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chemical analysis Flashcards

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1
Q

What is a pure substance in chemistry?

A
  • In chemistry, a pure substance is a single element or compound, not mixed with any other substance.
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2
Q

How can we distinguish pure substances from mixtures?

A
  • Melting point and boiling point data can be used to distinguish pure substances from mixtures
  • This is because pure elements and compounds melt and boil at specific temperatures
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3
Q

How can you test the purity of a sample?

A
  • By measuring its melting point and boiling point and comparing it to that of a pure substance
  • The closer the measure value to the actual m.p/b.p the purer the sample
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4
Q

What happens when there are impurities in a sample?

A
  • It will lower the melting point and increasing the melting range of the substance
  • It will also increase the boiling point and may result in the sample boiling over a rang of temps
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5
Q

What is a pure substance in everyday language?

A
  • In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state
    e.g. pure milk.
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6
Q

What is a formulation?

A
  • A formulation is a mixture (with a precise purpose) that has been designed as a useful product.
  • Many products are complex mixtures in which each chemical has a particular purpose.
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7
Q

How are formulations made?

A
  • Formulations are made by mixing the components in carefully measured quantities to ensure
    that the product has the required properties.
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8
Q

Give some examples of formulations

A
  • Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
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9
Q

Why are formulations important in the pharmaceutical industry?

A
  • By altering the formulation of a pill, chemists can make sure that it delivers the drug to the correct part of the body at the right concentration, that it’s consumable and has a long shelf life
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10
Q

What information shows use a product’s formulation?

A
  • Ratio or percentage of each component
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11
Q

What can chromatography be used for?

A
  • Chromatography can be used to separate mixtures and can give information to help identify substances.
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12
Q

What does separation depend on in chromatography?

A
  • Separation depends on the distribution of substances between the phases.
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13
Q

What are the two phases in chromatography?

A
  • Chromatography involves a stationary phase and a mobile phase
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14
Q

What is the mobile phase?

A
  • Where molecules can move - this is always a liquid or a gas
  • this is the solvent
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15
Q

What is the stationary phase?

A
  • Where the molecules can’t move - this can be a solid or a really thick liquid
  • this is the chromatography paper
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16
Q

Explain how chromotography works

A

1) During chromatography, the substances in the sample constantly move between the mobile and stationary phase - and equilibrium is formed between the two phases
2) The MP moves through the SP and anything dissolved in the MP moves with it
3) How quickly a chemical moves depends on how it’s distributed between the two phases
4) The chemicals that spend more time in the MP than SP will move further in the SP - the more soluble a substance is the further up the paper it goes
5) The components in a mixture will normally separate through the stationary phase so long as all the components spend different amounts of time in the mobile phase
6) The separated components form spots
7) A pure substance will only for one spot in any solvent since there is only one substance in the sample

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17
Q

What does the amount of time the molecules spend in each phase of paper chromatography depend on?

A
  • How soluble they are in the solvent
  • How attracted they are to the paper
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18
Q

How does solubility and attractation to paper affect chromatography results?

A
  • Molecules with a higher solubility in the solvent and which are less attracted to the paper will spend more time in the mobile phase and will be carried further up the paper
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19
Q

What do you call the result of a chromatography analysis?

A
  • a chromatogram
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20
Q

Describe an Rf value

A
  • the ratio between the distance travelled by the dissolved substance (the solute) (Centre of spot from origin) and the distance travelled by the solvent
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21
Q

State the equation used to find the retention factor (Rf)

A

Rf = distance moved by substance/
distance moved by solvent

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22
Q

What’s the relationship between the Rf value and the distance travelled in the stationary phase?

A
  • The further the stationary phase a substance moves, the larger the Rf value
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23
Q

How can we identify compounds using Rf values?

A
  • Different compounds have different Rf values in different solvents, which can be used to help identify the compounds.
  • The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.
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24
Q

Explain how paper chromatography separates mixtures

A

explain how paper chromatography separates mixtures

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25
Q

Suggest how chromatographic methods can be used for distinguishing pure substances from impure substances

A
  • If one spot shows then the substance is pure
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26
Q

How do we test for hydrogen + positive result?

A
  • The test for hydrogen uses a burning splint held at the open end of a test tube of the gas.
  • Hydrogen burns rapidly with a pop sound. (the noises comes from the hydrogen burning quickly in the oxygen in the air to form water)
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27
Q

How do we test for oxygen + positive result?

A
  • The test for oxygen uses a glowing splint inserted into a test tube of the gas.
  • The splint relights in oxygen.
28
Q

How do we test for carbon dioxide + positive result?

A
  • The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water).
  • When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy)
29
Q

How do we test for chlorine + positive result?

A
  • The test for chlorine uses litmus paper.
  • When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white. (this is because a solution of chlorine is acidic)
30
Q

What can flame tests be used for?

A
  • Flame tests can be used to identify some metal ions (cations).
  • Lithium, sodium, potassium, calcium and copper compounds
    produce distinctive colours in flame tests
31
Q

How do you conduct a flame test?

A
  • Clean a nichrome wire loop by rubbing with fine emery paper and then holding it in a blue flame
  • Dip the nichrome loop into the sample you want to test and put it in the flame
  • Record the colour of the flame
  • You can use the colours to detect and identify different ions
32
Q

How do you know the nichrome loop is clean?

A
  • When you hold it in a blue flame and it doesn’t change colour
33
Q

What flame colour is shown when lithium burns?

A
  • Lithium compounds result in a crimson flame
34
Q

What flame colour is shown when sodium burns?

A
  • Sodium compounds result in a yellow flame
35
Q

What flame colour is shown when potassium burns?

A
  • Potassium compounds result in a lilac flame
36
Q

What flame colour is shown when calcium burns?

A
  • Calcium compounds result in an orange-red flame
37
Q

What flame colour is shown when copper burns?

A
  • Copper compounds result in a green flame
38
Q

What happens when there is a sample containing a mixture of ions in a flame test?

A
  • If a sample containing a mixture of ions is used some flame colours can be masked.
39
Q

How do you test for (some) metal ions? (cation)

A
  • By adding a few drops of sodium hydroxide solution - this may form a coloured insoluble hydroxide
  • The sodium hydroxide solution can be used to identify some metal ions (cation)
40
Q

What metal solutions form white precipitates when sodium hydroxide solution is added?

A
  • Solutions of aluminium, calcium and magnesium ions form white
    precipitates when sodium hydroxide solution is added
41
Q

What metal solutions form coloured precipitates when sodium hydroxide solution is added?

A
  • Copper(II), iron(II) and iron(III) ions form coloured precipitates when sodium hydroxide solution is added.
42
Q

What does copper(II) give in the presence of NaOH?

A
  • A blue precipitate is formed
43
Q

What does iron(II) give in the presence of NaOH?

A
  • A green precipitate is formed
44
Q

What does iron (III) give in the presence of NaOH?

A
  • A brown precipitate is formed
45
Q

What does magnesium give in the presence of NaOH?

A
  • a white precipitate is formed
46
Q

What does aluminium give in the presence of NaOH?

A
  • a white precipitate is formed at first, but then redissolves in excess NaOH to form a colourless solution
47
Q

What does calcium give in the presence of NaOH?

A
  • a white precipitate is formed
48
Q

Which precipitate dissolves in excess sodium hydroxide solution?

A
  • Only aluminium hydroxide precipitate dissolves in excess sodium
    hydroxide solution.
49
Q

How can you test for carbonate ions? (anion)

A
  • Using a pipette to add a few drops of dilute acid - this causes the carbonates to react with the dilute acid to form carbon dioxide gas
50
Q

How can carbon dioxide be identified?

A
  • CO2 can be identified with limewater
  • If carbonate ions are present, the reaction with acid will release CO2 which turns limewater cloudy
51
Q

How do you test for sulfates? (anion)

A
  • Use a pipette to add a couple of drops of dilute hydrochloric acid, followed by a few drops of barium chloride solution
52
Q

How can sulfates be identified?

A
  • If sulfates are present, a white precipitate of barium sulfate will form
  • This is because sulfate ions in a solution produce a white precipitate with barium chloride solution in the presence of dilute HCl
53
Q

When testing for sulfates, why is HCl added?

A
  • To get rid of any traces of carbonate ions as this would also produce a precipitate and confuse the results
54
Q

How do you test for halides?

A
  • To identify a halide ion, add a few drops of dilute nitric acid followed by a few drops of silver nitrate solution
55
Q

How do you identify a halide?

A
  • Halide ions in solution will produce precipitates with silver nitrate solution in the presence of dilute nitric acid.
56
Q

What does chloride produce in the presence of silver nitrate and dilute nitric acid?

A
  • A white precipitate of silver chloride
57
Q

What does bromide produce in the presence of silver nitrate and dilute nitric acid?

A
  • A cream precipitate of silver bromide
58
Q

What does iodide produce in the presence of silver nitrate and dilute nitric acid?

A
  • A yellow precipitate of silver iodide
59
Q

How can elements and compounds be detected?

A
  • Elements and compounds can be detected and identified using instrumental methods.
60
Q

What are the advantages of instrumental methods?

A
  • Instrumental methods are:
  • accurate - reliably identify elements and compounds
  • sensitive - can detect very small amounts of a substance in a small mount of sample
  • rapid
61
Q

What are the disadvantages of instrumental methods?

A
  • expensive
  • less readily available
62
Q

State advantages of instrumental methods compared with chemical tests

A
  • More accurate
  • More sensitive
  • Faster
63
Q

Define flame emission spectroscopy

A
  • Flame emission spectroscopy is an example of an instrumental method used to analyse metal ions in solutions
  • It can be used to identify different ions in mixtures
64
Q

What happens during flame emission spectroscopy ?

A
  • The sample is put into a flame and the light given out is passed through a spectroscope.
  • As the ions heat up, their electrons move to higher energy levels
  • When they drop back to their original energy levels, t hey release energy as light
  • The light passes through a spectroscope which can detect different wavelngths of light to produce a line spectrum
65
Q

What can the output of a flame emission spectroscopy tell us?

A
  • The output is a line spectrum that can be analysed to identify the metal ions in the solution and measure their
    concentrations.
66
Q

What do the wavelengths emitted by an ion depend on?

A
  • The combination of wavelengths emitted by an ion depends on its charge and its electron arrangement
67
Q

What does the intensity of the spectrum indicate?

A
  • The concentration of that ion in the solution

ZZ Chem 2 (5 decks)

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