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Chemistry GCSE 2020 > C8: Chemical analysis > Flashcards

C8: Chemical analysis Flashcards

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1
Q

How would you tell whether a substance is pure?

A
  • Testing the melting/boiling point. If not what expected, it is a mixture.
  • For an ink/dye, chromatography can be used
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2
Q

What are formulations?

A

Mixtures, designed to be useful products, made by mixing the components in exact quantities.

E.g. fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

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3
Q

What is chromatography used for?

A

Can be used to separate mixtures (inks/dyes) and can give information to help identify substances.

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4
Q

What are the mobile and stationary phases in chromatography?

A
  • Mobile phase = solvent (bc it moves up the paper).
  • Stationary phase = pencil line (it doesn’t move).
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5
Q

Practical

Explain how paper chromatography can be used to separate mixtures.

A
  • Draw a line near the bottom of a sheet of filter paper in pencil (since it’s insoluble).
  • Add a spot of ink to this line.
  • Place the sheet in a beaker of solvent (depends on what’s being tested), making sure the ink doesn’t touch the solvent.
  • Put a lid on the container to stop the solvent evaporating.
  • The mobile phase (solvent) seeps up the paper, carring the ink with it.
  • Each dye within the ink moves up the paper at a different rate; the dyes separate out into spots.
  • If any of the dyes are insoluble, they stay on the stationary phase (pencil line).
  • When the solvent has nearly reached the top, the paper is taken out and allowed to dry; the result is a chromatogram.
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6
Q

How would you calculate Rf value in chromatography?

A

distance moved by substance / distance moved by solvent

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7
Q

How would you test for hydrogen?

A
  • Hold a burning splint over a test tube of the gas
  • Positive result: burns rapidly and makes a “pop” sound
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8
Q

How would you test for oxygen?

A
  • Insert a glowing splint into a test tube of the gas
  • Positive result: the splint relights
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9
Q

How would you test for carbon dioxide?

A
  • Use an aqueous solution of calcuim hydroxide (limewater)
  • Bubble the gas through the limewater
  • Positive result: limewater goes cloudy/milky
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10
Q

How would you test for chlorine?

A
  • Place damp litmus paper into the gas
  • Positive result: litmus paper turns white (is bleached)
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11
Q

Practical

How would you carry out a flame test?

A
  • Clean a nichrome wire loop by dipping into hydrochloric acid, holding over a blue bunsen burner flame and redipping in the acid.
  • Dip the loop into the metal compound being tested.
  • Hold the loop in the flame and record the colour to identify the ion.

• Only works with one type of ion in the sample, because some flame colours may be masked in mixtures

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12
Q

What colour flame is produced by heating lithium ions?

A

Crimson

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13
Q

What colour flame is produced by heating sodium ions?

A

Yellow

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14
Q

What colour flame is produced by heating potassium ions?

A

Lilac

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15
Q

What colour flame is produced by heating calcium ions?

A

Orange-red (scarlet)

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16
Q

What colour flame is produced by heating copper ions?

A

Green

17
Q

What may happen if you flame tested a mixture of cations?

A

Some flame colours may be masked by others.

18
Q

What colour precipitate do dissolved calcium ions form when sodium hydroxide solution is added to the solution?

A

White

19
Q

What colour precipitate do dissolved magnesium ions form when sodium hydroxide solution is added to the solution?

A

White

20
Q

What colour precipitate do dissolved aluminium ions form when sodium hydroxide solution is added to the solution?

A

White, but redissolves in excess sodium hydroxide solution.

21
Q

What colour precipitate do dissolved copper (II) ions form when sodium hydroxide solution is added to the solution?

A

Blue

22
Q

What colour precipitate do dissolved iron (II) ions form when sodium hydroxide solution is added to the solution?

A

Green

23
Q

What colour precipitate do dissolved iron (III) ions form when sodium hydroxide solution is added to the solution?

A

Brown

24
Q

Write the balanced ionic equation for the reaction between dissolved iron (III) ions and sodium hydroxide solution. Include state symbols.

A

Fe3+(aq) + 3OH-(aq) →Fe(OH)3 (s)

25
Q

How would you test for carbonate ions?

A
  • They react with dilute acids to form carbon dioxide gas
  • CO2 can be identified by bubbling it through limewater
  • If CO2 is present, the limewater goes cloudy
26
Q

How would you test for chloride ions?

A
  • Add dilute nitric acid to a solution of the ions tested
  • Then add silver nitrate solution
  • Positive result: a white silver chloride precipitate forms
27
Q

How would you test for bromide ions?

A
  • Add dilute nitric acid to a solution of the ions tested
  • Then add silver nitrate solution
  • Positive result: a cream silver bromide precipitate forms
28
Q

How would you test for iodide ions?

A
  • Add dilute nitric acid to a solution of the ions tested
  • Then add silver nitrate solution
  • Positive result: a yellow silver iodide precipitate forms
29
Q

How would you test for sulfate ions?

A
  • Add dilute hydrochloric acid to a solution of the ions being tested
  • Then add barium chloride solution
  • Positive result: a white precipitate forms
30
Q

Elements and compounds can be detected and identified using instrumental methods, such as flame emission spectroscopy, instead of chemical tests.

Name 3 advantages of using instrumental methods.

A
  • accurate
  • sensitive - only an tiny sample needed
  • rapid + can be automated
31
Q

Explain how flame emission spectroscopy works.

A
  • Used to analyse metal ions in solutions
  • Sample is put into a flame
  • The light given out is passed through a spectroscope
  • The output is a line spectrum
  • This can be analysed to identify the metal ions in the solution, and measure their concentrations
32
Q

Why is flame emission spectroscopy more useful than flame tests?

A

It can be used to identify different ions in mixtures, whereas flame tests can only identify single ions.

33
Q

In flame emission spectroscopy, each type of ion produces an individual line spectrum. What do the combination of wavelengths emitted by an ion depend on?

A
  • its charge
  • its electron arrangement
34
Q

What indicates the concentration of an ion in flame emission spectroscopy?

A

The intensity of the colours.

Chemistry GCSE 2020 (10 decks)

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