C12 - Chemical Analysis

Question 1

What is a formulation?

Answer 1

• A useful mixture with a precise purpose
• Each component is present in a measured quantity, and contributes to the properties of the formulation so that it meets its required function.
• (e.g. Pigment, solvent, binder, additives)

Question 2

What are the two phases called in chromatography?

Answer 2

• A mobile phase (molecules can move): always a liquid or a gas
• A stationary phase (molecules can’t move): either solid or thick liquid.

Question 3

In paper chromatography, how many spots will a pure substance form on the paper?

Answer 3

• One spot

- Only one substance in the sample

Question 4

Give the formula for working out the R(f) value of a substance.

Answer 4

R(f) = distance travelled by substance / distance travelled by solvent

Question 5

Would you expect the R(f) value of a substance to change if you changed the solvent used in the chromatography experiment?

Answer 5

Yes

Question 6

What colour does litmus paper turn in the presence of chlorine

Answer 6

Damp litmus paper gets bleached

Question 7

Describe the test you could use to test for the presence of carbonate ions.

Answer 7

=> Put sample of solution in test tube
=> Add dilute acid
=> Connect test tube to a test tube of limewater
=> Carbonates present = limewater turns cloudy when solution is bubbled through

Question 8

What is the positive test for oxygen?

Answer 8

Oxygen will relight a glowing splint

Question 9

What will happen when a lighted splint comes into contact with hydrogen?

Answer 9

It will make a ‘squeaky pop’

Question 10

Describe a chemical test for sulfates.

Answer 10

=> Dilute HCl and barium chloride solution into mystery solution.
=> If sulfate ions present = white precipitate of barium sulfate is formed

Question 11

How do you test for the halides and what are the positive results?

Answer 11

=> Dilute nitric acid then silver nitrate solution
=> Chloride = white precipate (silver chloride)
=> Bromide = cream precipitate (silver bromide)
=> Iodide = yellow precipitate (silver iodide)

Question 12

Why is hydrochloric acid added for the test for sulfates?

Answer 12

=> Gets rid of any traces of carbonate ions

=> These would also produce a white precipitate so would confuse the results

Question 13

How do you conduct a flame test?

Answer 13

=> Clean a platinum wire loop by dipping it in dilute HCl then burn in blue flame until it burns with no colour
=> Dip the loop into the sample you want to test then and put it back in the flame.

Question 14

What are the colours of the flame tests for Lithium, Sodium, Potassium, Calcium and Copper?

Answer 14

• Lithium = crimson
• Sodium = yellow
• Potassium = lilac
• Calcium = orange/red
• Copper = green

Question 15

What is the main disadvantage of flame tests?

Answer 15

• Only work for samples that contain a single ion

- If sample tested has mixture of metal ions, flame colours of ions may be hidden by colours of others

Question 16

Describe the precipitates that are formed from NaOH and Calcium, Copper (II), Iron (II), Iron (III), Aluminium and Magnesium.

Answer 16

• Calcium = White
• Copper (II) =Blue
• Iron (II) = Green
• Iron (III) = Brown
• Aluminium = White at first but dissolves to form colourless solution
• Magnesium = White

Question 17

Give the ionic equation for the formation of a precipitate when a solution containing magnesium ions is reacted with sodium hydroxide.

Answer 17

Mg(2+) + 2OH(-) –> Mg(OH)(2)

Question 18

Describe how flame emission spectroscopy works.

Answer 18

=> Sample placed in flame
=> Ions heat up - electrons become excited
=> Drop back to original levels and release energy as light
=> Light passes through spectroscope - detects different wavelengths of light - produces line spectrum

Question 19

What does the intensity of a flame emission spectrum of a solution relate to?

Answer 19

The concentration of that ion in the solution

Question 20

Give three advantages and three disadvantages of using machines to conduct chemical analysis.

Answer 20

ADVANTAGES:
- Very sensitive: can detect tiny amounts
- Very fast and tests can be automated
- Very accurate
DISADVANTAGES:
- Equipment is very expensive
- Equipment requires special training to use
- Line spectrums can only be identified through comparison with pre-existing data.