrumental methods

A
  • they are rapid
  • they are sensitive, can work on tiny samples
  • they are accurate
27
Q

how to carry out a flame test

A
  1. Dip a clean wire loop into a solid sample of the compound being tested
  2. Put the loop into the edge of the blue flame from a Bunsen burner
  3. Observe and record the colour of the flame produced
28
Q

what colour is the flame if lithium is present

A

Crimson

29
Q

what colour is the flame if sodium is present

A

yellow

30
Q

what colour is the flame if potassium is present

A

Lilac

31
Q

what colour is the flame if calcium is present

A

orange - red

32
Q

what colour is the flame if copper ion is present

A

green

33
Q

how can we test for metal hydroxides

A

using dilute sodium hydroxide

34
Q

how to carry out a flame emission spectroscopy

A
  1. Put a sample of the metal ion into a flame
  2. The light given out is then passed into a machine called spectroscope
  3. The spectroscope converts light into a light spectrum
  4. The positions of the lines in the spectrum are specific for each metal ion
35
Q

what colour precipitates are formed when we add dilute sodium hydroxide to calcium, aluminium or magnesium ions

A

white

36
Q

what colour precipotate is formed when we add dilute NAOH to iron(ii) ions

A

green

37
Q

what colour precipitate is formed when we add dilute NAOH to iron(iii) ions

A

brown

38
Q

what colour precipitate if formed when we add dilute NAOH to copper(ii)

A

blue

39
Q

how can we distinguish between aluminium, calcium and magnesium ions

A
  • if we add excess sodium hydroxide solution to Al ions, the Al precipitate redissolves
  • Calcium and magnesium are unchanged so we carry out a flame test to see which one is calcium
40
Q

how to test for carbonate ions

A
  1. Add dilute acid to our sample
  2. When the carbonate ions react with the acid, a we see effervescence, if carbonate ions are present, CO2 gas is given off
  3. To confirm that this is co2, bubble the gas given off into limewater, if it turns cloudy, co2 is present and so are the carbonate ions
41
Q

how to test for sulfate ions

A
  1. Add dilute HCL to our sample
  2. Add barium chloride solution
  3. If sulfate ions are present, we will see a white precipitate
42
Q

why, in a test for sulfate ions, must the sample be acidified with HCL

A

because carbonate ions also produce a white precipitate when reacted with barium chloride solution. the acid reacts with any carbonate ions present. this removes them and stops them from giving a false positive result

43
Q

when testing for sulfate ions, why cant dilute sulfuric acid be used

A

because sulfuric acid contains sulfate ions, these would react with the barium ions in the test, giving an incorrect positive result

44
Q

how to test for hallide ions

A
  1. Add a few drops of dilute nitric acid to the sample
  2. Add a few drops of dilute silver nitrate solution to the sample
  3. Halide ions produce a precipitate of the silver halide, and each halide makes a different colour precipitate
45
Q

what colour precipitate does bromine form

A

a cream precipitate to form silver bromide

46
Q

what colour precipitate does chlorine form

A

white, to form silver chloride

47
Q

what colour precipitate does iodine form

A

yellow, to form silver iodide

48
Q

why is the sample acidified with nitric acid when testing for halide ions

A

Carbonate ions also produce a white precipitate with silver nitrate solution. The acid reacts with any carbonate ions present. This removes them, so stopping them giving an incorrect positive result for chloride ions.

AQA Chemistry GCSE (27 decks)

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