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