Chemical Tests

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  • Created by: Snuffles
  • Created on: 25-04-14 16:54

Flame tests

Flame tests can be used to identify metal ions. Lithium, sodium, potassium, calcium and bariumcompounds produce distinctive colours in flame tests:

  • lithium compounds result in a crimson flame
  • sodium compounds result in a yellow flame
  • potassium compounds result in a lilac flame
  • calcium compounds result in a (brick)red flame
  • barium compounds result in a green flame. 
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Precipitates

Aluminium, Calcium or Magnesium +Sodium hydroxide -> white preciptates

Aluminium hydroxide +sodium hydroxide -> disolves 

Copper(II), Iron (II), or Iron (II) +Sodium hydroxide --> coloured precipitates 

Carbonates+Dilute acids -> Carbon Dioxide

Carbon Dioxide + Limewater -> White precipitate (cloudy)

Halide ions(aq) + Silver nitrate(aq) +Nitric Acid -> precipitates (Increasingly yellow down table from Cl-I)

Sulfate ions(aq) + Barium chloride(q) +Hydrochloric acid -> White precipitate

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Mixtures

A mixture consists of two or more elements or compounds not chemically combined together.

The chemical properties of each substance in the mixture are unchanged.

It is possible to separate the  substances in a mixture by physical methods, including

  • distillation,
  • filtration
  • crystallisation. 

 Paper chromatography can be used to analyse substances present in a solution, eg food colourings and inks/dyes.

Elements and compounds can be detected and identified using instrumental methods. Instrumental methods are accurate, sensitive and rapid and are particularly useful when the amount of a sample is very small. 

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Gas chromatography

Gas chromatography linked to mass spectroscopy (GC-MS) is an example of an instrumental

method:

  • gas chromatography allows the separation of a mixture of compounds
  • the time taken for a substance to travel through the column can be used to help identify the substance
  • the output from the gas chromatography column can be linked to a mass spectrometer, which can be used to identify the substances leaving the end of the column
  • the mass spectrometer can also give the relative molecular mass of each of the substances separated in the column.
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Gas chromatography

Candidates need only a basic understanding of how GC-MS works, limited to:

  • different substances, carried by a gas, travel through a column packed with a solid material at different speeds, so that they become separated
  •  the number of peaks on the output of a gas chromatograph shows the number of compounds present
  • the position of the peaks on the output indicates the retention time
  • a mass spectrometer can identify substances very quickly and accurately and can detect very small quantities
  • the molecular mass is given by the molecular ion peak. 
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