C12 Chemical Anaylsis

Question 1

what is a pure substance?

Answer 1

a substance which is made up of just one substance (an element or compound)

Question 2

what are the fixed points of a substance?

Answer 2

the melting and boiling points of an element or compound

Question 3

what do impurities do to the melting and boiling points of a substance?

Answer 3

lower the melting point
increase the boiling point

Question 4

what is a formulation?

Answer 4

a mixture that has been designed as a useful product with set proportions of substances in the mixture

Question 5

what is the Rf value of a substance?

Answer 5

a measurement from chromatography: the distance a spot of substance has been carried above the base line divided by the distance of the solvent front

Question 6

what is a positive test for hydrogen?

Answer 6

a lighted splint ‘pops’

Question 7

what is the positive test for oxygen?

Answer 7

a glowing splint relights

Question 8

what is a positive test for carbon dioxide gas?

Answer 8

limewater turns milky (cloudy white)

Question 9

what is a positive test for chlorine gas?

Answer 9

damp blue litmus paper turns white (as it gets bleached)

Question 10

what is the flame colour for lithium ions?

Answer 10

crimson

Question 11

what is the flame colour for sodium ions?

Answer 11

yellow

Question 12

what is the flame colour for potassium ions?

Answer 12

lilac

Question 13

what is the flame colour for calcium ions?

Answer 13

orange-red

Question 14

what is the flame colour for copper ions?

Answer 14

green

Question 15

how do you carry out a flame test?

Answer 15

• dip a nichrome wire loop in HCl and heat it. dip it again in HCl
• dip wire in metal compound being tested
• hold wire in roaring blue flame of a bunsen burner
• use the colour of the flame to identify the metal ion

Question 16

what is a precipitate?

Answer 16

an insoluble solid formed by a reaction taking place in a solution

Question 17

what positive metal ions produce a white precipitate when reacted with sodium hydroxide?

Answer 17

aluminium
calcium
magnesium

Question 18

how can you tell that aluminium ions are in a compound when sodium hydroxide is added?

Answer 18

it forms a white precipitate
if more sodium hydroxide is added, the precipitate dissolves

Question 19

what colour precipitate is formed when copper (II) ions are present?

Answer 19

blue

Question 20

what colour precipitate is formed when iron (II) ions are present?

Answer 20

green

Question 21

what colour precipitate is formed when copper (III) ions are present?

Answer 21

brown

Question 22

how do you test for carbonates?

Answer 22

add a dilute acid and test the gas given off using limewater to see if it is carbon dioxide

Question 23

what is the test to see if halide ions are present?

Answer 23

add dilute nitric acid to remove carbonate ions
add silver nitrate and see if a precipitate forms

Question 24

what colour precipitate is formed when iodide ions (I-) are present?

Answer 24

yellow

Question 25

what colour precipitate is formed when bromide ions (Br-) are present?

Answer 25

cream

Question 26

what colour precipitate is formed when chloride ions (Cl-) are present?

Answer 26

white

Question 27

how do you test for sulfate ions?

Answer 27

• add dilute HCl to remove any carbonate ions
• add barium chloride solution
• a white precipitate tells you that sulfate ions are present

Question 28

what are the advantages of modern instrumental methods of chemical analysis?

Answer 28

• highly accurate and sensitive
• quicker
• small samples can be analysed

Question 29

what are the disadvantaged of modern chemical analysis?

Answer 29

• very expensive
• requires specialist training to use
• often gives results that can only be interpreted by comparison with data from known substances

Question 30

what is flame emission spectroscopy?

Answer 30

an example of an instrumental method of chemical analysis

Question 31

how does flame emission spectroscopy work?

Answer 31

• the sample is heated in a flame which excites electrons and makes them jump to higher energy levels
• when they jump back down the energy is released as light
• the wavelengths of light produced are analysed by passing it through a spectroscope

Question 32

how does flame emission spectrosopy identify metal ions?

Answer 32

each metal ion absorbs and gives out its own characteristic radiation pattern called a line spectrum
this can then be compared with a database

Question 33

how can flame emission spectroscopy be used to find out the concentration of a metal ion?

Answer 33

the spectrometer can measure the intensity of light with a specific wavelength for a metal ion
this can then be compared with intensities of light from known concentrations of metal ions