topic 8 - chemical analysis Flashcards
what is a pure substance?
a single element or compound, not mixed with any other substance
what is a formulation?
- 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
what are examples of formulations?
- fuels
- cleaning products
- paints
- medicines
- alloys
- fertilisers
- food
what is the purpose of paper chromatography?
- allows us to separate substances based on their different solubilities
what phase is the paper? (paper chromatography)
- the paper is in the stationary phase because it does not move
what phase is the solvent? (paper chromatography)
- the solvent is the mobile phase because it does move
how do pure compounds and compounds differ in the results of paper chromatography?
- 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
why do we draw our starting line in pencil? (paper chromatography)
- if we drew the line in pen, the pen ink would move up the paper, with the solvent
how do we use paper chromatography to identify an unknown substance?
- 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
describe how to identify substances using paper chromatography - required practical
- 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
how do you test for hydrogen gas?
- 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
how do you test for oxygen?
- if we place the glowing splint into a test tube of oxygen, the splint relights ( bursts into flames)
how do you test for carbon dioxide?
- 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
what is limewater?
- an aqueous solution of calcium hydroxide
how do you test for chlorine?
- insert a damp litmus paper into the mouth of the test tube
- chlorine bleaches the litmus paper and turns it white
describe how you would carry out a flame test.
- 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
why is it difficult to identify metal ions from the colour of the flame if the sample contained a mixture of metals?
- the flame colours are masked
what colour flame would lithium ions produce?
crimson
what colour flame would sodium ions produce?
yellow
what colour flame would potassium ions produce?
lilac
what colour flame would calcium ions produce?
orange-red
what colour flame would copper ions produce?
green
what colour precipitate is formed if sodium hydroxide solution is added to a solution containing aluminium ions?
- white precipitate of aluminium hydroxide formed
what colour precipitate is formed if sodium hydroxide solution is added to a solution containing calcium ions?
- white precipitate of calcium hydroxide formed
what colour precipitate is formed if sodium hydroxide solution is added to a solution containing magnesium ions?
- a white precipitate of magnesium hydroxide formed
how would you differentiate between aluminium ions in one solution, calcium ions in a second solution and magnesium ions in a third solution?
- 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
what colour precipitate is formed if sodium hydroxide solution is added to a solution containing copper (II) ions?
blue precipitate of copper hydroxide
what colour precipitate is formed if sodium hydroxide solution is added to a solution containing Iron (II) ions?
the green precipitate of iron (II) hydroxide
what colour precipitate is formed if sodium hydroxide solution is added to a solution containing Iron (III) ions?
brown precipitate of iron (III) hydroxide
how do you test for the presence of carbonate ions?
- add dilute acid to the sample
what are two observations seen if carbonate ions are present?
- effervescence
- bubble the gas into a test tube containing limewater. The limewater will turn cloudy showing the presence of carbon dioxide
how do you test for the presence of sulphate ions?
- 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
how do you test for the presence of halide ions?
- 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
