C8 Chemical analysis

Question 1

Plan an investigation to determine the Rf value for the dye in this food colouring

Answer 1

• Draw start line in pencil on bottom of chromatography paper and add dye with a capillary tube as small dots
• Fill beaker with a solvent
• Place chromatography paper in solvent ensuring solvent is below the start line
• use a lid to prevent evaporation
• remove once solvent travels 3/4 up the paper
• mark the solvent front
• dry chromatography paper
• measure distance from starting line to centre of spot and to solvent front using a ruler, use these measurements to calculate Rf value (distance by substance/distance by solvent)

Question 2

What does Rf value depend on?

Answer 2

• the solubility of the dye in the solvent: more soluble means will spend longer in solvent and travel further up
• the attraction of the dye to the paper: as if more attracted substance will move slower

Question 3

Why may less colours show up that the number of compounds a dye really has?

Answer 3

Some compounds may be colourless

Question 4

Explain how the different dyes are separated by paper chromatography

Answer 4

Solvent moves up the paper
Different dyes have different solubilities in solvent and different attractions for the paper so are carried at different distances

Question 5

Why may a colour not appear in a flame test testing two samples?

Answer 5

Not cleaned wire before reusing so colour appears to be ‘masked’ as they are mixed (as two different colours present)

Question 6

Describe the test for carbonates

Answer 6

Add hydrochloric acid and wait until effervescence given out. Collect and bubble through limewater which will turn cloudy in the presence of carbon dioxide (as a precipitate of carbonate has formed)

Question 7

What is the problem with the solvent being above the starting line?

Answer 7

Dye will just dissolve into the solvent and so not travel up the paper

Question 8

What is the problem with the starting line being drawn in ink?

Answer 8

Ink is soluble, and so would travel (‘run’) up the paper

Question 9

Why does limewater turn cloudy?

Answer 9

Presence of carbon dioxide reacting with limewater forming a precipitate

Question 10

How could the water be tested to show it is pure?
Give the expected result of the test for pure water

Answer 10

Determine boiling point by using a thermometer, should be at a fixed temperature of 100 degrees

Question 11

Give 2 reasons why some questions cannot be answered by science alone

Answer 11

• based on personal opinion
• cannot be done by experiment

Question 12

What does chromatography do?

Answer 12

Identify solvents (by separating mixtures based on their solubilities)

Question 13

Explain how paper chromatography could be used to confirm that something is a permitted additive.

Answer 13

use solvent to separate dye. calculate Rf value and compare to permitted additive.

Question 14

What is a formulation?

Answer 14

A mixture measured in careful quantities designed as a useful product

Question 15

How can you tell an ink spot is a mixture?

Answer 15

There is more than one spot

Question 16

How can you tell an ink spot contains the same colour as another?

Answer 16

Rf value matches

Question 17

How can you tell an ink spot is the most soluble?

Answer 17

Has the highest Rf value

Question 18

How do you know an ink spot is insoluble?

Answer 18

Remains on the starting line

Question 19

Plan an investigation to show presence of lithium ions and carbonate ions in a tablet

Answer 19

• Crush tablet using a pestle and mortar
• Clean a metal wire and dip it in the crushed sample then pass through the roaring flame and observe
• Flame will turn crimson red in presence of lithium ions
• Remaining sample of lithium carbonate is reacted with dilute hydrochloric acid until effervescence is seen
• Collect the gas and bubble it through limewater
• Limewater will turn cloudy in the presence of carbon dioxide gas, showing carbonate ions were present

Question 20

Describe the test and result for oxygen

Answer 20

Hold a glowing splint in the open end of a gas tube
The splint relights in the presence of oxygen

Question 21

Give the test and result for sulphate ions

Answer 21

React with barium chloride then add dilute hydrochloric acid
White precipitate forms

Question 22

What conclusions can you make with a chromatogram?

Answer 22

• a pure substance produces one spot on the chromatogram
• an impure substance, or mixture, produces two or more spots
• substances likely to be the same if spots have the same Rf value and colour
• most soluble spots have the highest Rf value

Question 23

How can you increase distance between spots

Answer 23

Allow solvent to travel further
Use a different solvent

Question 24

Explain why a manufacturer will want a pen to always use the same proportions of dye

Answer 24

It is a formulation - the manufacturer wants to have the exact shade of the pen to sell