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Unformatted text preview: Answering Questions on Identifying Unknown Inorganic Compounds Number 63 1 C hem F actsheet www.curriculumpress.co.uk Before reading through this Factsheet you should: Be confident in assigning ionic formulae; Have gained practical experience of inorganic chemistry tests and preparations. (Factsheet 24); Know and understand the AS/A2 content on inorganic chemistry (Factsheets 13, 14, 19, 20, 38, 46, 47). After working through this Factsheet you will be able to: Identify unknown inorganic chemicals when supplied with the necessary information in a written exam or practical situation. The aim of this Factsheet is to provide candidates with the necessary information and experience to tackle questions involving the identification of unknown inorganic compounds. Such questions are commonplace in both written and practical exam situations, and also practical assessments. In practical situations candidates will most likely be required to carry out simple chemical tests and make observations. In an exam situation the observations may be supplied, or the question may involve selecting the correct reagents for particular situations. This Factsheet will provides summaries of the common tests (see tables at the end of the Factsheet) and help in making inferences from the test results. Strategy Read the question carefully, assume any information about the unknown chemical supplied is important and must be considered. It is worth "ticking off" the information to make sure you have used it all. Unknown inorganic compounds are likely to be ionic - identification is usually achieved through chemical testing for both the anion and cation. Although some pieces of information can be used in isolation - eg a flame test - be aware that you will often have to make deductions from the results of more than one test - for example, a white precipitate with acidified barium chloride solution indicates a sulphate or hydrogen sulphate, but you will need to combine this with a further test (addition of sodium carbonate solution) to distinguish between the two. Chemical tests are not the only things to provide useful information. You should also note the colour of the compound, and whether it is soluble or not:- Colour- a coloured compound suggests a transition metal, most group 1 and 2 compounds are white Solubility alkali metal, ammonium salts and nitrates are soluble halides are soluble except for lead, mercury and silver sulphates are soluble except for calcium, strontium, barium, silver, mercury and lead carbonates and hydroxides are insoluble except for group 1 and ammonium compounds. These can provide a useful check on the results of chemical tests - if you think you have a solution of copper carbonate, for example, you cannot be right, as it is insoluble. Likewise, a blue compound is unlikely to be a sodium salt....
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This note was uploaded on 03/08/2011 for the course CHEM 101 taught by Professor Hard during the Spring '11 term at UT Arlington.
- Spring '11