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Analysing Substances

Analysing substances is a major part of GCSE Chemistry. This is the first of three quizzes on the topic and it takes a look at some of the many methods which are used in analytical testing.

Finding out which elements are present in a substance can be helpful to scientists in many fields, for example forensic science, hospitals and food science. Current methods used by professional scientists for analysing substances are very quick, extremely accurate and ultra-sensitive. There are, however, some disadvantages too. The machines tend to be extremely expensive, can only be used by highly trained operatives and a known result is required for comparison. Thankfully, not all methods of chemical analysis rely on machines to get the job done! There are many analytical tests that you can carry out which enable you to find out what is in a chemical.

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In your chemistry lessons you will have learned some methods in KS3 including chromatography, the flame test for metals and the iodine test for starch, but during your GCSE you will use several more. For your exams and practical work, you need to learn these and know how to interpret the results.

The flame test is used to find out what metal ion (cation) is present in a compound. It works because different metal ions burn with different colours. The key to a good flame test is to get the equipment as clean as possible so that the colour can be seen. Even the tiniest quantity of sodium ions give a very strong orange/yellow flame and if the flame test wire is not thoroughly cleaned, all you will see is the sodium colour.

The flame test covers some of the most common cations. Precipitation reactions using sodium hydroxide can identify some that are not detectable using the flame test. The precipitate formed by aluminium will re-dissolve if you continue to add sodium hydroxide whereas those of calcium and magnesium do not. The colour of the precipitate can give you some information too, for example, a brown precipitate indicates that the cation is iron III.

The anion (non metal or group of non metals) can be discovered using silver nitrate solution (for halides), barium chloride (for sulfates) or acid and limewater (for carbonates).

Analysing substances needs to be carried out safely and methodically. Start with the flame test and work through the other tests until you have a result. Negative test results are almost as useful as positive results as they can eliminate certain cations and anions and narrow down the range of tests needed.

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  1. A chromatogram is produced in a process called chromatography. There are several types of chromatography. Which of the following is NOT a type of chromatography?
    UV-visible is a type of spectroscopy. Chromatography involves the movement of a solvent which separates the chemicals from one-another in a mixture
  2. Three unknown solutions are added to test tubes of dilute nitric acid and silver nitrate. Each shows a positive result for the test. What ions are present in each tube in order if the colours produced are white, cream and pale yellow?
    The precipitates are silver chloride, silver bromide and silver iodide
  3. A titration is used to accurately measure volumes of liquids required to carry out chemical reactions such as neutralisation reactions. Name the long glass tube which is required to carry out a titration.
    It can be used to measure the volume of any liquid with great accuracy
  4. A white precipitate results from the reaction between an unknown substance and sodium hydroxide. Which ion CANNOT be present in the unknown substance?
    Copper 2+ ions produce a light blue precipitate, the other three produce white precipitates
  5. Which element produces a green flame when burned in a flame test?
    The flame test wire must be very clean
  6. What metal should the wire loop be made of that is holding your unknown compound during a flame test?
    This is an inert metal and can be heated to a high temperature before it melts. Most school laboratories, however, use nichrome wire as it is cheaper
  7. The meniscus is the curve in the upper surface of a liquid caused by surface tension. When measuring the amount of liquid held in a burette, which part of the meniscus do you measure from?
    The meniscus is read at the bottom of the curve. Always read the volume with your eye at the meniscus level to avoid parallax error
  8. Which of the following compounds would burn yellow in the flame test?
    Sodium gives a distinctive yellow colour. This is where street lights get their colour from
  9. A reddish brown precipitate results from the reaction between an unknown substance and sodium hydroxide. Which ion is present in the unknown substance?
    Obtaining a white precipitate could indicate several different metals, however, coloured precipitates are more useful as they identify specific metals
  10. Which of the following can iodine NOT be used for testing for?
    Limewater is used to test for the presence of carbon dioxide

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