C8 - Chemical Analysis Flashcards
Define a pure substance
only contains one compound or element, not mixed with any other substance
How do you measure a substance’s purity?
measure its melting/boiling point and compare it with the melting/boiling point of the pure substance (found in a data book)
Impurities in a solid will …….. the melting point.
lower
Impurities in a liquid will ………. the boiling point.
raise
Impurities will ……….. the range of temperatures a substance melts or boils at.
increase
Define a formulation
Formulations are useful mixtures with a specific purpose that are made by following a formula.
Give some uses for chromatography
- detect bombs in airports
- compare fibres found at a crime scene
- fingerprinting
What is the mobile phase (chromatography) ?
the liquid/gas that moves (the solvent)
What is the stationary phase (chromatography) ?
the medium that doesn’t move (chromatography paper)
Describe the chromatography practical
- draw a pencil line near the bottom of a strip of chromatography paper
- add dots of different inks onto the line
- place the paper in a beaker with solvent, ensuring the solvent doesn’t touch the line
- wait for the solvent to absorb up the paper
- the colour will move up the paper and separate into different colours
- remove from beaker and add another pencil line where the solvent reached
How do you calculate Rf value?
distance travelled by substance / distance travelled by solvent
Why are Rf values useful when interpreting chromatograms?
it allows you to identify unknown substances
What is a solvent front?
the distance travelled by the solvent - starting from the baseline
Describe how you test for chlorine gas
chlorine bleaches damp litmus paper, turning it white
Describe how you test for oxygen
use a glowing splint - it will relight if present
Describe how you test for carbon dioxide
turns limewater cloudy
Describe how you test for hydrogen
use a lit splint - you’ll hear a squeaky pop
How do you test for carbonates?
- put a sample in a test tube and add a few drops of hydrochloric acid
- connect to a test tube of limewater
- warm with a bunsen burner
- if present, the limewater will turn cloudy
How do you test for sulphates?
- add hydrochloric acid, barium chloride and mystery solution into a test tube
- shake
- will form a white precipitate if present
How do you test for halides?
- add silver nitrate and nitric acid to mystery solution and shake
- forms a precipitate
- chloride = white
- bromide = cream
- iodide = yellow
Describe how you test for metal ions using a flame test
- dip nichrome wire loop into solution
- place in a roaring blue flame
- record colour of flame
What colour flame is observed for each metal?
lithium
potassium
sodium
calcium
copper
- lithium = crimson
- potassium = lilac
- sodium = yellow
- calcium = orange red
- copper = green
Why is the flame test not very accurate for identifying metal ions?
- mixture of ions are difficult to detect
- colour is subjective
- colours can be masked
Why is the nichrome wire loop dipped in acid between each test?
to avoid cross contamination
Which reagent is added to metal ions to perform a precipitate test?
sodium hydroxide solution
Give the result for each metal ion in a precipitate test
- calcium
- copper (II)
- iron (II)
- iron (III)
- aluminium
- magnesium
- calcium = white
- copper = blue
- iron (2) = green
- iron (3) = brown
- aluminium = white (dissolves in excess -> goes colourless)
- magnesium = white
Why is a precipitate test often not enough to confirm which metal ion is present in a solution?
lots of the results give the same colour (white) precipitate
Describe how flame emission spectroscopy works
- a sample is placed in a flame
- ions heat up and electrons become excited
- they drop back to their original energy levels and transfer energy as light
- this light passes through a spectroscope, which produces a line spectrum
What are advantages of flame emission spectroscopy?
- you can be certain of the element - unlike flame tests
- can be used on mixtures
- intensity of the spectrum tells you the concentration
What are advantages of using instrumental analysis (machines) to analyse unknown substances?
- very sensitive - can detect very small amounts of substances
- very fast
- very accurate
