Chemical Analysis Flashcards
why do we draw our starting line in pencil
because if we drew the line in pen, the pen ink would move up the paper with the solvent
how to find out an unknown substance using paper chromotography
After seeing how far the substance has moved and if it is pure :
- measure the distance from the pencil line to the centre of the spot to see how far the substance moved
- measure the distance that the solvent moved
- find the Rf value by dividing the distance moved by the substance over the distance moved by the solvent
Rf equation
distance moved by substance / distance moved by solvent
how to increase the reliability of a paper chromatography ecperiment
repeating the experiment using different solvents
problems with paper chromotography
- several different substances may have the same Rf values
- if the substance has never been analysed, then there will not be an Rf value on the database
what is limewater
the aqueous solution of calcium hydroxide
what is a pure substance
a substance that consists of only one element or compound
what is a mixture
a substance that contains two or more different elements or compounds that are not chemically joined together
what is an element
a substance that contains one type of atom
what is a compound
a substance that contains two or more types of atom chemically joined together
what are the differences between pure and impure substances
- a pure substance melts at a specific fixed temperature
- a pure substance has a specific boiling point
- impure substances melt and boil over a range of temperatures
what is a formulation
a mixture which has been designed as a useful product, In a formulation, every chemical has been added in specific amountst. Each chemical has a specific purpose in the formulation.
examples of formulations
- fuels
- cleaning products
- paints
- medicines
- alloys
- fertilisers
why is paper chromatography used
to separate mixtures of soluble substances and identify substances present in the mixture, often coloured substances such as food colourings, inks, dyes
What are the steps of paper chronatography
- Take a piece of chromatography paper and draw a pencil line near the bottom
- Put a dot of your first colour onto the pencil line, and next to that, put a dot of your second colour
- Place the piece of paper into a solvent
- The solvent moved up the paper and dissolves the ink and the two coloured dots
- The colour with only one spot is pure, the colour with more than one is impure
what is a solvent
a liquid that’ll dissolve substances
what is the stationary phase
the paper since it does not move
what is the mobile phase
the solvent as it moves up the paper
what is a chromotogram
the results of separating mixtures by chromatography
how to test for hydrogen
place a lighted splint into a test tube, if hydrogen is present, you’ll hear a squeaky pop sound
how to test for oxygen
put a glowing splint into the mouth of a test tube, if it relights, oxygen is present
how to test for carbon dioxide
if carbon dioxide is bubbled through limewater, it turns milky or cloudy white
how to test for chlorine
hold damp litmus paper into the mouth of the test tube, chlorine bleaches litmus paper and turns it white
what are some problems with doing flame tests
- the colour of the flame could be hard to distinguish, especially if there is a low concentration of the metal compound
- sometimes a metal compound contains a mixture of metal ions, which can mask the colour of the flame
what does it mean if a method is instrumental
it is carried out by a machine
advantages of instrumental methods
- they are rapid
- they are sensitive, can work on tiny samples
- they are accurate
how to carry out a flame test
- Dip a clean wire loop into a solid sample of the compound being tested
- Put the loop into the edge of the blue flame from a Bunsen burner
- Observe and record the colour of the flame produced
what colour is the flame if lithium is present
Crimson
what colour is the flame if sodium is present
yellow
what colour is the flame if potassium is present
Lilac
what colour is the flame if calcium is present
orange - red
what colour is the flame if copper ion is present
green
how can we test for metal hydroxides
using dilute sodium hydroxide
how to carry out a flame emission spectroscopy
- Put a sample of the metal ion into a flame
- The light given out is then passed into a machine called spectroscope
- The spectroscope converts light into a light spectrum
- The positions of the lines in the spectrum are specific for each metal ion
what colour precipitates are formed when we add dilute sodium hydroxide to calcium, aluminium or magnesium ions
white
what colour precipotate is formed when we add dilute NAOH to iron(ii) ions
green
what colour precipitate is formed when we add dilute NAOH to iron(iii) ions
brown
what colour precipitate if formed when we add dilute NAOH to copper(ii)
blue
how can we distinguish between aluminium, calcium and magnesium ions
- if we add excess sodium hydroxide solution to Al ions, the Al precipitate redissolves
- Calcium and magnesium are unchanged so we carry out a flame test to see which one is calcium
how to test for carbonate ions
- Add dilute acid to our sample
- When the carbonate ions react with the acid, a we see effervescence, if carbonate ions are present, CO2 gas is given off
- To confirm that this is co2, bubble the gas given off into limewater, if it turns cloudy, co2 is present and so are the carbonate ions
how to test for sulfate ions
- Add dilute HCL to our sample
- Add barium chloride solution
- If sulfate ions are present, we will see a white precipitate
why, in a test for sulfate ions, must the sample be acidified with HCL
because carbonate ions also produce a white precipitate when reacted with barium chloride solution. the acid reacts with any carbonate ions present. this removes them and stops them from giving a false positive result
when testing for sulfate ions, why cant dilute sulfuric acid be used
because sulfuric acid contains sulfate ions, these would react with the barium ions in the test, giving an incorrect positive result
how to test for hallide ions
- Add a few drops of dilute nitric acid to the sample
- Add a few drops of dilute silver nitrate solution to the sample
- Halide ions produce a precipitate of the silver halide, and each halide makes a different colour precipitate
what colour precipitate does bromine form
a cream precipitate to form silver bromide
what colour precipitate does chlorine form
white, to form silver chloride
what colour precipitate does iodine form
yellow, to form silver iodide
why is the sample acidified with nitric acid when testing for halide ions
Carbonate ions also produce a white precipitate with silver nitrate solution. The acid reacts with any carbonate ions present. This removes them, so stopping them giving an incorrect positive result for chloride ions.
