Topic 8 - Chemical analysis

Question 1

What is a pure substance?

Answer 1

a substance that only contains one compound or element throughout

Question 2

How can you assess purity using melting and boiling points?

Answer 2

• chemically pure substance melt or boil at a specific temperature
• test purity of sample by measuring melting/boiling point and comparing it witih those of pure substance
• closer measured value is to actual melting/boiling point - purer the sample is

Question 3

How do impurities affect the melting and boiling points of a sample?

Answer 3

• impurities in sample lower melting point - increase melting range
• impurities in sample increase boiling point - may result in sample boiling at range of different temperatures

Question 4

What are formulations?

Answer 4

• useful mixtures with precise purpose
• mixture that has been designed to produce useful product

Question 5

Paint is a formulation made of?

Answer 5

• pigment - gives paint colour
• solvent - alter viscositsy - help pigment and binder spread well during painting
• binder - forms protective film that golds pigment in place after its been painted on

Question 6

How are formulations important in the pharmaceutical industry?

Answer 6

altering formulation of pills can make them more effective and useful

Question 7

What is chromotography?

Answer 7

analytical method used to separate substances in a mixture

Question 8

What are the two phases in chromotography?

Answer 8

• mobile phases - molecules can move - always liquid or gas
• stationary phase -molecules cant move - can be solid or really viscous liquid

Question 9

What are the two stages in paper chromotography?

Answer 9

• stationary phase - is the chromotography paper
• mobiles phase - is the solvent

Question 10

What does the amount of time molecules spend on each phase in paperchromotography depend on?

Answer 10

• how soluble they are in the solvent
• their affinity to the paper - how attracted they are to the paper

Question 11

Which molecules move further up the page?

Answer 11

• chemicals that spend more time in mobile phase than stationary phase move further
• molecules with higher solubility in solvent and less attracted to paper spend more time in mobile phase - carried further up the paper

Question 12

What is an Rf value?

Answer 12

• ratio between distance travelled by dissolved substance and distance travelled by solvent
• further up paper substance moves - larger the Rf value

Question 13

How do you calculate Rf value?

Answer 13

distance travelled by substance / distance travelled by solvent

Question 14

What is the Rf value dependent on?

Answer 14

the solvent - if you change the solvent the Rf value for a substance will change

Question 15

Test for chlorine?

Answer 15

• chlorine bleaches damp litmus paper turning it white
• this is because a soltuion of chlorine is acidic

Question 16

Test for oxygen?

Answer 16

• put glowing splint inside test tube containing oxygen
• oxygen relights glowing splint

Question 17

Test for carbon dioxide?

Answer 17

bubbling carbon dioxide through aqueous solution of calcium hydroxide (limewater) causes solution to turn cloudy

Question 18

Test for hydrogen?

Answer 18

• if you hold a lit splint at open end of test tube containing hydrogen you will get a squeaky pop
• noise comes from hydrogen burning quickly with oxygen in air to form H2O

Question 19

Testing for carbonates?

Answer 19

• carbonates can be tested for using dilute acid
• if carbonate ions are present - react with acid to form carbon dioxide - turns limewater cloudy

Question 20

Testing for sulfate ions?

Answer 20

• sulfate ions tested with HCL and barium chloride
• add couple drops of dilute HCL followed by couple drops of barium chloride solution
• if sulfate ions present - white precipitate formed (barium sulfate)

Question 21

Testing for halides?

Answer 21

• add few drops of dilute nitric acid (HNO3) followed by a few drops of silver nitrate solution (AgNO3)
• chloride gives WHITE precipitate of silver chloride
• bromide gives CREAM precipitate of silver bromide
• iodide gives YELLOW precipitate of silver iodide

Question 22

What colour flame does lithium ions burn with?

Answer 22

crimson flame

Question 23

What colour flame does sodium ions burn with?

Answer 23

yellow flame

Question 24

What colour flame does potassium ions burn with?

Answer 24

lilac flame

Question 25

What colour flame does calcium ions burn with?

Answer 25

orange-red flame

Question 26

What colour flame does copper ions burn with?

Answer 26

green flame

Question 27

Preperation for flame test?

Answer 27

- clean platinum wire loop by dipping it in some dilute HCL - hold it in blue flame in bunsen until it burns with no colour - dip loop into sample and put it back in flame - only works for samples containing single metal ion - if sample tested contains mixture of metal ions - flame colours of some ions may be hidden by colours of others

Question 28

What solution can be used to test for metals?

Answer 28

sodium hydroxide (NaOH)

Question 29

Colour of precipitate when sodium hydroxide solution is added to calcium ions?

Answer 29

white - Ca(OH)2 formed

Question 30

Colour of precipitate when sodium hydroxide solution is added to copper (II) ions?

Answer 30

blue - Cu(OH)2 formed

Question 31

Colour of precipitate when sodium hydroxide solution is added to iron (II) ions?

Answer 31

green - Fe(OH)2 formed

Question 32

Colour of precipitate when sodium hydroxide solution is added to iron (III) ions?

Answer 32

brown - Fe(OH)3 formed

Question 33

Colour of precipitate when sodium hydroxide solution is added to aluminium ions?

Answer 33

- white at first - then redissolves in excess NaOH to form colourless soltution - Al(OH)3 formed

Question 34

Colour of precipitate when sodium hydroxide solution is added to magnesium ions?

Answer 34

white - Mg(OH)2 formed

Question 35

Chromatography method?

Answer 35

- paper placed in container with solvent ensuring solvent level is below pencil line so dyes do not mix with solvent directly - solvent rises up the paper by capillary action, carrying the dyes along with it - different dyes move at different rates depending on their solubility in solvent and affinity to the paper - more soluble dyes travel further up paper - less soluble dyes travel shorter distance - separation of dyes results in distinct spots forming on paper, allowing identification of individual components of mixture based on their Rf values