C12

Question 1

What is chemical purity ?

Answer 1

• substance made of just one substance, nothing else
• can either be element or compound

Question 2

How can you tell if a substance is chemically pure ?

Answer 2

• using its fixed points (mp + bp)
• a pure substance will have fixed points that can be found in a databank
• if the fixed points of the substance don’t adhere to what they’re supposed to be, the substance is not pure and is a mixture

Question 3

What effect does an impurity have on a substance’s fixed points ?

Answer 3

• lowers mp
• raises bp
• widens range of mps/bps (eg. an impure substance will have a range such as 180’C - 220’C for its mp, whilst the purer version may have 210’C - 220’C)

Question 4

What is a formulation ?

Answer 4

mixtures with fixed proportions of their components designed to be useful products
- paints
- drugs
- washing up liquids
- fuels
- alloys
- fertilisers
etc. are all formulations

Question 5

Explain simply how paper chromatography works.

Answer 5

• mobile phase (solvent chosen)
• stationary phase (paper)
• base line drawn in pencil at bottom of paper
• dots of substance being tested placed on base line
• paper submerged in solvent until just above base line
• wait
• draw solvent front in pencil
• measure distance from base to solvent front, and from base to the end of the substance you want to identify
• divide the base to substance length by base to solvent front length to get Rf value
• compare Rf value to a database to identify the substance

Question 6

What is the positive test for hydrogen gas ?

Answer 6

burns rapidly with a squeaky pop when lit splint is applied

Question 7

What is the positive test for oxygen gas ?

Answer 7

relights a glowing splint

Question 8

What is the positive test for carbon dioxide gas ?

Answer 8

turns limewater milky/cloudy

Question 9

What is the positive test for chlorine gas ?

Answer 9

bleaches DAMP blue litmus paper white

Question 10

Describe how to carry out a flame test for positive (metal) ions.

Answer 10

• dip nichrome wire loop in conc. hydrochloric acid
• burn to sterilise and clean
• do not set down or blow on it whilst cooling
• dip into acid again
• dip into metal compound being tested
• burn in ROARING BLUE flame of bunsen burner
• use colour of resulting flame to identify metal ion
• if it is a mixture, the colour will be muddy or may be masked by stronger colours of certain metal ions in the mixture

Question 11

What colour flame do lithium ions produce ?

Answer 11

crimson

Question 12

What colour flame do sodium ions produce ?

Answer 12

yellow

Question 13

What colour flame do
potassium ions produce ?

Answer 13

lilac

Question 14

What colour flame do calcium ions produce ?

Answer 14

orange-red

Question 15

What colour flame do copper (II) ions produce ?

Answer 15

green

Question 16

Describe how to conduct a test for positive ions using sodium hydroxide ?

Answer 16

• in a boiling tube, add the unknown metal compound
• add sodium hydroxide
• swirl to mix
• observe the precipitate formed
• if white precipitate, add more sodium hydroxide
• if this precipitate dissolves, positive test for aluminium
• if not, do a flame test to determine if it is magnesium or calcium
• if original precipitate is coloured, can either be copper (II), iron (III) or iron (II)

Question 17

What colour flame do magnesium ions produce ?

Answer 17

no colour

Question 18

What colour precipitate do calcium ions produce ?

Answer 18

white

Question 19

What colour precipitate do magnesium ions produce ?

Answer 19

white

Question 20

What colour precipitate do aluminium ions produce ?

Answer 20

white - dissolves in excess sodium hydroxide

Question 21

What colour precipitate do copper (II) ions produce ?

Answer 21

light blue

Question 22

What colour precipitate do iron (II) ions produce ?

Answer 22

light green, slowly turns brown

Question 23

What colour precipitate do iron (III) ions produce ?

Answer 23

reddish-brown

Question 24

Describe how to test for a metal carbonate.

Answer 24

• in one boiling tube, hydrochloric acid with the carbonate
• air tube connected to it leading to submerged in limewater
• if limewater turns milky, is carbonate
• this is because HCl + metal carbonate –> [metal] chloride + water + CO2 gas

Question 25

Describe how to test for halide ions in a compound.

Answer 25

• compound in boiling tube
• add dilute nitric acid to remove other ions and carbonate ions as would also form precipitates
• add silver nitrate solution
• observe precipitate formed

Question 26

What colour precipitate do iodide ions form in silver nitrate solution ?

Answer 26

yellow

Question 27

What colour precipitate do bromide ions form in silver nitrate solution ?

Answer 27

cream

Question 28

What colour precipitate do chloride ions form in silver nitrate solution ?

Answer 28

white

Question 29

Describe how to test for sulfates in a compound.

Answer 29

• add dilute hydrochloric acid to remove any carbonate ions that would also form a precipitate
• add barium chloride solution
• observe precipitate formed
• white precipitate = sulfate ions present (is insoluble salt barium sulfate)

Question 30

Give three advantages of instrumental analysis methods.

Answer 30

• faster than old methods
• more accurate
• can analyse very small samples (ideal for testing blood in a medicinal capacity)

Question 31

Give three disadvantages of instrumental analysis methods.

Answer 31

• expensive machinery
• can only be used in conjunction with a database
• takes special training to use machinery

Question 32

Explain how flame emission spectroscopy works.

Answer 32

• heats sample
• stops heating sample and sample dissipates the thermal energy as light energy
• each metal ion has characteristic patterns of radiation (line spectrum)
• machine records this
• can compare the line spectrum of the sample to metals from the database in order to identify the metal ion present