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GCSE Chemistry paper 2 > Chemical Analysis > Flashcards

Chemical Analysis Flashcards

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1
Q

What is a pure substance?

A

A single compound or a substance made of a single element, that is not mixed with any other.

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2
Q

What are some qualities of pure substance?

A
  • Have specific boiling points,

- Have specific melting points

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3
Q

What does the graph of a pure substance look like and why?

A
  • When you melt a pure substance (solid), the temperature will increases until the entire substance is a liquid,
  • There will then be a flat area on a graph, where the temperature does not change for a while because a pure substance has a fixed melting point,
  • Then the graph will start to increase (temp) again as the liquid becomes a vapour,
  • When the liquid is fully vaporised the temperature increase will stop and another straight section on the graph will appear, this is the fixed boiling point of the substance.
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4
Q

How does the graph of an impurity differ from a pure substances graph?

A

The impure substances graph has multiple changes in the rate temperature increase because there are lots of different substances that have different melting and boiling points s the graph pauses in lots of places.

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5
Q

What is a formulation?

A
  • A complex mixture that has been designed as a useful product.
  • In a formulation the quantities of each substance is carefully measured so that the product has the properties we need.
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6
Q

What are some examples of formulation?

A
  • Fuel,
  • Cleaning products,
  • Paints,
  • Medicines,
  • Alloys,
  • Fertilisers,
  • Food.
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7
Q

What are the 5 ways to separate mixtures?

A
  • Filtration,
  • Crystallisation,
  • Simple distillation
  • Fractional distillation,
  • Chromatography.
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8
Q

What type of process is paper chromatography?

A
  • Physical,

- This means that no chemical reactions happen and no new substances are made.

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9
Q

Why does chromatography work?

A

Some substances are more soluble than others, so the ink will seperate as the substances move up further than others within the mixture.

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10
Q

What are the names of the 2 phases in paper chromatography and where do they occur?

A
  • The paper is called the stationary phase because it does not move,
  • The solvent is called the mobile phase because it does move.
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11
Q

Why do substances separate in chromatography?

A
  • Each chemical in the mixture will be attracted to the stationary phase (the paper), but to different extents,
  • Chemicals with a strong attraction wont move up the paper much,
  • But chemicals with a weak attraction will move up further,
  • This separates the mixture into different substances, as they all have different. strengths of attraction to the stationary phase.
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12
Q

What happens when a different solvent is used with the same mixture in paper chromatography?

A
  • Their will be the same colours/ spots but the position of the spot on the paper will change.
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13
Q

Why do we draw a starting line in pencil?

A

If we drew it in pen, the pen ink would move up the paper with the solvent and make the results invalid.

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14
Q

How do you use paper chromatography to identify an unknown substance in a mixture?

A
  • First draw a horizontal pencil line, 2cm from the bottom of the paper
  • Then put 5 dots (using the pencil) on this pencil line, each 1cm apart from each other,
  • Next use a capillary tube (a very thin glass tube) to place the 4 known colours and the 1 unknown colour onto the pots,
  • These spots must be incredibly small so the inks don’t mix,
  • Now pour a solvent into a beaker with 1cm depth,
  • Attach the top of the paper to a glass rod using tape and balance the rod on the beaker,
  • The paper should dip into the solvent,
  • The solvent should move up the paper and the colours will be carried up,
  • remove the paper when the solvent has travelled 3/4 up,
  • Use a pencil to mark where the solvent got up to and leave the paper out to dry,
  • You should see that the unknown mixture has separated into its inks, these may line up with other spots from the known substances and therefore, be the same substance.
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15
Q

What are some cautions that you must take when doing a paper chromatography practical?

A
  • The pencil line must be above the solvent so the ink is not washed off the line/paper,
  • The sides of the paper must also not touch the sides of the beaker because it can interrupt the solvent movement,
  • Put a lid on the beaker to reduce evaporation of the solvent.
  • You should try not to touch the beaker so that none of the inks make contact when the solvent moves up the paper.
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16
Q

How do you find the chemicals in an unknown ink sample after paper chromatography?

A
  • Calculate the RF values,
  • Measure the distance form the pencil lines to the centre of each spot,
  • Then measure the distance moved by solvent,
  • To calculate the RF value, divide the distance the spot moved by the distance the solvent moved,
  • Then compare to a database and find the identity of the chemical.
  • Repeat this experiment with other solvents in order to find the correct RF value because some substances have the same in particular solvents,
  • If the substance does not have an RF value because it has not been analysed before then further analysis will need to be used to identify the substance.
17
Q

How do you test for Hydrogen?

A
  • If you put a lit flame near hydrogen gas it makes a pop sound,
  • This is called the squeaky pop test.
18
Q

How do you test for Oxygen?

A
  • If you put a glowing splint in a test tube of oxygen, the splint will relight,
  • This is because oxygen is a fuel fro combustion.
19
Q

How do you test for CO2?

A
  • Draw a some of an unnkonw gas using a pippete,
  • Then bubble the gas into lime water ( Calcium hydroxide),
  • If it turns cloudy, that gas is CO2.
20
Q

How do you test for Chlorine?

A
  • Insert damp litmus paper into a test tube with an unknown substance,
  • If the paper bleaches, chlorine is present.
21
Q

How do you carry out a flame test?

A
  • First place a small amount of chemical onto a wire mounted on a handle (like an inoculating loop),
  • Then place the end of this onto a blue Bunsen burner flame,
  • The change of colour will tell you what chemical is on the end of your wire.
22
Q

What are the coloured flames associated with: lithium, sodium, potassium, calcium and Copper ions

A
  • lithium = crimson
  • sodium = yellow
  • potassium = lilac
  • calcium = orange-red
  • copper = green
23
Q

What are issues with using the flame test?

A
  • The colour of the flame can be difficult to distinguish, especially with low concentration of the metal compound,
  • Sometimes a sample contains a mixture of metal ions, some colours can mask other (sodium’s yellow mask all other colour flames).
24
Q

What is the process of flame emission spectroscopy?

A
  • A sample of metal ion in a solution is placed into a flame,
  • The light given out is then passed into a machine called a spectroscope,
  • This converts the light into a line spectrum,
  • Each metal has a different line spectrum so it is easier to distinguish between each metal ion this way.
25
Q

What else can flame emission spectroscopy be used for, along side finding out an unknown metal ion?

A
  • It can also tell us the concentration of the metal ion,

- The lines on the line spectrum are more intensive at higher concentrations.

26
Q

What is an instrumental method?

A

An experiment that is carried out by a machine (e.g. spectroscopy)

27
Q

What are the advantages of using instrumental methods?

A
  • Very fast,
  • Sensitive to low concentrations,
  • Accurate.
28
Q

How can you use sodium hydroxide in order to find out what metal ion is present in a mixture?

A
  • Add sodium hydroxide to the metal ion solutions,
  • This will from a metal precipitate,
  • you can then use the flame test with the precipitates.
29
Q

What happens to an aluminium ion solution that is mixed with excess sodium hydroxide?

A
  • First the solution will have a white precipitate of aluminium,
  • Then the excess sodium hydroxide redissolves the aluminium precipitate back into the solution.
30
Q

What happens when a copper (II) ion solution is mixed with sodium hydroxide?

A

A blue precipitate forms

31
Q

What happens when a iron (II) ion solution is mixed with sodium hydroxide?

A

A green precipitate forms,

32
Q

What happens when a iron (III) ion solution is mixed with sodium hydroxide?

A

A brown precipitate forms

33
Q

How do you test for carbonate ions?

A
  • Add dilute acid to the
    unknown sample,
  • The acid will react with the carbonate (if present) to make CO2 gas, the solution will fizz,
  • Then bubble the gas into lime water using capillary tubes through a bong, if the lime water goes cloudy, the solution was a carbonate.
34
Q

How do you test for halide ions?

A
  • First add nitric acid to the unknown sample,
  • Then add silver nitrate solution,
  • Each halide ion produces a different colour precipitate of the silver halide.
35
Q

What colour precipitate do chloride, bromide and iodide ions make in the halide test?

A 
Chloride = white
Bromide = cream
Iodide = Yellow
36
Q

How do you test for sulfate ions?

A
  • Add hydrochloric acid to the unknown sample,
  • Then add barium chloride solution,
  • If sulfate ions are present then a white precipitate will from.

GCSE Chemistry paper 2 (5 decks)

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