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chemistry > C8 (chemical analysis) > Flashcards

C8 (chemical analysis) Flashcards

1
Q

what is a pure substance?

A

a substance that’s not mixed with any other substance (i.e. made of a single element/compound).

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2
Q

how do you check if a substance is pure using its melting/boiling point?

A
  • pure substances melt and boil at fixed temperatures.
  • impure substances melt and boil over a range of different temperatures.
  • e.g. if a beaker of water melts at a temperature other than 0 degrees, or boils at a temperature other than 100 degrees, it’s impure.
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3
Q

what is a formulation?

A

a complex mixture that has been designed as a useful product.
- in this, the quantity of each component is carefully measured, so the product has the desired properties.
- examples include: fuels, cleaning products, paints, medicines, alloys, fertilisers, food.

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4
Q

what is the purpose of paper chromatography?

A

allows us to separate substances based on their different solubilities.

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5
Q

what is the method for chromatography?

A
  1. get chromatography paper, draw pencil line near the bottom
  2. put dot of first colour on pencil line, dot of second colour next to it on the line
  3. place the bottom of the paper into a solvent
  4. solvent moves up the paper, and dissolves the ink and two coloured dots. they’re carried up the paper
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6
Q

what are the stationary and mobile phases in chromatography?

A

stationary phase: the paper, as it doesn’t move

mobile phase: the solvent, as it moves

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7
Q

what are the different results in chromatography?

A
  • pure compounds will produce a single spot in all solvents
  • the compounds in a mixture will produce several spots, depending on the solvent
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8
Q

how does a substance act based on their solubility in chromatography?

A

a more soluble substance will travel further up the paper than a less soluble substance

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9
Q

why do we draw the chromatography starting line in pencil?

A

drawing the line in pen would mean the pen ink would move up the paper with the solvent, so the starting point wouldn’t be clear, and it would also make our results unclear

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10
Q

what is the method for chromatography to identify an unknown substance?

A
  1. place a dot of our chemical on our pencil line
  2. place the bottom of the paper in the solvent, meaning the chemical will start to move up the paper
  3. record with a line where the solvent moved to, compared to the chemical
  4. measure the distance from the pencil line to the centre of the chemical spot
  5. measure the distance moved by the solvent
  6. divide the distance moved by the substance, by the distance moved by the solvent, to get your Rf value
  7. look the Rf value up in a database, and it should help you identify the chemical
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11
Q

what is the possible disadvantage of using paper chromatography to identify an unknown chemical?

A
  • several different substances may have the same Rf values, so we may need to repeat this experiment with a different solvent to narrow it down further
  • if the substance has never been analysed before, then there will not be an Rf value for it on the database, so you’d need to carry out more analysis to identify it
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12
Q

what is the Rf value?

A
  • the ratio of how far up the paper a certain spot moves compared to how far the solvent has travelled
  • number between 0 and 1
  • depends on the solvent and its temperature, and different substances will have different values

to calculate it, use:
Rf = distance moved by substance / distance moved by solvent

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13
Q

what are the two ways to collect hydrogen gas?

A
  • react zinc with dilute hydrochloric acid
  • hydrogen is insoluble in water, so you can collect it using a gas jar in a trough of water
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14
Q

how do you test for hydrogen gas?

A
  1. remove the bung of a test tube potentially containing hydrogen and insert a burning splint
  2. hydrogen gas will burn rapidly and produce a popping sound
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15
Q

what are the uses of hydrogen?

A
  • making the fats in margarines solidify
  • weather balloons
  • an alternative to hydrocarbon fuels
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16
Q

how do you test for oxygen gas?

A
  • to produce oxygen, collect 15cm^3 of hydrogen peroxide solution in a small conical flask. add a small amount of manganese oxide from the end of the spatula. DECOMPOSITION OF HYDROGEN PEROXIDE SOLUTION.
  1. place a glowing splint into a test tube potentially containing oxygen
  2. if oxygen is present, the splint will relight
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17
Q

what solution do you use to test for carbon dioxide?

A

limewater. it’s an aqueous solution of calcium hydroxide

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18
Q

what is the word equation for the production of carbon dioxide?

A

calcium carbonate (marble chips) + hydrochloric acid = calcium chloride + water + carbon dioxide

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19
Q

how do you test for carbon dioxide?

A
  1. have a test tube potentially containing carbon dioxide and a test tube containing limewater
  2. draw some of the gas into a plastic pipette
  3. bubble the gas through the limewater
  4. repeat this several times, and if carbon dioxide is present, the limewater will turn cloudy
20
Q

how do you produce chlorine gas?

A

add concentrated hydrochloric acid (corrosive) to a spatula of moistened potassium manganate crystals in a boiling tube inside a fume cupboard.

21
Q

how do you test for chlorine gas?

A
  1. insert damp litmus paper into the mouth of a test tube potentially containing chlorine
  2. the chlorine will bleach the litmus paper and turn it white (from blue)
22
Q

what is a flame test?

A
  • used to identify metal ions
  1. place a small amount of the chemical onto wire mounted in a handle
  2. place the end of this into a blue bunsen burner flame
  3. the colour of the flame can be used to work out the metal ion present
23
Q

why do you keep the end of the splint wet during flame tests?

A
  • wet with the ion solution, otherwise you’d see the colour of the wood burning, not the ion
24
Q

what colour flame does lithium create?

A

lithium produces a crimson flame

25
Q

what colour flame does sodium create?

A

sodium creates a yellow flame

26
Q

what colour flame does potassium create?

A

potassium creates a lilac flame

27
Q

what colour flame does calcium create?

A

calcium creates an orange-red flame

28
Q

what colour flame does copper create?

A

copper creates a green flame

29
Q

what are the problems with using flame tests?

A
  • colour of the flame can be difficult to distinguish, especially if there’s only a low concentration of the metal compound
  • sometimes a sample contains a mix of metal ions which can mask the colour of the flame
30
Q

what happens in flame emission spectroscopy?

A
  1. sample of the metal ion in solution is placed into a flame
  2. the light given out is then passed into a machine called a spectroscope
  3. the spectroscope converts the line into a line spectrum. the position of the lines in the spectrum are specific to a given metal ion
  4. we can use this to identify the metal ion in the sample
31
Q

what else can flame spectroscopy tell us?

A

the concentration of the metal ion, as the lines become more intense at a higher concentration

32
Q

what is an instrumental method, and what are its advantages?

A

flame emission spectroscopy is an instrumental method, as it’s carried out by a machine

  1. rapid. analyses samples more quickly than manual flame tests
  2. sensitive. can work on even the tiniest sample of a compound
  3. accurate. FES is more likely to identify a metal ion accurately than flame tests
33
Q

how do you use sodium hydroxide solution to identify calcium, magnesium and aluminium?

A
  1. have solutions of aluminium, magnesium, and calcium ions in 3 test tubes
  2. add sodium hydroxide solution to these ions, they all produce a white precipitate
  3. add excess sodium hydroxide solution, and the aluminium precipitate will re-dissolve, and we can distinguish the aluminium solution
  4. to distinguish between the other two (show which one is calcium), you’d have to do a flame test
34
Q

what are the reaction equations for reacting sodium hydroxide with calcium, magnesium, and aluminium?

A

calcium nitrate (also works for other solid calcium compounds) + sodium hydroxide = sodium nitrate + calcium hydroxide precipitate

same equation for magnesium and aluminium

35
Q

what happens when copper II ions react with sodium hydroxide?

A

form a blue precipitate of copper II hydroxide

36
Q

what happens when iron II ions react with sodium hydroxide?

A

form a green precipitate of iron II hydroxide

37
Q

what happens when iron III ions react with sodium hydroxide?

A

form a brown precipitate of iron III hydroxide

38
Q

how do you test for the carbonate ion?

A
  1. add a dilute acid to the sample
  2. if carbonate is present, the acid will react with it to make carbon dioxide gas. we will see effervescence (fizzing). (however, this doesn’t prove that we have carbon dioxide gas)
  3. therefore we must bubble the gas through limewater, and if it goes cloudy we’ll know we have carbon dioxide and that we started with a carbonate ion
39
Q

how do you test for halide ions?

A
  1. add dilute 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
40
Q

what colour precipitate does a chloride ion produce?

A

white precipitate of silver chloride

41
Q

what colour precipitate does a bromide ion produce?

A

cream precipitate of silver bromide

42
Q

what colour precipitate does an iodide ion produce?

A

yellow precipitate of silver iodide

43
Q

how do you test for sulfate ions?

A
  1. add dilute hydrochloric acid to the sample
  2. add barium chloride solution
  3. if sulfate ions are present, we’ll see a white precipitate
44
Q

what is the difference between a pure substance in chemistry and a pure substance in everyday life?

A

chemistry: single element/compound
everyday: substance that’s had nothing added to it

45
Q

what causes different substances to move up the paper differently?

A

the different substances in a mixture are attracted to the two phases in different proportions. they therefore move at different rates throughout the paper

e.g. a substance that’s greatly attracted to the mobile phase will move more rapidly

chemistry (12 decks)

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