topic 8 - chemical analysis

Question 1

what is a pure substance?

Answer 1

a single element or compound, not mixed with any other substance

Question 2

what is a formulation?

Answer 2

• a complex mixture that has been designed as a useful product
• in a formulation, the quantity of each component is carefully measured so that the product has the properties we need

Question 3

what are examples of formulations?

Answer 3

• fuels
• cleaning products
• paints
• medicines
• alloys
• fertilisers
• food

Question 4

what is the purpose of paper chromatography?

Answer 4

• allows us to separate substances based on their different solubilities

Question 5

what phase is the paper? (paper chromatography)

Answer 5

• the paper is in the stationary phase because it does not move

Question 6

what phase is the solvent? (paper chromatography)

Answer 6

• the solvent is the mobile phase because it does move

Question 7

how do pure compounds and compounds differ in the results of paper chromatography?

Answer 7

• a pure compound will produce a single spot in all solvents
• the compounds in a mixture may separate into different spots depending on the solvent

Question 8

why do we draw our starting line in pencil? (paper chromatography)

Answer 8

• if we drew the line in pen, the pen ink would move up the paper, with the solvent

Question 9

how do we use paper chromatography to identify an unknown substance?

Answer 9

• measure the distance moved by the unknown chemical - measure from the pencil line to the centre of the spot
• measure the distance moved by the solvent
• use these values to find the Rf value of the substance
  Rf = distance moved by the substance / distance moved by solvent

Question 10

describe how to identify substances using paper chromatography - required practical

Answer 10

• use a ruler to draw a horizontal pencil line on the chromatography paper
• the line should be 2cm from the bottom of the paper
• mark five pencil spots at equal spaces across the line
• use a capillary tube (thin glass tube) to put a small spot of each of the known food colours and the unknown colour onto the pencil spot
• keep the spots relatively small - this prevents the colours from spreading into each other later
• pour water into a beaker to a depth of 1cm
• attach the paper to a glass rod using tape and lower the paper into the beaker
• the pencil line with spots of ink must be above the surface of the water - otherwise the water will wash the ink off the line
• the sides of the paper must not touch the side walls of the beaker - interfere with the way the water moves
• put a lid on the beaker to reduce evaporation of the solvent
• the water will move up the paper and the colours will be carried up - do not move the beaker
• remove the paper when the water has travelled around 3/4 up
• use a pencil to mark the point where the water reached
• hang the paper up to dry
• match the substances used to make the mixture

Question 11

how do you test for hydrogen gas?

Answer 11

• to test for hydrogen, we remove the bung of the test tube and insert a burning splint
• hydrogen gas burns rapidly and produces a pop sound

Question 12

how do you test for oxygen?

Answer 12

• if we place the glowing splint into a test tube of oxygen, the splint relights ( bursts into flames)

Question 13

how do you test for carbon dioxide?

Answer 13

• draw some of the gas into a plastic pipette
• bubble the gas through the limewater
• if we repeat this several times, the limewater may turn cloudy
• if the limewater turns cloudy then the gas was carbon dioxide

Question 14

what is limewater?

Answer 14

• an aqueous solution of calcium hydroxide

Question 15

how do you test for chlorine?

Answer 15

• insert a damp litmus paper into the mouth of the test tube
• chlorine bleaches the litmus paper and turns it white

Question 16

describe how you would carry out a flame test.

Answer 16

• clean the nichrome wire by dipping it into acid and then heat it over the blue flame of a Bunsen burner
• Dip the wire into the metal sample
• hold the wire in the roaring blue flame of a bunsen burner and observe the flame colour

Question 17

why is it difficult to identify metal ions from the colour of the flame if the sample contained a mixture of metals?

Answer 17

• the flame colours are masked

Question 18

what colour flame would lithium ions produce?

Answer 18

crimson

Question 19

what colour flame would sodium ions produce?

Answer 19

yellow

Question 20

what colour flame would potassium ions produce?

Answer 20

lilac

Question 21

what colour flame would calcium ions produce?

Answer 21

orange-red

Question 22

what colour flame would copper ions produce?

Answer 22

green

Question 23

what colour precipitate is formed if sodium hydroxide solution is added to a solution containing aluminium ions?

Answer 23

• white precipitate of aluminium hydroxide formed

Question 24

what colour precipitate is formed if sodium hydroxide solution is added to a solution containing calcium ions?

Answer 24

• white precipitate of calcium hydroxide formed

Question 25

what colour precipitate is formed if sodium hydroxide solution is added to a solution containing magnesium ions?

Answer 25

- a white precipitate of magnesium hydroxide formed

Question 26

how would you differentiate between aluminium ions in one solution, calcium ions in a second solution and magnesium ions in a third solution?

Answer 26

- only the aluminium hydroxide precipitate will redissolve in excess sodium hydroxide - the solutions containing calcium and magnesium can be distinguished using a flame test. the solution containing calcium will produce an orange-red flame colour

Question 27

what colour precipitate is formed if sodium hydroxide solution is added to a solution containing copper (II) ions?

Answer 27

blue precipitate of copper hydroxide

Question 28

what colour precipitate is formed if sodium hydroxide solution is added to a solution containing Iron (II) ions?

Answer 28

the green precipitate of iron (II) hydroxide

Question 29

what colour precipitate is formed if sodium hydroxide solution is added to a solution containing Iron (III) ions?

Answer 29

brown precipitate of iron (III) hydroxide

Question 30

how do you test for the presence of carbonate ions?

Answer 30

- add dilute acid to the sample

Question 31

what are two observations seen if carbonate ions are present?

Answer 31

- effervescence - bubble the gas into a test tube containing limewater. The limewater will turn cloudy showing the presence of carbon dioxide

Question 32

how do you test for the presence of sulphate ions?

Answer 32

- add dilute hydrochloric acid to the sample to remove any carbonates - add barium chloride to the sample - white precipitate of barium sulphate formed if the sample contained sulphates

Question 33

how do you test for the presence of halide ions?

Answer 33

- add dilute nitric acid to remove any carbonates - add silver nitrate - chloride ions: white precipitate of silver chloride forms - bromide ions: cream precipitate of silver bromide forms - iodide ions: yellow precipitate of silver iodide forms