CHEMICAL ANALYSIS and IDENTIFICATION TESTS

Doc Brown's Chemistry  Qualitative Methods of Analysis Revision Notes

PART 1 INTRODUCTION and chemical identification test index (repeated on each page)

Introduction to qualitative analysis tests to identify inorganic gases & ions (cations/anions) and organic molecule functional groups

Note - Chemical Calculations and Quantitative Analysis Index

QUALITATIVE CHEMICAL TEST INDEX

Part 1 Introduction to chemical testing and analysing substances (this page)

Part 2 Qualitative tests to identify organic molecule functional groups of homologous series

Part 3 TESTS for Metal cations (positive ions), metal carbonates, ammonium ion, hydrogen ions (acids)

Part 4 TESTS for Gases, water and non–metallic elements

Part 5 TESTS for Anions (negative ions) including hydroxide (alkalis)

APPENDIX 1. IDENTIFYING ELEMENTS from LINE SPECTRA (non-chemical test method)

PART 1 INTRODUCTION to CHEMICAL TESTS and ANALYSING SUBSTANCES

Of what use is chemical analysis and chemical testing?

Well, rather a lot, as it happens! The production of many products we use in our society involved some kind of chemical analysis at some stage or other. It might analysing mineral ores for their metal content or testing the final purity of some drug/medicine from the pharmaceutical industry.

For chemical analysis analysts have developed a wide range of qualitative tests to detect specific chemicals which may be molecules or ions. Chemical tests are based on reactions that produce a gas with distinctive properties, or a colour change produced by adding a reagent or the production of an insoluble solid that appears as a precipitate. Modern instrumental methods provide a fast, sensitive and accurate means of analysing chemicals, and are particularly useful when the amount of chemical being analysed is small. Forensic scientists and drug control scientists rely heavily on such instrumental methods in their work.

Environmental agencies monitor levels of oxygen and pollutants in rives and lakes. Hospitals carry out complex blood analysis for iron, proteins, cholesterol etc. and this is very important diagnostic information for clinicians and doctors seeking to get you well again prevent diseases developing. Blood can be analysed for specific proteins to indicate particular medical conditions e.g. cancer, and ions such as sodium, chloride and iron compounds. At water treatment plants test are done to determine the levels of metal ions, insecticides and lots of other substances to check that their levels are not high enough to be harmful to humans.

Apart from a multitude of forensic tests for DNA, powder burns from guns, explosives etc. the police use breathalyser kits to test for alcohol levels in your breath and may request a blood sample for analysis too.

Elements and compounds can be detected and identified using instrumental methods which are accurate, sensitive and rapid and are particularly useful when the amount of a sample is very small

There are two types of chemical analysis

Qualitative tests and quantitative analysis. Whatever the nature of the investigation, all tests of analyses should be carried out by using 'tried and tested' standard procedures. They should be the most accurate, reliable and safest methods that can be devised. It means, whatever laboratory you work in, anywhere in the world, you should get the same results as long as the sample is collected, stored and analysed by the same standard procedures.

(a) QUALITATIVE CHEMICAL ANALYSIS

Qualitative chemical analysis indicates whether a particular substance is present or not. It does not tell how much of the substance is there or its concentration. However, if a substance is potentially harmful, even toxic, its a good idea to know whether the substance is there at all. The larger the sample you have, the better. With more to work with you are more likely to able to detect minute traces of substances with qualitative tests AND have spare material to repeat tests several times if the results seem uncertain at first.

At school/college level, the simple tests you learn enable to identify the cation (+ve ion e.g. metal ions) and anion (-ve ion, e.g. chloride, sulfate) in a salt, and the salt usually does contain only two ions, but some salts do have three ions e.g. iron(II) ammonium sulfate which might take a bit of sorting out. Most tests at this level are done on soluble substances so that you can dissolve the substance in water and carry out tests on the aqueous solution. Aqueous means a solution in water. You may come across a non-aqueous solution using a non-aqueous solvent like alcohol or hexane.

AND don't forget, negative test results are just as important as positive results, you may need to eliminate possibilities as well as confirm the presence of a particular ion or gas etc.

UNFORTUNATELY, not all tests are unique for a particular ion, but this shouldn't be a problem in school chemistry!

Associated qualitative analysis links

See index at top of page Typical qualitative tests are described in Parts 2 to 5.

Summary of some cation and anion tests for GCSE/IGCSE/O Level students

TOP OF PAGE

(b) QUANTITATIVE CHEMICAL ANALYSIS

Quantitative analysis gives you (hopefully) a precise measure of how much of a substance is present or its concentration in a sample being analysed e.g. ore analysis tells you whether it is worth exploiting for a metal, the purity of drug ensure no harmful impurities in it, blood sample analysis for alcohol allows the police to decide to prosecute for 'being over the limit'. With a large sample you have spare material to repeat the quantitative analysis several times to get the most statistically valid result.

In schools and colleges you can do quite accurate titrations to illustrate quantitative analysis.

Associated quantitative links

% purity of a product (GCSE/IGCSE/O level, introduction for Advanced Level)

Volumetric titration analysis methods and calculations (GCSE/IGCSE/O level, introduction for Advanced Level)

Various non-redox titration methods and questions (Advanced Level, acid-alkali, EDTA, silver nitrate etc.)

Various redox titration methods and questions (Advanced Level, potassium manganate(VII), thiosulfate/iodine etc.)

Other Associated Qualitative or Quantitative Analysis LINKS (GCSE/IGCSE Level)

Instrumental Methods of Chemical Analysis

Chromatography - Paper/Thin Layer Chromatography and Gas Chromatography

GCSE/IGCSE Revision QUIZ on chemical tests for identifying ions, gases and compounds

Part 1 contd. Alphabetical TEST INDEX for cations, anions, organic functional groups etc.

Full list of KEYWORDS for inorganic/organic identification methods in alphabetical order e.g. test/reagent for: * acid ==> H⁺ * acid/acyl chloride RCOCl * alcohols – general ROH/prim RCH₂OH/sec R₂CHOH/tert R₃COH) * aldehyde RCHO * prim aliphatic amine R–NH₂ * aliphatic/aromatic carboxylic acids * alkali ==> OH^– * alkane/alkene >C=C</alkyne –CC– (saturated versus unsaturated) * aluminium/aluminum ion Al³⁺ * amide RCONH₂ * prim aliphatic amines R–NH₂ * ammonia gas NH₃ * ammonium ion NH₄⁺ * prim aromatic amine C₆H₅–NH₂ etc. * barium ion Ba²⁺ * Benedict's solution * Brady's reagent * bromide ion Br^– * bromine Br₂ * caesium ion Cs⁺ * calcium ion Ca²⁺ by flame or hydroxide ppt. * carbonate CO₃^2–/hydrogencarbonate HCO₃^– with acid or effect of heating metal carbonate e.g. MCO₃ * carbon dioxide gas CO₂ * carboxylic acid RCOOH * carboxylic acid (aliphatic) salts e.g. RCOO^–Na⁺ * chloride ion Cl^– * chlorine gas Cl₂ * Chromate(VI) ion CrO₄^2– * copper(II) ion Cu²⁺ by flame or hydroxide ppt. * 24DNPH (for aldehydes/ketones test) * esters RCOOR * Fehlings test/solution * flame test for metal ions * fluoride ion F^– * haloalkanes/halogenoalkanes R–X * hydrogen gas H₂ * hydrogen sulphide H₂S * hydrogen ion, acids H⁺ * hydrogen bromide gas/hydrobromic acid HBr * hydrogen chloride gas/hydrochloric acid HCl * hydrogen iodide gas/hydriodic acid HI * hydroxide ion, alkali OH^– * hydroxy/alcohol/phenol (organic) * iodide ion I^– * iodine I₂ * iodoform test – formation of CHI₃ * iron(II) ion Fe²⁺ * iron(III) ion Fe³⁺ * ketone R₂C=O * lead(II) ion Pb²⁺ * lithium ion Li⁺ * lime water Ca(OH)_2(aq) * magnesium ion Mg²⁺ * metal carbonates–heating e.g. MCO₃ * metal ions via hydroxide precipitate * nitrate or nitrate(V) NO₃^– * nitrite or nitrate(III) NO₂^– * nitrogen dioxide or nitrogen(IV) oxide NO₂ * oxygen gas O₂ * phenols C₆H₅OH etc. * potassium ion K⁺ * rubidium ion Rb⁺ * reducing sugars * saturated/unsaturated * silver nitrate AgNO₃ (see chloride, bromide, iodide tests) * sugars (reducing) * sodium ion Na⁺ * strontium Sr⁺ * 'sulphate/sulfate' or sulphate(VI) SO₄^2– * sulphide S^2– * 'sulphite/sulfite' or sulphate(IV) SO₃^2– * sulphur dioxide gas SO₂ * Tollen's Reagent * unsaturated/saturated * water H₂O * zinc ion Zn²⁺ *

Use the list above for identifying anions, cations, gases, molecules etc. to find what you require! for your KS3–KS4 Science–GCSE–IGCSE– Chemistry and GCE–AS–A2–IB–US grades 9–12 K12 advanced subsidiary chemistry course etc. and help you to identify unknown inorganic and organic compounds–molecules for qualitative analysis. Its also a good idea to read the brief notes after the alphabetical list.

EMAIL query? comment? test missing? * Gas Preparations *  Hazard warning signs/symbols–examples of labelling

• PLEASE NOTE:

• Most of the tests describe use simple apparatus like test tubes, teat pipette, wire for flame test (nichrome, platinum best but costly) and standard chemical reagents accessible in most school or college laboratories.

• Where possible balanced symbol equations are given for the reactions occurring in doing the test.

• Sometimes a precipitate (ppt) initially forms with a limited amount of a reagent, it may then dissolve in excess of reagent to give a clear solution. Both observations will be crucial for a positive id.

• There are no tests specific to identify a compound e.g.

  • (i) there is no test for calcium chloride, but there are tests for the calcium ion and the chloride ion, i.e. using specific ion tests.

  • (ii) Similarly, in organic tests, all you can do is identify a functional group i.e. a particular bit of the molecular structure of a member of a homologous series, rather a particular unique molecule.

  • Not all the reactions are good definitive tests, but they may well be important reactions of cations or anions you need to know about.

• The first tests in the 'inorganic' section are typical of GCSE Science level, but finally these overlap and extend into those needed for GCE Advanced AS or A2 level. In the organic section, only the alkene test is in GCSE double award science, but some others might be found in a full single or coordinated triple award GCSE syllabus. 

• If any GCSE/IGCSE/GCE/AS/A2/IB/US grade 8–12 K12 test seems missing, just let me know by email

• These days more emphasis is given to modern spectroscopic methods of analysis such as NMR, Infrared, Mass spectrometry, Atomic Emission etc.  Quite correctly, though updating A level chemistry is intellectually challenging at times, it isn't always as much fun!

• The methods described give no recipe details or risk assessment, just basically what is needed, what you see and what you can or cannot deduce. Consult teacher, 'practical' text books and Hazcards before attempting any analysis.

• Most tests involve 'standard' chemical reactions and few tests are totally specific so observations should be viewed in context, i.e. is this a realistic deduction in that particular situation? 

• Please remember each syllabus has its own 'list' of required tests – so do not 'over learn' – check out what is needed!

• There is a web page covering the methods some safety aspects of "Preparing and collecting gases".

• Use the alphabetical list to find the test you need.

A brief description of what the hazard might be.

Biohazard: Biohazardous materials include anything that may cause disease in  living organisms or cause significant impact to the environment or community.

NEW SECTION currently working on the table of examples below. 

e.g. chemicals toxic to aquatic wildlife an in general harmful to organisms and the environment e.g. toxic metals like mercury, old pesticides like DDT.

 Doc Brown's Chemistry 

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