chemical analysis

Question 1

What is a pure substance?

Answer 1

A pure substance is a single element or compound, not mixed with any other substances.

Question 2

How can you distinguish pure substances from mixtures?

Answer 2

Pure elements and compounds have a specific fixed melting and boiling point, which can be used to distinguish mixtures from pure substances as mixtures melt and boil over a range of temperatures.

Question 3

What is an impure substance?

Answer 3

An impure substance is a mixture.

Question 4

What is a formulation?

Answer 4

A formulation is a mixture that has been designed for a useful product. The quantity of each component is carefully measured to ensure that the product has the required properties.

Question 5

What is chromatography?

Answer 5

Paper chromatography is a separation technique, which is a physical process. It allows substances to be separated based on their solubilities.

Question 6

What is a physical process?

Answer 6

A physical process is a separation technique which does not involve chemical reactions and no new substances are made.

Question 7

What is the stationary phase in chromatography?

Answer 7

The stationary phase in chromatography is the paper as it does not move.

Question 8

What is the mobile phase in chromatography?

Answer 8

The mobile phase in chromatography is the solvent as it moves.

Question 9

What is a solvent?

Answer 9

A solvent is a liquid that will dissolve a substance.

Question 10

How can a mixture be identified in chromatography?

Answer 10

A mixture can be identified in chromatography as it will separate into multiple dots on the chromatogram. These spots may be different in different solvents.

Question 11

How can a pure substance be identified in chromatography?

Answer 11

A pure substance can be identified in chromatography as it will remain as one dot on the chromatogram. This is the same spot at the same level in all solvents.

Question 12

How does chromatography work?

Answer 12

Paper chromatography works because the chemical is attracted to the stationary phases to a different extent. If they are strongly attracted, they won’t move far.

Question 13

Why is the line on chromatography paper in pencil?

Answer 13

The line is in pencil as it does not travel up the solvent.

Question 14

What does it mean if two dots are at the same level on two different chromatograms?

Answer 14

They are the same substance.

Question 15

How do you calculate the Rf value?

Answer 15

Rf = distance moved by chemical / distance moved by solvent

• each substance has a different Rf value
• some may have the same Rf value, so repeat to narrow it down

Question 16

Why are flame tests not good for a mixture of ions?

Answer 16

Flame tests are not good for a mixture of ions as some of the flame colours may be masked.

Question 17

Describe how flame emission spectroscopy works.

Answer 17

flame emission spectroscopy:

• a sample of a metal ion is placed into a flame
• the light given out is passed through a spectroscope
• this coverts the light into a line spectrum, and these lines are specific to the metal ion
• the concentration can be told as higher conc. means thicker lines
• this is an instrumental method

Question 18

Give seven examples of formulations.

Answer 18

formulation examples:

• fuels
• cleaning agents
• paints
• medicine
• alloys
• fertilisers
• food

Question 19

Give three examples of why flame emission spectroscopy (instrumental method) is better than flame tests.

Answer 19

flame emission spectroscopy > flame tests as:

• they are more rapid
• they are more sensitive (FES will work on a tiny sample)
• they are more accurate