C8: Chemical analysis

Question 1

How would you tell whether a substance is pure?

Answer 1

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

Question 2

What are formulations?

Answer 2

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.

Question 3

What is chromatography used for?

Answer 3

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

Question 4

What are the mobile and stationary phases in chromatography?

Answer 4

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

Question 5

Practical

Explain how paper chromatography can be used to separate mixtures.

Answer 5

• 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.

Question 6

How would you calculate R_f value in chromatography?

Answer 6

distance moved by substance / distance moved by solvent

Question 7

How would you test for hydrogen?

Answer 7

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

Question 8

How would you test for oxygen?

Answer 8

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

Question 9

How would you test for carbon dioxide?

Answer 9

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

Question 10

How would you test for chlorine?

Answer 10

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

Question 11

Practical

How would you carry out a flame test?

Answer 11

• 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

Question 12

What colour flame is produced by heating lithium ions?

Answer 12

Crimson

Question 13

What colour flame is produced by heating sodium ions?

Answer 13

Yellow

Question 14

What colour flame is produced by heating potassium ions?

Answer 14

Lilac

Question 15

What colour flame is produced by heating calcium ions?

Answer 15

Orange-red (scarlet)

Question 16

What colour flame is produced by heating copper ions?

Answer 16

Green

Question 17

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

Answer 17

Some flame colours may be masked by others.

Question 18

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

Answer 18

White

Question 19

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

Answer 19

White

Question 20

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

Answer 20

White, but redissolves in excess sodium hydroxide solution.

Question 21

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

Answer 21

Blue

Question 22

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

Answer 22

Green

Question 23

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

Answer 23

Brown

Question 24

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

Answer 24

Fe³⁺_(aq) + 3OH^-_(aq) →Fe(OH)_{3 (s)}

Question 25

How would you test for carbonate ions?

Answer 25

* 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

Question 26

How would you test for chloride ions?

Answer 26

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

Question 27

How would you test for bromide ions?

Answer 27

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

Question 28

How would you test for iodide ions?

Answer 28

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

Question 29

How would you test for sulfate ions?

Answer 29

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

Question 30

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.

Answer 30

* accurate * sensitive - only an tiny sample needed * rapid + can be automated

Question 31

Explain how flame emission spectroscopy works.

Answer 31

* 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

Question 32

Why is flame emission spectroscopy more useful than flame tests?

Answer 32

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

Question 33

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?

Answer 33

* its charge * its electron arrangement

Question 34

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

Answer 34

The intensity of the colours.