8 - Chemical Analysis Flashcards
What are the four physical seperation techniques? (C CA Ch)
- filtration
- crystillisation
- distillation
- chromatography
What does paper chromatography allow us to do? (C CA Ch)
separate substances based on their different solubilities
What is the paper used in paper chromatography? (C CA Ch)
chromatography paper.
What is the set up for paper chromatography? (C CA Ch)
- take chromatography paper
- draw a pencil line near the bottom of the paper (2cm)
- dot substance A onto the line
- dot of substance B next to it along the line
- place the bottom of the paper into a solvent
What is a solvent? (C CA Ch)
a liquid that will disolve substances
How does paper chromatography work? (C CA Ch)
when we dip the paper into the solvent, it moves up the paper until it reaches your two substances which are then carried up as well. The substances will then seperate if they are impure. If the substance stays one dot/ colour then it is pure.
This works becuase different substances have different solubilities and a more soluble substance travels further up the paper.
What is the stationary phase in paper chromatography and why? (C CA Ch)
the paper becuase it does not move
What do we call the movile phase in paper chromatography? (C CA Ch)
the solvent becuase it does move
How do we know if the substance was pure from paper chromatography? (C CA Ch)
a pure compound will produce a single spot in all solvents.
What happens when we change the solvent in paper chromatography (C CA Ch)
The compounds in a mixture may separate into different spots depending on the solvent. But they will still separate.
Why do we draw the line in pencil in paper chromatography? (C CA Ch)
if we drew the line in pen, the pen ink would move up the paper, with the solvent.
How do you use paper chromatography to identify and unknown substance and what are two problems? (C CA Ch)
- we meausre the distance moved by the unknown chemical - from the pencil line to the centre of the spot (mm)
- then we measure the distance moved by the solvent - from the pencil line to the solvent (mm)
- calculate the Rf value for the substance.
Rf = distance moved by substance / distance moved by solvent
Rf has no units - look the Rf value up in a database and we can know the identity of the substance.
However several different substance could have this Rf value as well so we have to repeat with a different solvent
If the substance has never been analysed before, then there will bot be an Rf value on the database
What do all seperation techniques not invole? (C CA Ch)
they are all physical processes so there are no chemical reactions and no new substances made
How do you carry out chromatography for the required practical. One unkown substance and 4 others we know of. (RP6 CA Ch)
- use a ruker to draw a horizontal pencil line on the chromatography paper . 2cm from the bottom of the paper
- mark five pencil spots at equal spaces across the line with 1 cm clear on each side
- use a capillary tube to put a small spot (to prevent the colours spreading into each other) of each known and the unknown onto the pencil spots
- pour water into a breaker to 1cm deap (solvent)
- attach the paper to a glass rod using tape and lower the paper into the beaker and the bottom of the paper should dip into the water.
- pencil line with spots of ink must be above the water otherwise the water will wash the ink off the line
- sides of the paper must not touch the sides of the breaker otherwise it will interfere with the way that the water moves - place a lid on the beaker so that it reduces evaporation of the solvent.
- water will move up the paper and the colours will be carried up. do not move the beaker during this time
8 remove the paper when the water has travelled around three quarters up - mark a pencil where the water reached and hang the paper to dry
- see which spot/s the unknown match up to and we can see what goes into it.
Test for hydrogen: (TFG CA Ch)
Remove the bung and insert a burning splint.
Hydrogen burns rapidly and produces a squeaky pop
Test for oxygen (TFG CA Ch)
use a glowing splint.
If we place the glowing splint into a test tube of oxygen the splint relights (burst into flames)
Test for carbon dioxide (TFG CA Ch)
use limewater because it is an aqueous solution of calcium hydroxide
draw some gas into a plastic pipette and then bubble the gas through some lime water if we repeat this step and the limewater turns cloudy then it was carbondioxide
Test for Chlorine (TFG CA Ch)
we insert damp litmus paper into the mouth of the tube
chlorine bleaches the litmus paper and it turns white
What do flame tests descover? (FT CA Ch)
discover the identities of metal ions
How do you do a flame test? (FT CA Ch)
- place a small amount of our chemical onto wire mounted in a handle
- place the end of this into a blue flame
- the colour of the flame can be used to workout the metal ion present
What does lithium ion produce in a flame test? (FT CA Ch)
a crimson flame
what does sodium ion do in the flame test? (FT CA Ch)
yellow flame
what does potassium ion do in a flame test? (FT CA Ch)
lilac flame
what does calcium ion do in a flame test? (FT CA Ch)
orange - red flame
what does copper ion do in a flame test? (FT CA Ch)
green flame
What are the problems with using flame tests? (FT CA Ch)
- the colour of a flame test can be difficult to distinguish. especially if there is a low concentration of the metal compound
- sometimes a sample contains a miture of metal ions which can mask the colour of the flame
- instead of flame tests scientists do flame emission spectroscopy
What happens in flame emsision spectroscopy? (FT CA Ch)
a sample of the metal ion in solution is placed into a flame
the light given out is then passed into a machine called spectroscope
this converts the light into a line spectrum
the positions of the lines in the spectrum are specific for a given metal ion so we can identitfy a metal ion in a sample
how does flame emission spectrosocpy tell us the difference in concentration between metal (FT CA Ch)
the lines become more intense at a higher concentration
What are the three advantages of instrumental methods (flame emission spectrosocpy) (FT CA Ch)
- rapid (more than flame tests)
- sensitive and will work on really small samples
- accurate more likely to identitfy the metal ion correctly
which ions in solution produce a white precipitate when we add sodium hydroxdie? (MHP CA Ch)
calcium magnesium and aluminium
What happens to a solution of aluminium ions when we add excess sodium hydroxide solution? (MHP CA Ch)
redissolves
How do you work out the difference between calcium and magnesium when adding sodium hydroxide solution? (MHP CA Ch)
flame tests
What does a solution of copper ions mixed with a solution of sodium hydroxide do ? (MHP CA Ch)
Forms a blue precipitate of copper hydroxide
What does a solution of iron II ions form when mixed with a solution of sodium hydroxide? (MHP CA Ch)
A green precipitate of iron II hydroxide
What does a solution of iron III form when mixed with a solution of sodium hydroxide? (MHP CA Ch)
Brown precipitate of iron III hydroxide
How do you test for the carbonate ion? (INMI CA Ch)
- Add dilute acid to our sample
- The acid with react with the carbonate to make carbon dioxide gas. We will see effervescence. But that does not prove that we have carbon dioxide gas
- Bubble the gas through lime water and if this goes cloudy then this proves that we have carbon dioxide which means that we started with the carbonate ion.
What is the test for halide ions ? (INMI CA Ch)
- Add dilute nitric acid to the sample
- Add dilute silver nitrate solution
- Halide ions produce precipitate of the silver halide. Each halide makes a different coloured precipitate
Chloride ions -> white precipitate of silver chloride
Bromide ions -> cream precipitate of silver bromide
Iodide ions -> yellow precipitate of silver iodide
What is the test for sulfate ions? (INMI CA Ch)
- Add dilute hydrochloride acid to our sample
- Add barium chloride solution
- If sulfate ions are present, we will see a white precipitate.
