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Analysing Substances - Qualitative

In GCSE Chemistry students will look at the various methods and techniques used when analysing substances. In this, the second of three quizzes on the topic, we look at qualitative analysis. This helps us to identify what is present, but does not tell us how much of it there is.

Analysing is the identification of substances by investigating their physical and chemical properties. It is always important to be able to identify unknown substances, particularly in the the food industry and the field of medicine. Many metals are poisonous in large enough amounts and analysis can identify them should they accidentally find their way into the food chain. In cases of actual poisoning, medical staff need to be able to reliably analyse body tissue and blood to find out what is there. Only then can they decide on the correct treatment.

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There are two aspects to analysing substances - qualitative and quantitative. Qualitative analysis is the topic of this quiz and is only concerned with what is there. It does not even begin to answer the question of 'how much is there?'. A lot of qualitative analysis is simple and quick to carry out, for example the tests for sulphates and the test for chlorides can both be carried out in a few seconds. Some tests take a little longer, for example the flame test, as the equipment used needs careful preparation to ensure that it is very clean before starting.

Analysing substances such as organic chemicals is a lot more complicated because the chemicals of life are usually complex and can contain many different elements. They are also usually covalently bonded which makes it much more difficult too - substances that are ionically bonded are a lot easier to analyse. Your analysis of organic chemicals at GCSE will probably be limited to the biuret test for proteins and the bromine (or iodine) test for saturated compounds. If you study chemistry at higher levels, you will undoubtedly have to learn some of the more complex tests for the qualitative analysis of organic substances.

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  1. A positive test for nitrate ions would give which of the following observations?
    Red litmus turning blue shows that an alkali has been formed
  2. The copper oxide that is produced when copper carbonate is heated is what colour?
    The copper is reduced to copper (I) and the carbonate decomposes, releasing carbon dioxide into the air
  3. If iodine solution is added to ethane, what observation would be made?
    Ethane is saturated and does not have a double bond that can react with the iodine
  4. Some metal ions form coloured precipitates with sodium hydroxide. Which metal ion forms a light blue precipitate when reacted with sodium hydroxide?
    Transition metal ions often form coloured compounds, copper is no exception
  5. If dilute nitric acid and silver nitrate are added to an unknown solution and a cream precipitate appears, we can tell that the ions that are present are...
    You need to know the results of this test off by heart for the exams
  6. What is used to identify an unsaturated compound?
    This is used to test for unsaturated hydrocarbons and oils. Iodine can be used instead of bromine
  7. How do we test for sulphate ions?
    When sulphate ions combine with barium ions, they form barium sulphate which is insoluble and therefore appears as a precipitate
  8. If ammonia gas is produced when sodium hydroxide is added to an unknown solution, what ions were present in the unknown solution?
    The ammonium ions have the formula NH4+. They donate a proton to the hydroxide forming water and NH3 which is a gas at room temperature
  9. Which of the following ions dissolves in excess sodium hydroxide solution?

     Both magnesium and calcium ions are insoluble in excess sodium hydroxide solution

  10. Flame tests can be used to identify metals in groups 1 and 2 of the Periodic Table. Which metal burns with an intense yellow/orange flame?
    Street lights are yellow/orange due to the sodium they contain

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