Test for positive ions. Flame Test.
Some of the metals in Group 1 and 2 can be identified by a flame test, as they produce different coloured flames:
- lithium = bright red
- sodium = yellow
- potassium = lilac
- calcium = brick red
- barium = green
How to carry one out:
- put a small amount of the unknown substance in a platinum wire loop (which has been cleaned in HCl)
- then hold the loop in the 'blue' flame of a bunsen burner and record the colour the flame turns
Test for positive ions. (using sodium hydroxide)
If an unknown substance is added to a sodium hydroxide solution and it forms a white precipitate (solid material produced from a solution) then the unknown substance contains aluminium, magnesium or calcium ions.
If more sodium hydroxide is added and the precipitate formed dissolves, then it contains aluminium ions. Al3+(aq) + 3OH-(aq) => Al(OH)3(s) (ionic equation)
Calcium and magnesium can be identified by a flame test. Magnesium give no colour in a flame test.
Some metals form coloured precipitates.
- copper(II) ions => copper hydroxide (light blue)
- iron(II) ions => iron(II) hydroxide (green to brown)
- iron(III) ions =>iron(III) hydroxide (reddish brown)
- ammoniun ions => ammonia (turns damp red litmus paper blue)
Test for negative ions. Carbonates.
When acid is added to a carbonate it fizzes and CO2 gas is produced.
To test for this gas we bubble the gas through lime water to see if it turns milky. If it does, CO2 is present.
Copper carbonate and zinc carbonate go through colour changes when the are heated.
CuCO3(s) ==(heat)==> CuO(s) + CO2
(green ==(heat)==> black + turns lime water milky)
ZnCO3(s) ==(heat)==> ZnO(s) + CO2
(white ==(heat)==> yellow when hot, white when cold + turns lime water milky)
Halides, Sulfates and Nitrates.
Halide ions (chloride, bromide and iodide):
- add dilute nitric acid and then silver nitrate solution
chloride = white precipitate
bromide = cream precipitate
iodide = yellow precipitate
- add dilute HCl and then barium chloride solution, see if a white precipitate forms.
- add aluminium powder and then sodium hydroxide solution, then gently warm. Test gas given off with damp red litmus paper, if it turns blue nitrate ions are present.
Testing for organic substances.
Organic substances come from mainly living things and they burn or char when heated.
The modern definition is that they are substances based on the element carbon.
To test hydrocarbons to see if they contain carbon-carbon double bonds:
- unsaturated hydrocarbon(double) + bromine water(orange) => colourless solution
- saturated hydrocarbon(single) + bromine water(orange) => no reaction
Iodine is used in titrations to find the number of double bonds in organic substances.
The empirical formula of an organic substance can be worked out by burning it and measuring the amounts of products formed:
An organic substance Z contains carbon and hydrogen. It is burnt in oxygen, producing 1.80g of water and 3.52g of carbon dioxide. What is the empirical formula of Z?
1) Calculate moles of CO2: mass / RFM = 3.52 / 44 = 0.08 moles
2) Calculate moles of H2O: mass / RFM = 1.80 / 18 = 0.10 moles
2 Hs for every 1 carbon so 0.10 x 2 = 0.20
- 0.08 : 0.20 = 2 : 5
=> Z = C2H5
Instrumental analysis.. pros and cons.
- highly accurate
- enables small quantities of chemicals to be analysed
- very expensive
- it takes special training to use it
- gives results that can only be interpreted by comapring with already available known specimens
Instrumental Analysis 1.
Two main ways of detecting and measuring elements in samples:
- Atomic absorption spectroscopy (AAS) is a technique used to easure the concentration of metal in a liquid. It can detect very small amounts in samples as small as 0.02cm3.
- Mass spectrometers compare the mass of different atoms. This helps determine Relative Atom Masses (RAM). Also used to identify elements in a substance.
Instrumental analysis 2.
There are chemical instruments that can be used to identify unknown chemical compounds:
One example is chromatography, it is used to separate compunds with in a mixture. It is based on how well they dissolve in a particular solvent.
Spectroscopic methods use radiation:
- Infra-red spectroscopy: gives information about bonds in a molecule
- Nuclear magnetic resonance spectroscopy: is used to find the structures of organic molecules
- Ultraviolet and visible light spectroscopy: is used to fine the amount of substance in a solution