C8: Chemical analysis

Question 1

What is chemical analysis?

Answer 1

The process of establishing what chemicals are present in a substance

Question 2

What is a pure substance?

Answer 2

• a pure substance is a single element or compound , not mixed with any other substance

Question 3

What is a formulation and how is it made? Give an example

Answer 3

• a mixture of compounds in measured quantities that has been designed as a useful product
• formulations include fuels, cleaning agents, paints , medicines, alloys

Question 4

What is chromatography?

Answer 4

A process to separate the constituents of a mixture

Question 5

In paper chromatography, what is
the stationary phase and what is
the mobile phase

Answer 5

Paper is stationary, solvent (usually water or ethanol) is mobile

Question 6

How can chromatography show
the difference between pure and
impure substances?

Answer 6

Pure ones will not separate into a number of spots

Question 7

How is the Rf value calculated?

Answer 7

Rf = distance moved by spot/distance moved by solvent

Question 8

Describe the tests for hydrogen, oxygen, carbon dioxide and chlorine.

Answer 8

hydrogen: pop with burning splint over gas
oxygen: glowing splint relights
carbon dioxide: turns limewater milky
chlorine: bleaches damp litmus paper and makes it white

Question 9

Describe the 5 flame test results

Answer 9

lithium compounds - crimson red flame
sodium compounds - yellow flame
potassium compounds - lilac flame
calcium compounds - orange red flame
copper compounds - green flame

Question 10

In chromatography, why must the
substances be placed on a pencil
line?

Answer 10

Pencil will not dissolve in the solvent

Question 11

In chromatography why must the
solvent height be lower than the
pencil line?

Answer 11

So that the substances do not dissolve into the solvent off the paper

Question 12

Describe the sodium hydroxide test results for
- aluminium
- calcium
- magnesium

Answer 12

they all form white precipitates of ( metal) hydroxide

Question 13

How would you
differentiate between
aluminium ions in one
solution , calcium ions in
a second and
magnesium ions in a
third solution

Answer 13

● Only the aluminium hydroxide precipitate will
redissolve in excess sodium hydroxide
● The solutions containing calcium and
magnesium can be distinguished using a flame
test. The solution containing calcium will
produce an orange-red flame colour

Question 14

Describe the sodium hydroxide results for:
- copper (||) ions
- iron (||) ions
- iron (|||)

Answer 14

copper - blue precipitate of copper hydroxide
iron (||) - Green precipitate of iron (II) hydroxide
iron (|||) - Brown precipitate of iron (III) hydroxide

Question 15

Write the balanced and ionic
equation for the reaction
between copper (II)
sulphate and sodium
hydroxide

Answer 15

balanced: CuSO₄ (aq) + 2NaOH (aq) ➝ Cu(OH)₂ (s) + Na₂SO₄ (aq)

ionic: Cu₂₊ (aq)+ 2OH- (aq) ➝ Cu(OH)₂ (s)

Question 16

Write the balanced and ionic equation for the reaction between magnesium sulphate and sodium hydroxide

Answer 16

balanced : MgSO₄ (aq) + 2NaOH (aq) ➝ Mg(OH)₂
(s) + Na₂SO₄ (aq)
ionic: Mg₂₊ (aq) + 2OH- (aq) ➝ Mg(OH)₂ (s)

Question 17

Write the balanced and ionic equation for the reaction between iron (|||) chloride and sodium hydroxide?

Answer 17

balanced: FeCl₃ (aq) + 3NaOH (aq) ➝ Fe(OH)₃ (s) + 3NaCl (aq)
ionic: Fe₃₊ (aq) + 3OH-(aq) ➝ Fe(OH)₃ (s)

Question 18

2. Describe how to
   test for the presence of
   carbonate ions.

Answer 18

Add dilute acid to sample

Question 19

Give 2 observations
seen if carbonate ions
are present

Answer 19

● Effervescence
● Bubble the gas into a test tube containing limewater.
- The limewater will turn cloudy
showing the presence of carbon dioxide

Question 20

Describe how to test for the presence of
sulphate ions. Give the observation seen for a
positive result

Answer 20

● Add dilute hydrochloric acid to the sample to
remove any carbonates
● Add barium chloride to the sample
● White precipitate of barium sulphate formed if the sample contained sulphates

Question 21

Describe how to test for the presence of
halide ions. Give the observation seen for the
presence of chloride,bromide and iodide ions

Answer 21

● Add dilute nitric acid to remove any carbonates
● Add silver nitrate
● Chloride ions: white precipitate of silver
chloride forms
● Bromide ions: cream precipitate of silver
bromide forms
● Iodide ions: yellow precipitate of silver iodide
forms

Question 22

Describe the flame emission spectroscopy

Answer 22

a) instrumental method used for identifying metal ions in solution or measuring their concentration
b) Spectroscope measures the exact wavelength of the light emitted by a metal ion
c) That allows for definite identification - sometimes colours are difficult to distinguish
d) Concentrations are found by measuring the intensity of light emitted. The more intense light, the greater the concentration of the metal ion in a solution
e) from the intensity vs concentration graph, you can read off a relevant concentration value at a given intensity

Question 23

what are cations and anions?

Answer 23

cations: positive ions
anions: negative ions