Unit 8 - Chemical Analysis


Purity and Formulations

A pure substance is something that only contains one compound or element throughout:

The boiling or melting point tells you how pure a substance is:

  • A chemically pure substance will melt or boil at a specifc temperature.
  • You can test the purity of a sample by measuring its melting or boiling point and comparing it with the melting or boiling point of a pure substance.
  • The closer the measured value is to the actual melting or boiling point, the purer your sample is.
  • Impurities in your sample will lower the melting and increase the melting range of your substance. And the same for the boiling point.

Formulations are useful mixtures with a precise purpose that are made by following a 'formula.' Each component in a formulation is present in a measure quantity, and contributes to the properties of the formulation so that it meets its required function. 

Formulations are really important in the pharmaceutical industry. In everyday life, formulations can be found in cleaning products, fuels, cosmetics, fertilisers, metal alloys and even food and drink.

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Paper Chromatography

Chromatography is an analytical method used to seperate the substance in a mixture. You can then use it to identify the substances. There are different types of chromatography, but they all have two phases:

  • mobile phase - where the molecules can move, this is always a liquid or a gas
  • stationary phase - where the molecules can't move, this can be a solid or really thick liquid

During a chromatography experiment, the substances in the sample constantly move between the mobile and the stationary phase - an equilibrium is formed between the two phases. The mobile phase moves through the stationary phase, and anything dissolved in the mobile phase moves with it. How quickly a chemical moves depends on how it's 'distributed' between the two phases - whether it spends more time in the mobile or stationary phase.

During paper chromatography, the stationary phase is the chromatography paper and the mobile phase is the solvent. The amount of time the molecules spend in each phase depends on:

  • how soluble they are in the solvent
  • how attracted they are to the paper
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Paper Chromatography

The result of chromatograpgy analysis is called a chromatogram. An Rf value is the ratio between the distance travelled by the dissolved substance and the distance travelled by the solvent. The further through the stationary phase a substance moves, the larger the Rf value.

The Rf value is dependent on the solvent - if you change the solvent the Rf value for a substance will change.

You can calculate the Rf values uing this formula:

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Tests for Gases and Anions

There are tests for 4 common gases:

  • chlorine bleaches damp litmus paper, turning it white - it may turn red for a moment as the solution of chlorine is acidic
  • oxygen relights a glowing splint if you put the glowing splint in a test tube containing oxygen
  • carbon dioxide will turn lime water cloudy
  • hydrogen will produce a squeaky pop when placed into a test tube with hydrogen

Tests for Anions often give precipitates:

  • dilute acids can help detect carbonates - if a carbonate is present carbon dioxide will be released that will turn the limewater cloudy when it bubbles through
  • test with sulfates HCl and Barium Chloride - if sulfate ions are present a white precipitate of barium sulfate will form
  • test for halides with Nitric Acid and Silver Nitrate - a chloride gives a white precipitate of silver chloride, a bromide gives a cream precipitate of silver bromide and an iodide gives a yellow precipitate of silver iodide
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Test for Cations

Flame Tests identify metal ions:

  • lithium ions burn with a crimson flame
  • sodium ions burn with a yellow flame
  • potassium ions burn with a lilac flame
  • calcium ions burn with an orange-red flame
  • copper ions burn with a green flame

To do the test, you first need to clean a nichrome wire loop by dipping it into dilute HCl and then holding it in a blue flame from a Busen burner until it burns without any colour. Then, dip the loop into the sample you want to test an put it back into the flame, record the colour of the flame. You can use these colours to detect and identify ions.

However, it only works for samples that contain a single metal ion. If the sample tested contains a mixture of metal ions, the flame colours of some ions may be hidden by the colours of others.

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Test for Cations

Some metals form a coloured precipitate with NaOH:

  • Calcium forms a white precipitate
  • Copper(2) forms a blue precipitate
  • Iron(2) forms a green precipitate
  • Iron(3) forms a brown precipitate
  • Aluminium forms a white precipitate at first but then redissolves in excess NaOH to form a colourless solution
  • Magnesium forms a white precipitate

The ionic equation for precipitate formation are:

  • Calcium - 
  • Copper(2) - 
  • Iron(2)  - 
  • Iron(3) - 
  • Aluminium - 
  • Magnesium - 
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Flame Emission Spectroscopy

During flame emission spectroscopy a sample is placed in a flame:

  • As the ions heat up their electrons become excited.
  • When the electrons drop back to their original energy levels, they transfer energy as light.
  • The light passes through a spectroscope, which can detect different wavelengths of light to produce a line spectrum.
  • The combination of wavelengths emitted by an ion depends on its charge and its electron arrangement. Since no two ions have the same charge and the same electron arrangement, different ions emit different wavelengths of light. So each ion produces a different pattern of wavelengths, and has a different line spectrum.
  • The intensity of the spectrum indicates the concentration of that ion in a solution.
  • This means that line spectrums can be used to identify ions in a solution and calculate their concentrations.

Advantages of using machines:

  • Very accurate
  • Very fast and tests can be automated
  • Very sensitive - they can detect even the tiniest amounts of substances
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